An introduction to the thresher shark

Introduction

The thresher shark is an all-encompassing term referring to three surviving species from the family Alopiidae: the pelagic thresher (Alopias pelagicus), the bigeye thresher (Alopias superciliosus), and the common thresher (Alopias vulpinus).

Pelagic thresher shark (Alopias pelagicus). By Thomas Alexander – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=50280277

Bigeye thresher shark (Alopias superciliosus). By PIRO-NOAA Observer Program – http://ias.pifsc.noaa.gov/lds/obs_training/SharkThresherNew.pdf, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6534429

Common thresher shark (Alopias vulpinus). By NOAA/PIER – http://www.nmfs.noaa.gov/gallery/images/photos/6472384727.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=18920049

Comparison between the three thresher species. By FactZoo.com – https://www.factzoo.com/fish/pelagic-thresher-shark-longtail-smack.html

Morphology

Thresher jaws are small. They have clearly not evolved to attack prey like tuna and seals; if anything, they look like they should be eating small crabs or scavenging chunks of floating debris that someone else took the time to kill. They would seemingly struggle to catch a live fish because the long snout overhangs their tiny mouth so much. That’s where the notorious tail comes in. The thresher tail is designed to be used like a whip to strike and immobilise prey [1], [2]. Such force occurs with a single tail slap that gas is diffused out of the seawater and rises to the surface in bubbles [1]. Typically, pelagic sharks hunt and capture one fish at a time; this strategy enables the shark to catch on average 3 fish, and sometimes more, in one go.

A sequence of still images taken from an overhead tail-slap hunting event [1].

The easiest way to differentiate between the three species is the colouration. Common threshers are likely to be more grey and they lack any colouration above their pectoral fins; sometimes, they have white dots on the tips of their fins. Pelagic threshers have distinct colouration above their pectoral fins. The bigeye’s eye is visible from the top of the head and has characteristic groves above the eyes and gills.

A common thresher shark, identified by the lack of colouration above the pectoral fin and white dots on the tips of the pectoral and dorsal fins. By Paul E Ester at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=19628266

A pelagic thresher shark, identified by the colouration above its pectoral fin. By NOAA Observer Program – http://www.fpir.noaa.gov/Graphics/OBS/obs_sharks/obs_pelagic_thresher_sharks/obs_pelagic_thresher_shark5.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5553329

A bigeye thresher shark, distinguished by the birds-eye view of the large eyes and the lateral grooves. By PIRO-NOAA Observer Program – http://www.fpir.noaa.gov/Graphics/OBS/obs_sharks/obs_bigeye_thresher_sharks/obs_bigeye_thresher_shark5.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6534392

A bigeye thresher shark is distinguished by its large eyes and lateral groove above the eye and gills. By PIRO-NOAA Observer Program – http://www.fpir.noaa.gov/Graphics/OBS/obs_sharks/obs_bigeye_thresher_sharks/obs_bigeye_thresher_shark4.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6534388

Distribution

Threshers are pelagic, meaning they live in the deep, open ocean. The only thresher known to inhabit New Zealand waters is the largest of the trio, the common thresher [3]. The bigeye may also be found in NZ, but due to its habit of staying hundreds of metres below the ocean surface during the day, it is unlikely to be encountered if it is there at all [4].

Threats

The common and bigeye thresher are ‘Vulnerable’ to extinction under the IUCN Red List [3], [5], and the pelagic thresher is further threatened and classified as ‘Endangered’ [6]. Estimated global population reductions are as follows:

• Common thresher: 30–49% over 3 generations (76.5 years)
• Bigeye thresher: 30–49% over 3 generations (55.5 years)
• Pelagic thresher: 50–79% over 3 generations (55.5 years)

All three species are globally targeted for their meat, fins, skin, and liver oil [7], the latter commonly used in modern cosmetics and supplements. All species are usually retained if accidentally caught on commercial vessels [8]–[11]. In one study [15] conducted in the Indian Ocean, 13 out of 19 bigeye threshers caught on a commercial vessel’s longline were dead upon retrieval. In the same study, in the Atlantic Ocean, 412 out of 849 bigeye threshers caught on longlines were dead. On the same ships were deaths of stingrays and manta rays as well as blue, shortfin mako, silky, and smooth hammerhead sharks. One of the largest hubs for shark fin trading globally is Hong Kong [9], and threshers accounted for up to 3% of the total fin mass in the past [8].

Threshers are also popular among recreational, big game anglers. Although a tag and release method is more commonly practised nowadays, there is also a risk of post-release mortality. With threshers likely to be hooked from their tail due to their hunting style, a study of common threshers found 78% of tail-hooked sharks died after release [12].

The New Zealand commercial fishery is not exempt from thresher shark landings. A recent report by Fisheries New Zealand [13] saw that in 5 years from the 2014/15 to 2018/19 commercial fishing seasons, 149,916 tonnes of common thresher shark were caught by the core deep-water fleet. With the average 5-metre common thresher shark usually weighing in at 230 kg, we can infer that in the 5 years, the deep-water fleet landed approximately 651 individual thresher sharks.

Thresher vs swordfish

In April 2020, a dead, 4.5 metre, female bigeye thresher was found beached on a Libyan coast [14]. A 30 cm swordfish (Xiphias gladius) rostrum was found embedded in its head. It is understood that the rostrum severely injured some of the shark’s nerves, arteries, and gill arches. It was concluded that the impalement was what led to the ultimate death of the shark.

A young swordfish (Xiphias gladius). By Michael Landress, CC BY-NC-ND 2.0 – https://www.flickr.com/photos/myfwc/26802909040

Taken from Jambura et al (2021) [14]. “Female bigeye thresher Alopias superciliosus (TL = 445 cm) stranded on the Libyan coast (Mediterranean Sea), with a swordfish Xiphias gladius rostrum embedded deep in the branchial chamber. Scale bars indicate 50 cm (b) and 10 cm (c and d). Photo (a and b) and video content (c and d) courtesy of Faraj Habrisha and Abdalhakim Ahmed Al sebaihe”.

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[1]        S. P. Oliver, J. R. Turner, K. Gann, M. Silvosa, and T. D’Urban Jackson, “Thresher Sharks Use Tail-Slaps as a Hunting Strategy,” PLoS One, vol. 8, no. 7, 2013, doi: 10.1371/journal.pone.0067380.

[2]        S. A. Aalbers, D. Bernal, and C. A. Sepulveda, “The functional role of the caudal fin in the feeding ecology of the common thresher shark Alopias vulpinus,” J. Fish Biol., vol. 76, no. 7, pp. 1863–1868, 2010, doi: 10.1111/j.1095-8649.2010.02616.x.

[3]        C. L. Rigby et al., “Alopias vulpinus,” IUCN Red List Threat. Species 2019, vol. e.T39339A2, 2019.

[4]        H. Nakano, H. Matsunaga, H. Okamoto, and M. Okazaki, “Acoustic tracking of bigeye thresher shark Alopias superciliosus in the eastern Pacific Ocean,” Mar. Ecol. Prog. Ser., vol. 265, pp. 255–261, 2003, doi: 10.3354/meps265255.

[5]        C. L. Rigby et al., “Alopias superciliosus,” IUCN Red List Threat. Species 2019, vol. e.T161696A, 2019.

[6]        C. L. Rigby et al., “Alopias pelagicus,” IUCN Red List Threat. Species 2019, vol. e.T161597A, 2019, doi: https://dx.doi.org/10.2305/IUCN.UK.2019-3.RLTS.T161597A68607857.en.

[7]        R. W. Jabado et al., “The trade in sharks and their products in the United Arab Emirates,” Biol. Conserv., vol. 181, pp. 190–198, 2015, doi: 10.1016/j.biocon.2014.10.032.

[8]        S. C. Clarke, J. E. Magnussen, D. L. Abercrombie, M. K. McAllister, and M. S. Shivji, “Identification of shark species composition and proportion in the Hong Kong shark fin market based on molecular genetics and trade records,” Conserv. Biol., vol. 20, no. 1, pp. 201–211, 2006, doi: 10.1111/j.1523-1739.2005.00247.x.

[9]        A. T. Fields et al., “Species composition of the international shark fin trade assessed through a retail-market survey in Hong Kong,” Conserv. Biol., vol. 32, no. 2, pp. 376–389, 2018, doi: 10.1111/cobi.13043.

[10]      S. C. Clarke et al., “Global estimates of shark catches using trade records from commercial markets,” Ecol. Lett., vol. 9, no. 10, pp. 1115–1126, 2006, doi: 10.1111/j.1461-0248.2006.00968.x.

[11]      F. Dent and S. Clarke, “State of the global market for shark products,” Rome, Italy, 2015.

[12]      C. A. Sepulveda et al., “Post-release survivorship studies on common thresher sharks (Alopias vulpinus) captured in the southern California recreational fishery,” Fish. Res., vol. 161, pp. 102–108, 2015, doi: 10.1016/j.fishres.2014.06.014.

[13]      Fisheries New Zealand, “Annual Review Report for Deepwater Fisheries for 2018/19,” 2020.

