Sharks and Ocean Acidification in the California Current System.
What is Ocean Acidification?
The ocean acts as the world’s largest carbon sink, meaning it helps to regulate the amount of carbon dioxide (CO2) in the atmosphere by taking in approximately 50% of carbon emissions through a reaction involving CO2 and H2O which forms carbonic acid. The global and constant occurrence of this process is lowering the pH of the ocean in a process known as ocean acidification.
Why should we care?
Each species has its respective range of tolerance, a range of conditions in which it can survive. Calcifying organisms, who build structures out of calcium carbonate, require a level of carbonate saturation, and thus high enough pH, to maintain and create vital parts of their anatomy. Ocean acidification lowers the carbonate saturation state, lessening the availability of necessary calcium carbonate minerals needed for the calcification process. Furthermore, acidic waters can lead to the dissolution, malformation, and weakening of calcium carbonate structures, leading to a higher mortality rate among calcifying species.
What do we know so far?
Multiple studies have been dedicated to observing the effect of lowered carbonate saturation state on organisms occupying low trophic levels on their respective marine food chain, such as oysters and coccolithophores. Through this bottom-up approach, researchers have concluded that as carbon emissions continue and ocean acidification intensifies, marine organisms higher up in the food chain will lose part of their food source to relocation in search of a less acidic environment or high mortality rates due to dissolution. Only two studies, however, have been done on the effect of ocean acidification on sharks.
Why sharks?
Sharks are keystone predators, meaning that their contributions to their surrounding ecosystem is vital to its function. More specifically, sharks dictate the spatial distribution of surrounding organisms, maintain the ecosystem’s structure, and regulate population sizes though consumption. They are also calcifying organisms, as their teeth and skin, segmented into tooth-like scales known as denticles, are made up of calcium carbonate. Thus it is reasonable to assume that ocean acidification and accompanying lowered levels of carbonate saturation would cause dissolution, malformation, or weakening of sharks’ teeth and skin, which could contribute to a higher mortality rate. This would affect not just shark populations, but the species reliant on their ecological contributions. A study done by Leon Green and Fredrik Jutfelt in 2014, however, found that the denticles on small-spotted cat sharks were unaffected by increased dissolved CO2 levels. This was then contradicted by research by Auerswald et al. in 2019 which concluded that sharks which are adapted to low carbonate saturation levels are more susceptible to denticle, and likely tooth, erosion due to ocean acidification. The discrepancy between these studies and emphasis made by Auerswald et al. on the vulnerability species of shark which are already exposed to waters undersaturated in carbonate point to the need investigate the risk of ocean acidification to the sharks endemic to the California Current System.
Why the California Current System?
The California Current System (CCS) is one of the most biologically productive and therefore industrially lucrative regions in the world. Thus the maintenance of its health and function is incredibly important to climate change researchers and industry connections. Additionally, the CCS is characterized by seasonal upwelling which brings deoxygenated water low in carbonate saturation levels to the surface. This means that the sharks endemic to the CCS are regularly exposed to low carbonate levels in their local environment and are therefore according to Auerswald et al.’s study, more susceptible to damage from ocean acidification in the form of erosion of their teeth and skin. Furthermore, the sharks in the CCS are responsible for regulating ecosystems of great importance to millions of people who depend on their function, so the CCS’s sharks’s wellbeing in the face of advancing climate change should be a priority for industry investors and scientific researchers.
Conclusion
Overall, due to the importance of sharks in the California Current System and the threat ocean acidification poses to their calcified structures and therefore population sizes and health, it is imperative that researchers more deeply investigate the effect that climate change can be expected to have on these keystone predators, such that a more accurate understanding of the structure of future marine ecosystems can be constructed and used to inform present conservation, economic modeling, and climate change mitigation efforts.