Ocean acidification — the term for how carbon emissions, which affect our air and atmosphere, are making the oceans more acidic — may drive substantial change in aquatic ecosystems during the 21st century, authors of a recently published paper have found. Ultimately, the long-term survival ability of certain species may be compromised, and this could impact the food resources found within the ocean.

A Shifting Balance

According to Woods Hole Oceanographic Institute, scientists believe the ocean absorbs about one-quarter of total carbon dioxide emissions. Although concentration of carbon dioxide in the atmosphere was roughly 280 parts per million (ppm) prior to industrialization, now concentration levels are approaching 400 ppm. Such ocean acidification, which has occurred over the past 200 years (and continues to occur), impacts not only the ocean but also the many species living within it.

After compiling and examining 167 studies, which tested 153 ocean species' ability to withstand varying levels of carbon dioxide exposure, investigators found evidence to suggest that increasing acidity may lead to potential long-term shifts in the composition of corals, while also affecting echinoderms (such as sea urchins and ocean floor species), molluscs (including clams, squid, and octopus), and crustaceans (such as crabs, lobsters, and shrimp). That said, short-term responses to acidification vary across a range of ocean organisms, according to the researchers from the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research in Germany. For instance, they discovered that corals, echinoderms, and molluscs would be more sensitive than crustaceans to the scientific estimates of carbon dioxide emissions and their effects by the year 2100. Unfortunately, fish larvae may also be among those species found to be highly sensitive to projected levels of acidification.

This could signal substantial change and potentially lead to "long-term shifts in species composition,” the authors wrote in their paper published online in Nature Climate Change.

What Is Too Much?

Among the many studies they examined is a 2004 paper entitled "Biological Impact of Elevated Ocean CO2 Concentrations: Lessons from Animal Physiology and Earth History.” In this paper, the authors attempt to evaluate when exposure to carbon dioxide becomes detrimental — what exact level of concentration becomes a problem — and their findings broaden the scope of this discussion.

“CO2 elicits acidosis not only in the water, but also in tissues and body fluids,” wrote the authors who acknowledge that different species find ways to compensate for such physiological effects. Because of this, the number of species suffering from acute carbon dioxide toxicity "will be limited,” the authors noted. They hypothesize that sensitivity is related to the organizational level of an animal. Therefore, aquatic species “with a low activity mode of life” or those “pre-adapted to large fluctuations in environmental parameters” may be able to tolerate steady increases in acidification.

Though danger is implicitly suggested should changes in the ocean suddenly speed up, overall this idea of compensation and adaptability seems hopeful. However, this positive outlook is limited to short- and medium-term survival rates for individuals within a given population of any species. The researchers found, for instance, that ommastrephid squid are highly sensitive to acid-base disturbances as they have a high metabolic rate and an extremely pH-sensitive blood oxygen transport system. Ommastrephid squid are currently fished commercially. Worse, the authors noted that virtually nothing is known about the degree to which deep-sea fishes may be affected, especially if increasing acidity begins to speed up. Although deep sea fishes are not a currently popular food resource, who knows what knowledge or rare nutrients they might offer us in the future? Because the ocean is an ecological system, any shift in a single population of inhabitants might directly impact species we currently fish and eat.

“It is difficult, if not impossible, to give numbers for a critical threshold and to really qualify animal species as being permanently tolerant to elevations in ambient CO 2 since long-term effects have not yet been sufficiently investigated,” the authors wrote.

Sources: Portner HO, Langenbuch ML, Reipschlager AR. Biological Impact of Elevated Ocean CO2 Concentrations: Lessons from Animal Physiology and Earth History. Journal of Oceanography. 2004.

Wittmann AC, Portner HO. Sensitivities of extant animal taxa to ocean acidification. Nature Climate Change. 2013.