[14]      P. L. Jambura, J. Türtscher, J. Kriwet, and S. A. A. Al Mabruk, “Deadly interaction between a swordfish Xiphias gladius and a bigeye thresher shark Alopias superciliosus,” Ichthyol. Res., vol. 68, no. 2, pp. 317–321, 2021, doi: 10.1007/s10228-020-00787-x.

[15]      R. Coelho, J. Fernandez-Carvalho, P. G. Lino, and M. N. Santos, “An overview of the hooking mortality of elasmobranchs caught in a swordfish pelagic longline fishery in the Atlantic Ocean,” Aquat. Living Resour., vol. 25, no. 4, pp. 311–319, 2012, doi: 10.1051/alr/2012030.

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Estuaries: how habitats and food webs create transitional estuarine ecosystems of high productivity

Introduction

An estuary is a coastal passage of water where tides from the open sea meet river currents, and salty seawater mixes with fresh riverine water to create an environment of high productivity (Correll, 1978; Pritchard, 1967). The intermediate location of estuaries causes it to act as a transitional environment between terrestrial and marine ecosystems. Estuaries can be viewed as a continuum, whereby further upstream, the calmer, fresher water of the river progresses into the tidal, saline water of the ocean. This estuarine continuum – due to the range of its attributes, including salt content, depth, temperature, tidal energy, et cetera – allows for the diversification of the habitats within it. Three key estuarine habitats include salt marshes, mangroves, and seagrass beds. The organisms within these habitats interact with one another in numerous ways, creating not only food chains but food webs of complex interactions and energy exchanges.

Correll (1978) states that estuaries are areas of unusually high productivity because the conditions that create estuaries result in high levels of photosynthate from in situ primary production byphytoplankton and plants. Estuarine food webs are sustained by their habitats; therefore, it is important to analyse the relationship between habitats and food webs and how they contribute to the overall high productivity of estuaries. Furthermore, because estuaries are closely linked with terrestrial and marine ecosystems, estuarine productivity may be attributed to or contribute to adjacent environments.

In this essay, I will do the following: describe the features of estuarine habitats and food webs; determine how estuarine habitats and food webs are both linked with terrestrial and marine ecosystems, making estuaries a transitional environment; and critically analyse how habitats and food webs contribute to a highly productive estuarine system.

Estuarine habitats

In order to analyse the high productivity of estuaries, it is crucial first to explain the features of estuarine habitats and the organisms which inhabit them. Links to food webs from the habitats can then be made to understand better how energy and nutrients enters the estuarine system. Three important estuarine habitats exist, including salt marshes, mangroves, and seagrass beds; each possesses unique attributes and organisms living there.

Salt marshes are wetlands dominated by few species of halophytic vegetation and are alternately inundated and drained by the tides (Christiansen, Wiberg, & Milligan, 2000; Gedan, Altieri, & Bertness, 2011; Richardson, Swain, & Wong, 1997). The halophytic plants add stability to the marsh by trapping and binding sediment and can also withstand stressful conditions like the constant flooding and draining of the marsh as well as the salty, waterlogged, and, sometimes, hypoxic soil (Gedan et al., 2011; Pennings, Grant, & Bertness, 2005). Three types of fauna utilise the salt marsh habitat. (1) Specialist species inhabit marshes as their primary home, utilise the high vegetation biomass for energy, and are well adapted against desiccation; these specialist animals tend to be invertebrate species (Kon et al., 2012; Minello & Webb, 1997; Richardson et al., 1997). (2) Another type of fauna uses marshes as an extension of their home and are therefore regular visitors to the marsh. As a general example, a crab species uses the marsh for protection while their home in the mudflat is under heavy predation, then returns home once predation has eased. (3) Terrestrial or ariel species venture into the marsh for certain activities such as gathering food, courtship, or nesting (Burger, 1979; Hanson & Shriver, 2006; Teixeira, Duarte, & Caçador, 2014). Overall, the conditions that define a salt marsh create a habitat dominated by marine invertebrates (Daiber, 1982).

Restored Tomago salt marsh.
By John Turnbull – Own Work, CC BY-NC-SA 2.0, https://www.flickr.com/photos/johnwturnbull/31538875578/

Mangroves are woody plants that can withstand excessive salt concentrations and water inundation, and “true” mangroves are from the genus Rhizophora. Mangroves are distributed globally in tropical and sub-tropical regions (Parida & Jha, 2010; Simard et al., 2019). Mangrove plants contend with adverse conditions like saline water and anoxic substrates, which are difficult, if not impossible, environments for terrestrial plants to live in. Mangroves have combatted this by developing salt glands, roots with low permeability, and adventitious roots with pneumatophores (Liang, Zhou, Dong, Shi, 2008; Medina, 1999; Parida & Jha, 2010; Yáñez-Espinosa, Terrazas, & Angeles, 2008). The foliage, trunks, and roots of mangroves all provide unique niches for organisms to inhabit (Nagelkerken et al., 2008). Above the water, the foliage provides a habitat for terrestrial fauna like birds, mammals, reptiles, and insects. The trunks and pneumatophores provide a rigid substrate for organisms to settle on. Under the water, the roots can support sponges, bivalves, and algae, and provide a labyrinth of microhabitats for aquatic species like fish. Due to the abundance of food and shelter, mangroves may function as a nursery for juvenile fish and invertebrates species (Nagelkerken et al., 2008).

Mangrove roots.
By Jonathan Wilkins – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=40213291

Above and below water view of mangroves and roots.
Public Domain, https://commons.wikimedia.org/w/index.php?curid=142126

Seagrasses are the only flowering plants that have evolved to survive submersion in saltwater (Orth et al., 2006). The seagrasses, anchored to the sediment by underground runners called rhizomes, congregate to form vast beds that provide a large habitat area for organisms to inhabit. The stability of seagrass habitats means it is a popular environment for transient species to lay eggs and is one of the most productive and diverse marine ecosystems (Constanza et al., 1997; Duarte & Chiscano, 1999). Seagrass habitats provide shelter, food, and a nursery area for fish and marine invertebrates as well as food for waterfowl and marine mammals like dugongs and manatees (Heck, Hays, & Orth, 2003; Holm & Clausen, 2006; Rybick & Landwehr, 2007; Thayer, Bjorndal, Ogden, Williams, & Zieman, 1984; Weisner, Strand, & Sandsten, 1997).

Seagrass meadow (Zostera marina) in the Dzharylhach Bay, Ukraine.
By Sofia Sadogurska – Own work, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=97165229

Carbon uptake and photosynthesis in a seagrass meadow. Special cells within the seagrass, called chloroplasts, use energy from the sun to convert carbon dioxide and water into carbohydrates (or sugar) and oxygen through photosynthesis. Seagrass roots and rhizomes absorb and store nutrients and help to anchor the seagrass plants in place.
By Cullen-Unsworth L, Jones B, Lilley R and Unsworth R – [1], CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=89279174

Estuarine food webs

Habitats contain niches that make way for a variety of interactions between organisms. In estuaries, salt marshes, mangroves, and seagrass beds create unique environments that induce complex food webs. These food webs begin with an input to the first trophic group known as the primary producers, a group that includes phytoplankton or plants like the vegetation of salt marshes, algae, and seagrasses. Trophic transfers take place from one group to another as the energy that began with primary production is moved up the food web to other trophic groups like herbivores or carnivores. During trophic transfers, some energy may be transported out of the food web into a cycle (such as the nitrogen cycle) or be expelled as a respiratory end product (Day, Kemp, Yáñez-Arancibia, & Crump, 2012).

A freshwater, marine, and terrestrial food web.
By File:FoodWeb.jpg: Thompsmaderivative work: Pixelsquid – This file was derived from: FoodWeb.jpg:, CC0, https://commons.wikimedia.org/w/index.php?curid=102527971

Detritus is dead, particulate, organic material, and can be processed repeatedly by organisms at most levels of a food web until it is either buried in sediment or exported (Darnell, 1961); therefore, detritus is an important input to food webs. Salt marsh vegetation, mangrove litter, and seagrasses are likely to provide significant amounts of plant detritus to estuarine ecosystems (Teal, 1962; Odum & de la Cruz, 1963).

Estuaries as transitional environments of high productivity

Estuaries are a transitional environment between terrestrial and marine ecosystems. Terrestrial species will use estuarine salt marshes for specific activities because the marshes provide an environment unlike any terrestrial habitat. Furthermore, fishes may also enter salt marshes at high tide to feed before returning to the marine environment (Heck et al., 2008). Liang et al. (2008) determined mangroves as a buffer for the terrestrial environment against marine energies such as erosion and damage from waves and typhoons. Seagrasses provide an essential link from estuaries to marine ecosystems because live seagrass leaves not consumed enter the detrital pool (Heck et al., 2008). The detrital seagrass can be transported by currents and waves over extensive distances and be used by organisms for shelter, habitat, or consumed along the way (Ochieng & Erftemeijer, 1999; Thresher, Nichols, Gunn, Bruce, & Furlani, 1992). The detrital seagrasses journey may conclude in deep-sea canyons, where it accumulates and supports organisms in an environment with poor habitat structure and low primary production (Josselyn et al., 1983; Vetter, 1998). Thus, seagrass meadows link estuaries with an unlikely marine environment, the deep sea.  Estuaries also receive significant detritus inputs from both terrestrial and marine ecosystems (Day et al., 2012).

