Nagelkerken, I. and Connell, S.D. Global alteration of ocean ecosystem functioning due to increasing human CO2 emissions. PNAS. 27 October 2015. Doi: 10.1073/pnas.1510856112
Pointillism is a painting technique in which the artist produces a whole from a series of dots of different colors and shades. What is from afar a coastal landscape is up close dots beside dots (see feature image).
In much the same way that a pointillist masterpiece only has meaning as a sum of its dots, a complex scientific question can begin to be understood as the sum of the answers to smaller scientific questions that make up its whole.
One big scientific question is that of how rising carbon dioxide levels are affecting the structures of complex marine ecosystems. Carbon dioxide emissions due to human activities drive increasing global temperatures as well as ocean acidification. While the scientific literature is replete with studies that examine the ecological effects of the warming, relatively little is understood on the ecological effects of ocean acidification, or the combined effects of warming and acidification.
Aiming to build a framework of understanding the effects of rising carbon dioxide emissions on marine ecosystems, two Australian ecologists Ivan Nagelkerken and Sean D. Connell have constructed metastudy (that’s a study of studies) that synthesizes the findings of 623 smaller studies. The resulting analysis, which appears in a recent issue of the Proceedings of the National Academy of Sciences, paints a dire picture for the future of marine ecosystems, predicting a downward spiral in species diversity that will leave behind ecosystems as a mere shadows of their former selves.
Nagelkerken and Connell examined studies that reflected the individual effects of increasing temperature or ocean acidification on species representing different levels of the food chain – primary producers at the base of the food chain (e.g., phytoplankton that fix carbon dioxide using the energy of the sun), herbivores, and carnivores. They then determined by statistical methods whether these studies suggested a positive, negative, or neutral effect on populations. They also assessed whether studies provided evidence for species acclimatization to environmental changes.
Overall, they found that acclimatization to global warming and ocean acidification was small, meaning that species are more likely to die out than to adapt to their new environments. Only bacteria and archaea emerged unscathed due to their abilities to occupy varied and extreme environments.
Phytoplankton, as the ocean’s main primary producers, are the foundation of the marine food chain. The metanalysis predicts no effect on global primary productivity as as a result of ocean acidification. However, ocean acidification and warming increase the metabolic demands on herbivores that feed on plankton, leading to a reduction in secondary production. Top predators, in turn, face metabolic stress and a shrinking food supply. Unable to meet their energetic demands, predators will likely die out. Herbivores, unchecked by predators, will over-graze the oceans, affecting the balance of primary producers. As a result of shifts in phytoplankton communities the study predicts a reduction in levels of dimethylsulfide (DMS), an important determinant of food web structure produced by phytoplankton. It also happens that DMS that escapes to the atmosphere plays a role in seeding clouds, so a reduction in population of DMS-farting phytoplankton might lead to a reduction in cloud cover, leading to greater exposure to damaging UV radiation that could (full circle) affect primary productivity in the tropics. In other words, ecosystem simplification appears to be an inevitable outcome of a downward spiral driven by both ocean acidification and warming.
Ocean acidification has a disproportionate effect on calcifying organisms, such as coral, which mineralize carbon dioxide to form calcium carbonate structures that serve as habitats that can be many times more species diverse than rainforests. Calcification rates are adversely affected by ocean acidification, resulting in habitat loss. Hence, the metanalysis also predicts a shift from species like corals and crustaceans toward soft-bodied species such as sponges and invertebrates.
Nagelkerken and Connell leave us with a framework for thinking about the various effects of two aspects of carbon dioxide emissions – ocean acidification and global warming. Through comparison and contrast of these effects overlaid on food web interactions, they show a future scenario where ecosystems will tend toward loss of resilience, simplification, and population shifts and decline. The implications are grave for coastal communities that rely on the ocean for food and livelihoods. Apart from painting a dire picture of the future of marine ecosystems, Nagelkerken and Connell raise the important point that no one stressor is enough to explain the fate of ecosystems, and that other stressors apart from much-hyped warming and acidification must be urgently identified studied if we are to cope with rising carbon dioxide emissions.