Estuaries are thought to be highly productive per unit area. This high productivity can mean that the plants are very productive, that more organic matter is produced than used, or that estuaries sustain a substantial amount of fauna compared to other environments (Day et al., 2012). Schelske and Odum (1962) considered the different contributors to one estuary’s productivity, and these can be extended to all estuaries in a general sense:

• Firstly, there is diversity in the sources of organic matter that enter an estuary. The three key estuarine habitats (salt marshes, mangroves, and seagrass beds) all provide organic matter to estuarine systems. These habitats provide substrata for epiphytic algae to grow on and shallow, photic waters for pelagic phytoplankton. Adjacent terrestrial environments also passively contribute organic matter into the estuarine system. More specifically, the many sources of organic matter in an estuary can be narrowed down to a diverse species abundance of photosynthetic organisms. This means that light can be utilised throughout all seasons, reducing the chance of a substantial drop in organic matter production by the photosynthesising species.
• Secondly, physical energy, such as tides, currents, wind, and waves, produces considerable water movement in estuarine ecosystems, increasing the surface area by which phytoplanktonic photosynthesis can occur. For example, the flooding and draining of a salt marsh transports material and nutrients in and out of the marsh, and tides and currents transport unused seagrasses out of the seagrass beds to enter a different part of the food web.
• Thirdly, an abundant supply of various nutrients is thought to be present in estuaries, especially when compared to marine and freshwater environments. Nutrient abundance in estuaries allows for a higher rate of photosynthesis (Nixon 1980, 1995; Rhyther & Dunstan, 1971).
• Finally, because estuaries are relatively shallow and well-mixed, both food and nutrients are available to all organisms in the water column and benthic and pelagic food webs can be coupled, creating interactions that may be unique to estuaries alone (Day et al., 2012). Bivalves and microorganisms also play a role in biogeochemical cycling of nutrients. The mixing of salt and freshwater provides high nutrients in both the water and the sediment.

Habitats could potentially be viewed as the most important contributor to estuarine productivity because they give way to the environments which allow estuaries to be productive; the complexities of estuarine food webs would cease to exist if it were not for the availability of diverse habitats in which they can operate. Salt marsh, mangrove, and seagrass bed ecosystems have relationships with terrestrial and marine ecosystems, increasing estuaries’ productivity. The value of these habitats to certain trophic levels depends on their needs for shelter, food, or to carry out species-specific activities, showing that the way these habitats are structured affects trophic transfers in estuarine food webs (Walters, 2000). Some estuarine habitats may increase production in other estuarine habitats and adjacent ecosystems. The diversity of the mangrove ecosystems gives way to a rich microbial community, supported by nutrient cycling, which may promote faster sponge growth than in coral reefs (Kathiresan & Bingham, 2001). Odum and Helad (1972) suggest that the production of mangroves is exported to the marine environment, which provides food for secondary consumers and may lead to higher production in the marine ecosystem. Heck et al. (2008) describes how the highly productive nature of seagrass meadows can spill over into other ecosystems. For example, some fish live in mangroves but feed on seagrass, creating a trophic link between the two habitats. Also, some fish that inhabit coral reefs feed on seagrass, shifting organic matter and nutrients into the reef habitat and food web.

Conclusion

Productivity of any ecosystem is easily measured by the quantity of photosynthate it can produce because primary production essentially supports entire food chains. Estuaries have such a plethora of trophic interactions that it is necessary to extend their food chain to a food web that spans multiple ecosystems. This food web would cease to exist without the dynamic habitats that support them, with the key habitats being salt marshes, seagrass beds, and mangroves. The features that make these habitats unique from any other habitat globally, are also the features that sustain the species that live in them. Because estuaries are a transitional environment between terrestrial and marine ecosystems, it is likely that they benefit from the high productivity of estuaries, and this may be shown through the trophic interactions and nutrient cycling that occurs. Estuaries are a dynamic environment, and further research is needed to understand the global effect of their high productivity to learn how to better protect them.

An estuary in Auckland, New Zealand, showing how closely estuaries and terrestrial (specifically urban) environments are linked.
By Getty Images, https://www.newsroom.co.nz/ideasroom/reducing-pressure-on-aucklands-estuaries

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The species-area relationship and Bergmann’s rule: attempts to explain global patterns in biodiversity

Introduction

Humans have noticed the differences between species worldwide from the time they evolved up until the present, with the term “biodiversity” first being used by Tangley (1985). A natural progression from these discoveries of global biodiversity was to explain them using such rules as the species-area relationship (SAR) and Bergmann’s rule. SARs describe the link between the area of a habitat and the number of species found within that area (Losos & Schulter, 2000; Preston, 1962). Bergmann’s rule states that species of larger size are found in colder regions, and species of smaller size are found in warmer regions (Ashton, Tracey, & de Queiroz, 2000; Mayr, 1956). Both rules are utilised in the field of biogeography, and I will be looking at evidence that both supports and opposes SARs and Bergmann’s rule to critique the validity of each.

The SAR model has ideas such as island biogeography theory (IBT) and spatial scales and studies that support it as an ecological law (Franzén, Scweiger, & Betzholtz, 2012; Losos, 1996; Losos & Schluter, 2000; Rand, 1969). IBT states that larger islands have greater numbers of species than smaller islands (MacArthur & Wilson, 2001; MacGuinness, 1984) and spatial scales keeps each SAR study relevant to the size at which the study was undertaken (Drakare, Lennon, & Hillebrand, 2006; Turner, 2005; Tylianakis, Klein, Lozada, & Tscharntke, 2006). Criticism of IBT says that its equilibrium theory is too simplistic, which raises questions about it being successfully used to support SARs (Heaney, 2000; Lomolino, 2000; Tangney, Wilson, & Mark, 1990; Toft & Schoener, 1983; Webb & Vermaat, 1990; Wu, 1995). A lack of real-life SAR studies poses the question: is SARs more valuable as a tool for biogeography rather than an ecological rule (Diamond, 1984; Zimmerman, 2000)? 

Bergmann’s rule is supported by real-life studies that not only expressed the rule but followed a logical reason as to why they adhered to it (Ashton, Tracy, & de Queiroz, 2000; Fujita, 1986; Gigliotti et al., 2019; James, 1983; Rhymer, 1992; Sand, Cederlund, & Danell, 1995; Weaver & Ingram, 1969). Conversely, there are some exceptions to Bergmann’s rule that also carry their own rational explanation (Gigliotti et al., 2019; Oishi, 2010; Takeuchi, 1995; Kaneko, 1988). Geist (1987, 1990) disagrees with Bergmann’s rule and pointed out that not only is body mass a poor conserver of heat, but that body size is also due to a multitude of different factors. 

Species-area relationship

Supportive evidence for species-area relationship

SARs depict the number of species found in an area of habitat and are plotted on a graph to show a species-area curve where x = habitat area and y = species richness (Rosenzweig, 1995). The typical outcome of plotting SARs on a graph is a sharp increase in species richness over the initial area that begins to taper off as distance increases; it shows that, over a habitat area, species richness increases. Franzén et al. (2012) regarded SARs as an ecological tool used to make predictions for areas that have little or no data behind them. This means SAR predictions can be used as a powerful instrument in conservation for remote habitats or when there is little time to collect data in the field. 

The theory of island biogeography was coined by MacArthur and Wilson in 1967, and it states that species richness increases with island size and decreases with island distance from the mainland (MacArthur & Wilson, 2001). These islands receive new colonising species from the mainland, and, eventually, established populations of species become extinct; this creates an equilibrium point where immigration equals extinction. A study by Riebesell (1982) found that the species richness of plants on 13 peaks in the Adirondack Mountains is due to the area of the alpine ecosystem and immigration between summits. This reflects IBT as the plant seeds have been spread by birds and dropped on mountaintops (more seeds dropped on a bigger area). Extinction was evident because the plants on the island were not genetically identified as relict populations, meaning that an equilibrium has been met between seed dispersal (immigration) and plants dying (extinction). 

Spatial scales are essential in verifying the use of SARs in the context of the study being undertaken (C. Bishop, personal communications, May 7, 2020). This amplifies SAR effectiveness because it gives a guideline as to how SARs should be studied and therefore reduces the likelihood of error; when spatial scales are considered during SAR research, it increases the integrity of the research. Turner and Tjørve (2005) discuss spatial bias, where smaller areas are likely to be surveyed more intensely than larger ones for practical reasons, meaning that the number of species being sampled can potentially decrease with an increase in area. In a scenario such as this, the data will less likely represent the actual species richness of the area, reducing the validity of the species-area rule. A study of anthropogenic influences on bees and wasps by Tylianakis et al. (2006) found that estimation of species diversity of a larger area based on evidence gathered at a smaller spatial scale underestimated the actual result; had this gone unnoticed, it would have meant that greater conservation attention would have been given to the wrong area. This verifies the importance of using spatial scales to enhance SARs’ effectiveness.

Studies, such as those undertaken by Losos (1996) of the Anolis lizard and Franzén et al. (2012) of butterflies and moths on true islands show SARs as real examples. Losos (1996) found in his study of Anolis lizards on 147 Caribbean islands that a significant relationship exists between the number of species and area of the island. Larger islands were found to have more species present than smaller islands that were more geographically isolated and did not support more than 2 Anolis species at a time. This supports IBT and shows that Anolis lizards on smaller, more isolated islands have reached an equilibrium point, such as the one discussed by MacArthur and Wilson (2001). A positive relationship between area and species richness of 1016 moth and butterfly species was recorded by Franzén et al. (2012), who also noted species with a smaller geographic range were more sensitive to area change than species with a larger range; this emphasises the importance of spatial scales within SAR studies.

Counter-evidence for species-area relationship

There is a discussion about MacArthur and Wilson’s (2001) IBT to be used as more of a framework than a theory. It needs to be modernised, especially as its equilibrium model appears too simplistic (Heaney, 2000; Lomolino, 2000). As IBT is a SAR, any fragility found within IBT raises questions about the reliability of SARs as a law. IBT’s equilibrium model’s simplicity lies in the fact that it only accounts for species richness due to immigration and extinction and no other factors. Even so, the idea of equilibrium being reached on an island seems far-fetched as ecological stochasticity and geographical and geological fluctuations can affect a perfectly balanced model. Tangney et al. (1990) listed five other possible explanations for any studies that showed results consistent with IBT. These include random placement hypothesis (Arrhenius, 1921), habitat diversity hypothesis (Williams, 1943), incidence function hypothesis (Diamond, 1975), small island effect hypothesis (Whitehead & Jones, 1969), and small island habitat hypothesis (Kelly, Wilson, & Mark, 1989).

Assumptions of IBT (MacArthur & Wilson, 2001), and therefore SARs, can be challenged:

1. Species richness balances immigration rates and extinction rates, excluding speciation over evolutionary time. For example, the adaptive radiation resulting in different finch species on the Galapagos Islands (Grant & Grant, 2008).
2. The assumption that immigration rates to an island result from island size and distance from the mainland fail to account for other variables. These variables include the role ocean and air currents play in transporting species to islands (Payn, Dvorak, & Myburg, 2007), anthropogenic introduction of species, or immigration from other islands or emigration from the island.
3. IBT ignores stochastic events and assumes that all species have an equal ability to immigrate or become extinct.
4. Natural disasters can cause extinction on islands; natural disasters can also function independently of island size or distance to the mainland. 
5. Climate can affect the species’ likelihood to immigrate to or leave an island. For example, a species that functions in colder weather on the mainland is unlikely to leave its niche to travel to a warmer island, and vice versa, regardless of the islands size or proximity to the mainland.

As mentioned before, SAR’s equations and models are used in conservation. Still, Zimmerman and Bierregaard (1986) introduce the thought of SAR data acting as a “blindman’s cane [to] show us roughly the way” (p. 141) after stating that predictions using IBT and SARs for conservation were nonbeneficial. Diamond (1984) mentions that the speed at which environments are being dismantled should cause ecologists to erect conservation plans faster for them to be preserved. A study of Amazonian frog conservation (Zimmerman and Bierregaard, 1986) was used as a case for the misuse of SARs and IBT and showed that these models may underestimate species richness. Therefore, mean areas receive less conservation than they need. Zimmerman and Bierregaard (1986) propose that time spent studying theory would be better used to understand the relationships that species have with their environment, which would, in turn, produce a better estimate of species richness in an area than using SAR equations. There were many articles available on SARs, yet it was all theory with little in-field evidence; lack of evidence challenges the integrity of the law. The questions here are: is the SAR model an excuse to use maths and models instead of physical evidence which could say otherwise? Is there too much theory and not enough tested hypotheses? Is the consequence of a lack of conservation, where it is needed, taxonomic extinction? In-field experimentation is necessary to answer these questions.

Bergmann’s rule

Supportive evidence for Bergmann’s rule

Bergmann’s rule says species of larger size are found in colder environments, whilst their smaller counterparts are found in warmer environments (Ashton, Tracey, & de Queiroz, 2000; Mayr, 1956). Many arguments have arisen about the validity of Bergmann’s rule (Geist, 1987, 1991; Paterson, 1990), yet, animals are not set aside from the laws of physics (Cambell, 1977), and Paterson (1988) states that Bergmann’s rule obeys mass and surface-area heat-transfer rules. (It is important to note that this refers to their mass or surface area when referring to smaller and larger individuals, rather than length unless noted (Paterson, 1988).)

Evidence of geographic variation in body size of birds has a solid environmental link, including that of the mallard ducks (Anas platyrhynchos) of America and Canada (Rhymer, 1992). It was found that the growth of the mallard ducklings was constrained by environmental conditions experienced during development; tarsus length was 3% longer in the colder Canadian population, and it was shown that this variation was due to the environment they were exposed to during maturation. James (1983) conducted a study with 12 avian species in the central and eastern United States. He found intraspecific size variation is related to a combination of temperature, moisture, and other climatic variables, rather than just temperate ones. A modification of Bergmann’s rule to include this will maintain the integrity and framework of the rule whilst adding more information and specificity to it.

Mammals in both controlled and natural environments displayed evidence of adhering to Bergmann’s rule. Pigs (Sus scrofa domesticus) raised at 5oC were bigger than pigs of the same species raised at 35oC (Weaver & Ingram, 1969). Swedish moose (Alces alces) not only had a 15-20% larger body mass in colder, northern areas than in the south, but also had higher growth rates and matured later than in warmer areas (Sand et al., 1995), indicating a slower metabolic rate for moose in colder environments. Bats (Myotic lucifugus) also showed a similar metabolic correlation to latitude and conforming to Bergmann’s rule in that the colder population grew slower, showed delayed aviation development, and were born larger (Fujita, 1986).

Northern American populations of snowshoe hare (Lepus americanus) conform to Bergmann’s rule to mitigate heat loss (Gigliotti et al., 2019). They can withstand the colder winter months by conserving heat and energy due to increased body mass. The strong selection pressure for larger female snowshoe hares is because females have higher energy demands due to gestation and lactation; the hares breed in winter months, so conserving energy during the cold period whilst also having maternal demands is essential for the hare. 

Counter evidence for Bergmann’s rule 

Contrarily, southern American snowshoe hare populations do not follow Bergmann’s rule as strongly as their northern counterparts (Gigliotti et al., 2019). This is an example of different selection forces acting on individuals, even within a species. In the northern population, a big driver for body mass was to conserve energy and mitigate heat loss, whilst, in the southern population, their large body mass was a result of an extended growing season. Gigliotti et al. (2019) found that the milder, southern winter meant heat conservation wasn’t an essential factor in determining hare body mass. Furthermore, southern populations experienced a long summer growing season and a shorter winter, meaning they had more time to accumulate body mass and less time to lose it.

Two subspecies of red fox (Vulpes vulpes) in Japan show a negative correlation to Bergmann’s rule as skull size is bigger in the subspecies inhabiting the warmer, southern regions than the subspecies inhabiting the colder, northern region (Oishi, 2010; Takeuchi, 1995). Oishi (2010) found the southern subspecies showed increasing skull size with decreasing latitude across its populations. Although the body length of the northern subspecies was larger than the southern subspecies, the male body weight was the same between both, and the female body weight was lighter in the northern species, countering Bergmann’s rule as colder populations had less body mass.

Geist (1987, 1990) opposes Bergmann’s rule with solid reasoning. Firstly, because Bergmann’s (1847) article is written in German, translation has become an issue, with many people following Mayr’s (1956) translation, which Geist (1989) claims is wrong. Secondly, using formulas derived from Bergmann’s rule, Geist (1987, 1990) predicts an exponential size increase with latitude, anticipating higher latitudinal populations of Calgarian sparrows to be the size of chickens, which they are not. Thirdly, Geist (1987) concludes that hair is a superior adaption than size to cold, predicting a hairless beast at different temperate intervals would finally have to weigh 15,000kg at a temperature of –32oC, whereas, for the same beast, 1cm of hair coat is worth a 5-fold increase in mass. Consequently, if it is cheaper to evolve a denser coat, evolution would favour this over body mass increase when subjected to cold stress. Finally, Geist (1990) concludes that body size depends not only on heat retention but also on other factors. These include: individuals surviving longer on larger fat stores; larger females bearing more and producing larger young; larger females producer richer or more milk, therefore sustaining healthier or more young respectively; large size favouring males in fighting and sexual selection; larger individuals can run for more extended periods (helpful in prey species were speed and endurance are necessary for survival). When considering these, it seems far-fetched that body size is solely due to selection for heat conservation at any latitude.    

Significance of ideas

In biogeography, it is essential to scrutinise these rules because it not only helps argue their place in science but uncovers more and more about the distribution of species in space and time. For SARs, it seems that more real-life evidence is needed to properly analyse this rule. For now, caution should be exercised when using it as a conservation tool. For Bergmann’s rule, it is clear there is a difference between and amongst species of different latitudes. Yet, it is naive and bold to assume these differences come down to something as simple as energy conservation. It seems far-fetched that Bergmann’s rule will hold true to species worldwide when you take in the complexities and specialised adaptations of the natural world.

I believe it is a scientist’s job to doubt rules (if only to end up strengthening their integrity in the end). Both SARs and Bergmann’s rule are a stepping-stone to the truth that nature hides. Any disparities should be indications of necessary modifications rather than a chance to disregard the rule entirely. 

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Rarotonga

What do you do when you’ve just finished a stressful semester at uni, and your tenancy application for a new house has been accepted? You ditch New Zealand and head to Rarotonga for 2 weeks! With the quarantine-free travel bubble opening between NZ and the Cook Islands and flights and accommodation being ridiculously affordable, trading in Auckland’s sub-zero temperatures for a tropical paradise seemed to be the only reasonable choice.

The climate was the first thing that got me about Raro. Snapchat was covered with 7am stories of frost-covered grass, and I’m getting a sweat on just walking to breakfast. I think the next thing I quickly noticed was the atmosphere. It’s a small island (67 km2; a population of 13,000), and the convivial ambience of the community was almost tangible. The way the bus driver greeted me, you’d think he was my uncle. After being in Raro for only a day, I felt the same feeling of being in Pukekohe; I felt like I was home.

Snorkelling

Obviously, the daily routine began with breakfast. Then, as soon as we could get our shit together, we went snorkelling. The water was so warm and so salty. One of the locals said that as kids, they all swam in the ocean with their eyes open to get used to the salt. I totally understand why they did that because water leaked into my mask one time and I thought I went blind. In NZ, in the height of summer, I can rarely go freediving in just a bikini for more than 30 minutes before the cold sets in; in Raro, I almost felt too hot when I was snorkelling in my 3 ml spring suit – crazy. The ocean would probably feel like a bath in summertime.

To me, the most interesting animal I saw while snorkelling in the lagoon was the octopuses. I feel like every time I go freediving and there’s an octopus, everyone but me manages to see it; not this time, suckers (pun intended). They inhabited the right-hand side of the lagoon, amongst an area of littered stones and rubble. They were day octopuses (Octopus cyanea) who, unlike their nocturnal counterparts, are active during the day, stealthily patrolling the reef under an impressive camouflage [1, 2]. One of them even attempted to steal my GoPro and take it into its den for further inspection.

Bluefin trevally (Caranx melampygus) are the top predators in the Rarotongan lagoon systems. I came across a school of them. When I first saw one of them in the distance, I honestly thought it was a juvenile reef shark trapped in the lagoon. Once I got closer and noticed more of them, I realised they were, unmistakeably, trevally. They outsized all the other reef fish by three-fold, and they proved their predator status as they confidently cruised past me at arms-length. The biggest members of this school appeared to be around 70cm, putting them at the age of approximately 8 years [3].

Moray eels would appear out of nowhere in crevices and holes in the reef. Sometimes, I’d sink to the sand on my knees to be face to face with them for a moment and appreciate their beautiful ugliness and imagine what it’s like to be a little fish caught in their pharyngeal jaws.

The coral was beautiful, and, like the octopus, I have never seen coral like that in the wild before. The variation in size and colour was magnificent. So was the fact that they live in a highly changing, intertidal environment; I couldn’t touch the sandy bottom at high tide compared to the exposed reef at dead low. There was an abundance of brain coral and table coral, as well as soft coral species.

Darting around were parrotfish, doing their crucial job of scraping and cleaning the coral to keep it healthy.

The coral themselves provide homes for thousands of reef fish and act as a barrier for incoming waves that intend to reach the island. It is said that even though coral reefs cover less than 1% of the ocean floor, they provide support for around 25% of all known marine species.

Burrowing urchin (Echinometra mathaei)

Lionfish (Pterois sp.)

Threadfin butterflyfish (Chaetodon auriga) and black triggerfish (Melichthys niger)

Giant clam (Tridacna sp.)

Starry pufferfish (Arothron stellatus)

Peacock grouper (Cephalopholis argus)

Giant clam (Tridacna sp.)

Mushroom coral (Fungiidae sp.)

This environment was like nothing I had ever seen before. I wanted to spend every moment under that water; if a genie granted me only one wish, it would be to breathe underwater. Then again, my wish partly comes true every time I strap on some SCUBA gear and descend down 5 metres, 10 metres, 20 metres… which is exactly what I did.

Diving

Our first dive was the Papua Passage. Literally, a passage between two tall cliffs that felt like I was doing some sort of underwater, horizontal rock climbing to pass through. But, oh boy, was it worth it. Turtles everywhere you looked, sleeping on coral, drifting lazily past with their heavy eyes watching you meticulously. Two species exist in the Cook Islands, the endangered green turtle (Chelonia mydas) [4] and the critically endangered hawksbill turtle (Eretmochelys imbricata) [5], and I had the pleasure of meeting both.

Look closely at that photo. You can see the rear right quarter appears to be missing. If I had to take a guess, and I’ve stared at this photo forever trying to decipher it, then I would say that a shark, most likely a tiger, managed to get itself a quick on-the-go snack. This turtle is living proof that what doesn’t kill you makes you stronger.

The next dive was the Avaavaroa drop off. My god, did this blow my mind. An extensive coral reef that just drops off into the abyss.

Schools of fish, an eagle ray, and a couple of grey reef sharks cruised beneath us in front of a dark blue background that looked like it could engulf me at any moment. In a way, it was humbling to be on the edge of the vast, blue ocean like that.

The rest of our dives were on the northern and north-western sides of the island. Here, every time I slipped under the water, it felt like I was submerged in a woodland fairytopia.

The landscape was stunning. Here and there were crown of thorns starfish, a giant, impressive echinoderm with thorn-like ossicles containing venom. There appeared to be a healthy population here. Still, at another island within the Cook Islands called Aitutaki, their population is rampant. They are decimating the very coral they feed on.

Interestingly, on these northern dives was the amount of trash that seemed to be pulled along the floor by currents and would accumulate in hotspots around the reef and in caves. I grabbed all I could, but part of me wished to do a few dives whose sole purpose was to find as much trash as possible.

All in all, my trip to Rarotonga was fascinating. All I wanted to do once I got there was spend as much time as possible underwater, and that’s what I did. Rarotonga is a magical place, and it is so worthy of preservation. I hope to return back one day soon, maybe even to do my own research. But, for now, I’ll just keep harassing everyone’s insta feeds with Raro throwbacks.

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References

1. Heukelem, W. V. (1973). Growth and life‐span of Octopus cyanea (Mollusca: Cephalopoda). Journal of Zoology, 169(3), 299–315.

2. Mather, J. A., & Mather, D. L. (2004). Apparent movement in a visual display: the ‘passing cloud’of Octopus cyanea (Mollusca: Cephalopoda). Journal of Zoology, 263(1), 89–94.

3. Sudekum, A. E., Parrish, J. D., Radtke, R. L., & Ralston, S. (1991). Life history and ecology of large jacks in undisturbed, shallow, oceanic communities. Fishery Bulletin, 89(3), 493–513.

4. Seminoff, J. A. (2004). Chelonia mydas. The IUCN Red List of Threatened Species 2004: e.T4615A11037468. http://dx.doi.org/10.2305/IUCN.UK.2004.RLTS.T4615A11037468.en

5. Mortimer, J. A. & Donnelly, M. (2008). Eretmochelys imbricata. The IUCN Red List of Threatened Species 2008: e.T8005A12881238. http://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T8005A12881238.en

The three carbon pumps of the ocean: biological, carbonate, and physical

Introduction

Carbon is the most critical component of all biological compounds and is exchanged around the Earth through a biogeochemical cycle (Archer, 2010). Although carbon is part of natural planetary systems, current concentrations of carbon dioxide (CO₂) are the highest they have been in 14 million years, and this increase is attributed to anthropogenic activity, specifically from the Industrial Revolution of the 1700s (Etheridge et al., 1996; Falkowski et al., 2000; Zhang, Pagani, Liu, Bohaty, & DeConto, 2013). The oceanic carbon cycle is comprised of processes that cycle carbon around different areas of the ocean, the seafloor, the Earth’s interior, and the atmosphere. In pre-Industrial Revolution times, the ocean provided a net source of CO₂ to the atmosphere, whereas now most of the carbon that enters the ocean comes from anthropogenic, atmospheric CO₂ (Raven et al., 2005). According to Falkowski et al. (2000), the ocean is a reservoir for ~38,400 gigatons (Gt) of carbon, a vast amount when compared to the terrestrial biosphere (~2,000 Gt) and the atmosphere (~720 Gt). CO₂ is diffused into the ocean’s surface waters and dissolves, now ready to enter the oceanic carbon cycle through three pumps: the biological pump, the carbonate pump, or the physical pump (Duan & Sun, 2003). The biological pump utilises autotrophy, such as photosynthesis by phytoplankton, to export carbon from the upper, sunlit ocean to the ocean interior or seafloor sediments and respire organic carbon into inorganic carbon (Emerson & Hedges, 2008). The carbonate pump is a process of ocean carbon sequestration driven by calcifying plankton, which releases CO₂ back into the atmosphere but sequesters it by sinking to the seafloor (Smooth & Key, 1975); this is why this process is also referred to as the carbonate counter pump. The physical pump is the physio-chemical process whereby carbon is transported from the ocean surface to its interior, where it can be stored for hundreds of years (Ito & Follows, 2003; Toggweiler, Murnane, Carson, Gnanadesikan, & Sarmiento, 2003).

The Biological Pump

The biological pump is a process of oceanic carbon sequestration that is driven mainly by autotrophic phytoplankton that inhabits the surface waters. This method of autotrophy, photosynthesis, converts CO₂ (dissolved inorganic carbon (DIC)) into organic biomass (particulate organic carbon (POC)) (Passow & Carlson, 2012; Sigman & Hain, 2012). Photosynthesis is the initial method of bringing carbon into the biological pump. It is further moved throughout the ocean by entering the food web after phytoplankton, which are primary producers at the lowest trophic level, are eaten by consumers. Carbon can then stay in the food web as higher trophic levels continuously consume organisms, or it can be released from the food web in the form of defecation or dead tissue (Passow & Carlson, 2012). This carbon sequestration process by primary production accounts for a vast majority of carbon fixation on Earth (Christina & Passow, 2007; De La Rocha, 2003).

Carbon is also moved into deep ocean currents or seafloor sediments by sinking organic matter. Organic material is formed by phytoplankton in the euphotic zone located at the surface level of the ocean. When plankton or other marine organisms eat, defecate, die, and decompose, this material, known as marine snow, begins to sink downwards (Passow & Carlson, 2012). One phytoplankton cell sinks at a rate of approximately 1 metre per day, meaning that, with an average depth of 4,000m, it can take over ten years for one carbon-carrying phytoplankton to reach the seafloor. Organic and inorganic matter, as well as expulsion of faecal matter from larger predators, aggregate to form marine snow that has a greater sinking velocity and can complete its journey to the seafloor in days (Heinze et al., 2015; Turner, 2015). Once this sinking organic biomass reaches deep-sea levels, it can enter the food web by becoming metabolic fuel for organisms that live there, including benthic organisms and deep-sea fish (Turner, 2015). Sinking matter transports an estimated 5–20 Gt of carbon to the deep ocean annually, where between 200 million–500 million tonnes of carbon is sequestered for thousands of years in seafloor sediment (Giering et al., 2020; Guidi et al., 2015; Henson et al., 2011). Any global warming-induced change on the integrity or function of phytoplanktonic populations will alter the efficiency at which POC is transported to ocean depths, with feedbacks on the rate of climate change.

With less than 0.5 Gt of sinking carbon reaching sequestration in seafloor sediment, between 44.5 Gt – 54.5 Gt of carbon is remineralised in the euphotic zone (Ducklow, Steinberg, & Buesseler, 2001) and between 5 Gt – 6 Gt of carbon is remineralised in midwater processes during particle sinking (Feely, Sabine, Schlitzer et al., 2004). Remineralisation occurs in the biological pump when heterotrophic organisms utilise organic matter produced by autotrophic organisms. They recycle the compounds from the organic form back to the inorganic form through respiration, making them available for reuse in primary production (Guidi et al., 2015). Remineralisation usually occurs with dissolved organic carbon (DOC) rather than POC because particles must typically be smaller than the organism taking it up for remineralisation (Lefevre, Denis, Lambert, & Miquel, 1996; Schulze & Mooney, 2012).

The particles that make it to the seafloor sediment may remain there for millions of years, trapping the carbon with them. Together, the processes that make up the biological pump ultimately remove carbon in its organic form from the ocean’s surface and return it to DIC in the deeper ocean. The thermohaline circulation (THC) returns deep-ocean DIC to the atmosphere on timescales that exceed millennia (the topic of THC will be explained further in “The Physical Pump”).

The Carbonate Pump

The carbonate pump is an extension of the biological pump but instead sequesters particulate inorganic carbon (PIC) and is driven by calcifying organisms (organisms that produce calcium carbonate (CaCO₃) shells). The leading contributor to the carbonate pump is the calcifying plankton known as coccolithophores due to the vast quantity of their global population. Coccolithophores are eukaryotic, unicellular phytoplankton that produces overlapping calcite platelets called coccoliths and are currently one of the most significant contributors to carbonate sediments in the deep sea (Hofmann et al., 2010; Renaud, Ziveri, & Broerse, 2002). Coccolithophore production of coccoliths through the uptake of dissolved inorganic carbon and calcium produces CaCO₃ and CO₂, hence the alternate name of carbonate counter pump.

Ca²⁺ + 2HCO_3- → CaCO₃ + CO₂ + H₂O

However, some of the CO₂ released in calcification can be used in photosynthesis (Mackinder, Wheeler, Schroeder, Riebesell, & Brownlee, 2012), and over extended periods coccolithophores contribute to decreased levels of atmospheric CO₂. It is currently unknown as to the function of the coccolith. However, many theories have been proposed, including protection from predators or grazing zooplankton (Young, Andruleit, & Probert, 2009) or ballasting the cell for vertical migration into deeper water (Raven & Waite, 2004). The latter would be of considerable advantage to the carbonate pump in getting the carbon trapped in coccoliths to the seafloor. The most abundant species of coccolithophore is Emiliania huxleyi. It is likely to be the greatest global producer of calcite, meaning this species is an essential organism in transporting carbon from the ocean to be buried in marine sediment; they play a crucial role in the global biochemical carbon cycle (Balch, Holligan, & Kilpatrick, 1992).

The production of CaCO₃ shells in calcifying organisms such as molluscs, foraminifera, coccolithophores, crustaceans, echinoderms, and corals (Zondervan, Zeebe, Rost, & Riebesell, 2001) is the central part of the carbonate pump. When CO₂ dissolves in the surface layer of the ocean, it combines with water molecules. It enters into a series of chemical reactions that result in ions that calcifying organisms combine with calcium ions (Ca²⁺) to form CaCO₃ (Zeebe & Wolf-Gladrow, 2001). Even though one CO₂ molecule is released in calcification, one carbon atom becomes trapped within the CaCO₃ molecule used in calcification and becomes part of the sediment once it sinks to the bottom of the ocean. This means calcification takes two carbon atoms from the environment and only releases one back into it, even though the formation of calcium carbonate shells is a source of CO₂ (Mackie, McGraw, & Hunter, 2011) over the long-term calcifying organisms provide a sink for CO₂.

Calcifying organisms provide a large mechanism for the downward transport of CaCO₃ (Smith & Key, 1975). Dead organisms sink to the seafloor and dissolve on the way down and release carbon into deep-sea currents or reach the seafloor and build up to form CaCO₃ sediments stored for large timescales. The scale at which CaCO₃ makes its way down varies from species to species. For example, calcifying zooplankton (pteropods, ostracods and foraminifera) promote fast particulate inorganic carbon sequestration to the deep ocean due to the relatively large mass of their shells, which makes them sink rapidly. In comparison, calcifying phytoplankton such as coccolithophores will hardly sink individually, and even still, they have a broad range in sinking rates when they assimilate into biological aggregates. The burial of CaCO₃ in deep-ocean sediment is one of the primary mechanisms to reduce atmospheric CO₂ on geological timescales related to silicate weathering processes (Cartapanis, Galbraith, Bianchi, & Jaccard, 2018). Eventually, tectonic processes, including heat and pressure, transform seafloor sediments containing CaCO₃ into limestone; this process locks carbon away for millions of years (Folk, 1980). Over time, these sediment layers eventually return carbon to the oceans by weathering and erosion (Gibb, 1978).

The Physical Pump

The physical pump uses different processes to transport DIC from the ocean surface to its interior. Firstly, the solubility of CO₂ in water is the initial process of getting carbon into the ocean as DIC (Duan & Sun, 2003). Carbon dioxide dissolves in oceanic water and, unlike many other gases, it reacts with water to form a collective of ionic and non-ionic species (DIC), which include dissolved free carbon dioxide (CO₂ (aq)), carbonic acid (H₂CO₃), bicarbonate (HCO₃₋), and CO₃²⁻ (Weiss, 1974). There is a strong inverse function of seawater temperature on the solubility of CO2, as solubility is greater in colder water (Toggweiler et al., 2003). As sea surface temperature (SST) increases, less CO₂ can be taken up by the ocean; the progressive warming of the oceans releases CO₂ in the atmosphere because of its lower solubility in warmer seawater.

The THC is part of a global, oceanic conveyor belt, driven by heat and freshwater fluxes, where thermo refers to temperature, and haline refers to salinity, which together determines seawater’s density (Rahmstorf, 2003). The model of THC was first described by Stommel and Arons (1959), where they explored how temperature and salinity moved ocean water around the globe. Seawater with higher temperatures expands and is less dense than seawater at lower temperatures (Millero, Gonzalez, & Ward, 1976). Seawater with higher salinity is denser than seawater with lower salt content (Millero et al., 1976). Seawater with lower density floats over denser seawater; this is known as stable stratification (Maiti, Gupta, & Bhattacharyya, 2008). When dense seawater masses are initially formed, they are not stably stratified and will seek to correctly locate themselves vertically by their density and become stably stratified. This stratification process is the main driving force behind deep ocean currents, which carry carbon that has sunk from surface layers on the global conveyor belt, essentially sequestering it for hundreds of years.

The process of denser seawater joining the global conveyor belt is called downwelling. Higher density water accumulates and sinks below lower density water at places within the ocean where warm rings spin clockwise and create surface convergence, pushing the surface water downwards (Rao, Joshi, & Ravichandran, 2008; Yang, 2009). These areas of downwelling bring large amounts of carbon from the surface waters to be sequestered down below. Alternately, upwelling brings dense, cooler water to the surface to replace the warmer surface water. This water is usually nutrient-rich, including dissolved CO₂, meaning that these areas of upwelling provide an ideal location for an abundance of phytoplankton to carry out primary production and ultimately recycle the carbon brought to the surface from the deep ocean (Sarhan, Lafuente, Vargas, Vargas, & Plaza, 2000)

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The effect of ocean acidification on calcareous phytoplankton due to human-induced climate change

Introduction

Humans have begun to drastically alter the atmospheric environment since the Industrial Revolution of the 1700s, where the burning of fossil fuels released exponential amounts of carbon dioxide into the air. Along with increasing the global temperature, affecting life cycles of organisms, and disassembling natural species interactions, this release initiated a process known as ocean acidification, whereby the pH of the ocean has decreased from 8.2 to 8.1 (Feely, Doney, & Cooley, 2009). The effect that ocean acidification has on marine organisms starts at the base of the food chain with primary producers, such as phytoplankton, and repercussions can then be seen throughout entire food webs. Calcareous phytoplankton (i.e., coccolithophores) may be at high risk of impact from ocean acidification due to the calcium carbonate plates that they produce through calcification. Coccolithophores are key players in the global biogeochemical cycle, the pelagic food chain, and oxygen production through photosynthesis, so any negative impacts that ocean acidification may have on them need to be understood and mitigated for the health and wellbeing of humankind.

Climate change

Climate change refers to global climatic shifts that were intensified in the mid-20th century through the burning of fossil fuels (Griffin, 2018). This anthropogenic influence has resulted in an increasing amount of carbon dioxide (CO2) in the atmosphere with a >90% probability that the observed average global temperature increase is due to human-induced greenhouse gas concentrations and around half of this increase occurring in the last three decades (Feely et al., 2009; Rosenzweig et al., 2008). Anderson, Hawkins, and Jones (2016) describe the greenhouse effect as infrared energy that has been re-emitted from solar radiation is absorbed by water vapour and CO2 to create a ‘blanket’ around planet Earth. As CO₂ concentrations increase, this greenhouse effect is enhanced, and the planet warms further beyond its natural average temperature.

Regional warming exhibits observable biological changes in terrestrial systems. Included amongst these changes are increases in coastal erosion (Beaulieu & Allard, 2003; Forbes, Parkes, Manson, & Ketch; 2004; Orviku, Jaagus, Kont, Ratas, & Rivis, 2003), melting permafrost (Frauenfeld, Zhang, Barry, & Gilichinsky, 2004), and glaciers shrinking in all seven continents (Oerlemans, 2005). Natural increases and decreases in Earth’s temperature have been recorded over millions of years (Savin, 1977), which can add scepticism to the idea that current global heating is due to anthropogenic activity rather than part of a natural cycle. A study by Reichert, Bengtsson, and Oerlemans (2002) into Swiss and Norwegian glacial retreat showed that the retreat could not be due to natural climatic change because it exceeds glacial fluctuations derived from the general circulation model, meaning that another force must be acting on the glaciers.

Climate change’s effects also reach the ocean. Edwards and Richardson (2004) describe phenological changes that occur in the wake of a warming ocean. An increase in sea surface temperature (SST) is used as an indicator for climate change, and it affected the seasonal development and phenology of plankton species. Massive phenological changes occurred at a 0.9oC increase of SST. The extent of these changes was more significant than those of terrestrial studies (Root et al., 2003), which may indicate that marine communities have heightened sensitivity to climate change. Furthermore, Edwards and Richardson’s (2004) study showed there was a different rate of response in communities to ocean warming, creating a “mismatch between successive trophic levels and a change in the synchrony of timing between primary, secondary and tertiary production” (p. 883). This mismatch will directly affect populations at higher trophic levels, including commercial fish species, marine mammals, and seabirds, and adaption will need to take place to realign these trophic levels with primary production. Not only are higher trophic levels at risk in response to changing phenology of plankton due to climate change, but other essential ecosystem services will be impacted, including the production of oxygen that we breathe, the sequestration of CO2, and the biogeochemical cycling of nitrogen, phosphorus, and silica (Richardson & Schoeman, 2004). Beaugrand and Reid (2003) study provided evidence for climate change-induced, long-term changes to the three trophic levels of phytoplankton, zooplankton, and salmon. Various declines and increases of phytoplankton and zooplankton ensued due to regional temperature increases over time, which caused the ultimate decline of salmon stocks in 1988, and this decline is expected to continue as the climate proceeds to change.

Ocean acidification

The ocean has absorbed approximately one-quarter of anthropogenically emitted CO2 over the industrial era, causing chemical reactions that reduce oceanic pH, concentrations of carbonate ions (CO₃^2-), and saturation states of two calcium carbonate (CaCO₃) minerals calcite and aragonite; this is the process of ocean acidification (OA) (Feely et al., 2009). The ocean absorbs CO₂ in two ways: through photosynthesis undertaken by marine phytoplankton (Rost, Riebesell, & Burkhardt, 2003) and through the dissolving of CO₂ in seawater (Feely et al., 2009). When CO₂ reacts with seawater, it creates carbonic acid (H₂CO₃), which dissociates to hydrogen ions (H⁺) that combine with carbonate to form bicarbonate ions (HCO_3-). As atmospheric CO₂ increases, the ocean continually absorbs greater amounts of CO₂, and as temperature increases, CO₂ leaks out of the ocean back into the atmosphere. As CO₂ is absorbed, carbonate gets used up and must be replaced by stocks from the deeper ocean. Currents bring water with fresh carbonate to the surface and circulate water carrying the captured carbon into the ocean. As SST increases, this circulation process becomes more difficult, and the ocean stratifies. The surface water begins to saturate with CO₂, decreasing support for phytoplankton, and photosynthetic CO₂ uptake slows. Feely et al. (2009) reported the average pH of the ocean since the industrial era to have decreased by 0.11 pH units (29% acid concentration increase) and predicted a further decrease of 0.3 pH units (150% acid concentration increase) by 2100.

Marine organisms (e.g. coral, bivalves) extract bicarbonate ions from seawater to form skeletons or shells in a process called calcification:

Ca²⁺ + 2HCO_3- → CaCO₃ + CO₂ + H₂O

Calcification is a source of CO2 and is balanced by weathering, a process where rainwater reacts with carbonate rocks and consumes atmospheric CO2 on its way to rivers:

CaCO₃ + CO₂ + H₂O → Ca²⁺ + 2HCO_3-

A decline in the availability of carbonate ions will affect the degree of carbonate polymorph saturation in seawater, therefore compromising marine organisms’ ability to construct their skeletons/shells (Gledhill, Wanninkhof, & Eakin, 2009). A study by Gattuso et al. (2009) examined OA’s ability to affect the calcification of calcareous plankton, realising that any changes experienced by calcified taxa may alter the oceans’ ability to act as a global carbon sink. Calcareous, planktonic foraminifera observations from the Southern Ocean showed a decrease in shell weight compared to older records, with the implication that OA is the causing factor (Moy, Howard, Bray, & Trull, 2009). Suppose OA can change the morphology of a species and consequently affect multiple species up the trophic levels. In that case, the question is whether these species will cope with the effects of acidification through adaptation. This was highlighted in an experiment by Bibby, Cleall-Harding, Rundle, Widdicombe, and Spicer (2007) who observed the effects of acidification on the defence mechanisms of the periwinkle Littorina littorea toward its predator the green shore crab Carcinus meanus. L. littorea can reinforce their calcified shells when experiencing extensive predation pressure, with snails exposed to C. meanus for 15 days producing shells that were 30% thicker than those that were not under predation pressure. At reduced seawater pH, L. littorea could not thicken their shell and compensated by altering their behaviour to avoid the crabs. This shows that adaptation is possible, but at what cost? Behavioural alteration in L. littorea could mean they spend less time feeding, or it could affect their interactions with other species with unknown consequences.

Coccolithophores

Diversity of coccolithophores. (A) Coccolithus pelagicus, (B) Calcidiscus leptoporus, (C) Braarudosphaera bigelowii, (D) Gephyrocapsa oceanica, (E) E. huxleyi, (F) Discosphaera tubifera, (G) Rhabdosphaera clavigera, (H) Calciosolenia murrayi, (I) Umbellosphaera irregularis, (J) Gladiolithus flabellatus, (K and L) Florisphaera profunda, (M) Syracosphaera pulchra, and (N) Helicosphaera carteri.
By Monteiro, F.M., Bach, L.T., Brownlee, C., Bown, P., Rickaby, R.E., Poulton, A.J., Tyrrell, T., Beaufort, L., Dutkiewicz, S., Gibbs, S. and Gutowska, M.A. – https://advances.sciencemag.org/content/2/7/e1501822, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=91145677

Umbilicosphaera sibogae coccolithophore conglomeration taken with ZEISS MERLIN Scanning Electron Microscope. By ZEISS Microscopy – CC BY-NC-ND 2.0, https://www.flickr.com/photos/zeissmicro/6908938729

Coccolithophores are unicellular, eukaryotic, calcareous phytoplankton that produces overlapping calcite platelets called coccoliths and are presently one of the main primary producers of the open ocean and significant contributors to carbonate sediments in the deep sea. (Hofmann et al., 2010; Renaud, Ziveri, & Broerse, 2002). Coccolithophores produce coccoliths through the uptake of dissolved inorganic carbon and calcium, and CaCO₃ and CO₂ are produced. Some of the CO₂ released in calcification can be used in photosynthesis (Mackinder, Wheeler, Schroeder, Riebesell, & Brownlee, 2012). It is possible that over long periods, coccolithophores may contribute to decreased levels of atmospheric CO₂. Even though one CO₂ molecule is released in calcification, one carbon atom becomes trapped within the CaCO₃ molecule used to make coccoliths and becomes part of the sediment once it sinks to the bottom of the ocean. This means calcification takes two carbon atoms from the environment and only releases one back into it. Even though the formation of calcium carbonate shells is a source of CO₂, over the long-term coccolithophores provide a sink for CO₂ (Mackie, McGraw, & Hunter, 2011). It is currently unknown as to the function of the coccolith, but many theories have been proposed. These include protection from predators or grazing zooplankton (Young, Andruleit, & Probert, 2009), ballasting the cell for vertical migration into deeper, more nutrient-rich water (Raven & Waite, 2004), protection from virus infection (Raven & Waite, 2004), or aiding in the filtering of non-photosynthetically active radiation at the surface to aid in photosynthesis or focussing light to the chloroplasts in deeper water (Raven & Crawfurd, 2012).

The most abundant species of coccolithophore is Emiliania huxleyi. It is likely to be the greatest global producer of calcite, meaning this species is an essential organism in transporting carbon from the ocean to be buried in marine sediment; they play a crucial role in the global biochemical carbon cycle (Balch, Holligan, & Kilpatrick, 1992). As the most abundant species, this also means that E. huxleyi is a key organism in the marine pelagic system, where the base of the food web is comprised of over 5,000 species of phytoplankton (Rost & Riebesell, 2004). Among this vast array of species, only a few select taxonomic groups are responsible for most of the pelagic systems primary production, one of these is coccolithophores and therefore reinforces their importance in the oceanic ecosystem. Coccolithophores are sensitive to nitrogen, phosphorus, and silicate ratios in the water, inducing competitive dominance between coccolithophores and other phytoplanktonic communities such as diatoms, microflagellates, and dinoflagellates. Anthropogenic interference with these ratios comes in the form of agricultural runoff leading to eutrophication and increasing nitrogen in seawater, causing coccolithophores to form blooms in these favourable environments and outcompete other species (Yunev et al., 2007). As agriculture and the use of nitrogen-based pesticides increases, more runoff makes its way to the ocean, which could see a tip in the balance of conditions to favour coccolithophore species of phytoplankton. From there, will other phytoplankton species be able to adapt in time, or will they be outcompeted and eventually become extinct?

As OA decreases carbonate saturation in seawater, coccolithophores ability to produce coccoliths may be inhibited as the increase in atmospheric CO₂ may affect their calcification mechanisms. When environments change, some organisms adapt to suit their changing environment, so as ocean pH decreases, will coccolithophores have an evolutionary response? Beaufort et al. (2011) found that coccolithophores have channels to pump out H⁺ ions during calcification to avoid acidosis. Furthermore, a feedback loop is created because when the function of these channels is disrupted, calcification is halted. This study provided evidence that increased oceanic CO^₂ concentrations decreased coccolith mass as OA impairs the normal function of ion channels and places selection pressure on coccolithophore’s calcification rates (Tyrell, Holligan, & Mobley, 1999). Flynn, Clark, and Wheeler (2016) found that coccolithophores put under OA conditions showed selection for lower calcification rates to avoid the risk of acidosis at higher H⁺ concentrations. They predicted coccolithophore calcification would decrease by 25% as OA continued and atmospheric CO^₂ reached 750 ppm (an increase of 335.62 ppm from current atmospheric CO₂ levels). Conversely, a long-term study of an E. huxleyi population that was allowed to reproduce for 700 generations under conditions similar to those predicted for the year 2100 showed their population could adapt and increase calcification and CaCO₃ content (Benner et al., 2013). This contrasts with other short-term experiments and shows that long-term OA exposure could alter calcification responses in E. huxleyi, and potentially other calcareous phytoplankton as well. Similarly to this experiment, Smith et al. (2012) found naturally forming populations of highly calcified coccolithophores in water with low CaCO₃ saturations.

The Paleocene-Eocene Thermal Maximum (PETM) occurred approximately 55.5 million years ago and saw a global temperature increase of 5–8 °C and ~12,000 gigatons of carbon released over 50,000 years (0.24 gigaton per year) (McInherney & Wing, 2011). Today, the anthropogenic release of carbon is equal to 10 gigatons of carbon per year; therefore, it will only take 1,200 years for 12,000 gigatons of carbon to be released as in the PETM. It has been shown that there was no change in coccolithophore distribution attributed to acidification in the PETM (Beaufort et al., 2011; Iglesias-Rodriguez et al., 2008), meaning that they were likely able to adapt over the 50,000 years of exposure to increasing carbon levels. Will 1,200 years be enough time for them to adapt again? Results from studies about the effect OA has on coccolithophores contradict one another. Different experiment environments show different outcomes, including short-term experiments vs long-term experiments, different coccolithophore species, and different regional populations of coccolithophores (Beaufort et al., 2011; Benner et al. 2013; Flynn et al., 2016; Smith et al., 2012). An alternative way to look at the discrepancies seen between experiments is that studies about the effects of OA are often hypothesised to have a negative outcome. If it is looked at that a decreased pH is a more favourable condition to coccolithophores, rather than the alternative, it can explain why highly calcified coccolithophores can be found in conditions of low CaCO₃. In this scenario, they would have bioengineered their higher pH environment to a more favourable, lower one through calcification and the release of CO₂, creating highly calcified individuals. Once they “created” an environment with favourable pH that now had low concentrations of CaCO₃, they would no longer need the CaCO3. In some studies, at low pH environments, coccolith growth appeared to be inhibited. However, perhaps the coccolithophores were satisfied with the pH in their environment and therefore did not need to alter it further through calcification. But, I digress. What are your thoughts on the matter?

Conclusion

Anthropogenically induced climate change has a clear impact on natural, global processes, specifically the ocean’s role in balancing CO₂ levels in the atmosphere. Coccolithophores play a part in this role by producing vast amounts of calcite and acting as a carbon sink. The effects of ocean acidification on coccolithophores have been explored through previous studies and experimentation, with multiple conclusions drawn. This makes it difficult to understand how catastrophic the effects of ocean acidification will be in the future as atmospheric CO₂ continues to rise, primarily because any changes to coccolithophores will have direct and indirect consequences for other taxa and ecosystem processes. Alternatively, coccolithophores might be environmental bioengineers and use calcification to alter oceanic pH levels to suit their environment. Further research is essential to narrow down coccolithophores precise role in the ecosystem, determine the reason for their coccoliths, discover if they can adapt to a changing environment and how quickly they can do this, and establish the effect ocean acidification has had on them and can potentially have on them in the future, and what will this mean for the world.

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What to expect from The Ocean: Explained

My name is Emily Jupp. I’m a studying Marine Biologist and Geospatial Scientist, and I’ve got some things I’d like to say. So, grab yourself a hot bevvy and buckle up because it’s all underwater from here.

There’s a plethora of information out there, and some of that information is about the ocean. The only real way to figure out the truth about something is to do your own research, which can be really hard when there’s so much research material to choose from. That’s where I come in. I hope to provide you with as much reliable, honest, and up-to-date oceanic information as possible, including references that will help you with your own study.

You don’t need to be a scientist to read about science; all you need is an inquisitive mind and a childlike curiosity about the world that surrounds you.