New studies show adaptation to ocean acidification under certain conditions

Due to anthropogenic emissions of carbon dioxide (CO2), ocean acidification – an increase in seawater acidity due to uptake of CO2 by the ocean - has become a serious threat for many species. Now scientists have found that some key animals may be able to adapt over time but only under certain conditions.

Females Pseudocalanus

IMPORTANCE AS FOOD FOR FISH LARVAE Females Pseudocalanus acuspes carrying eggs. This individual Is 2 mm in length. The females carry the eggs for up to 20 days before they hatch to nauplius larvae. These larvae then go through 12 molts before they emerge as adult copepods. Photo: Norwegian Polar Institute

Small crustaceans, copepods of the species Pseudocalanus acuspes are of major importance as food for fish larvae in the Arctic Ocean and the North Atlantic. When copepod stocks decline, fish stocks also diminish. Researchers Peter Thor, Norwegian Polar Institute, and Sam Dupont, University of Gothenburg, have carried out a number of experiments on these copepods to assess their sensitivity to ocean acidification. Most earlier studies showing that many species have problems coping with ocean acidification were conducted over short time spans – a couple of weeks. Very few studies have investigated whether animals may adapt through natural selection and evolution over generations.

The new study is unique because it focused on animals' potential for adaptation across two generations and at different levels of ocean acidification. For Pseudocalanus, a generation – from eggs to the adult stage – lasts approximately 60 days. The new findings show copepods are able to adapt to ocean acidification. However, in the experiments, adaptation happened only at extreme ocean acidification conditions and when pH – a measure of seawater acidity - deviates from present seasonal variations. Under moderate acidification, applying a scenario commonly used in ocean acidification experiments but within the range of present day seasonal variations, no adaptation could be observed.

"Our research highlights the need to revise how we define 'control' and 'future' scenarios. It is critical to consider the natural variability if you don't want to under- or over-estimate the impacts," says Sam Dupont.

This study also shows that adaptation to ocean acidification is not without a cost. Copepods exposed to ocean acidification still suffered from a 29% reduction in egg production.

"A reduction of 29% of the copepod production can have severe effects for the fish in the future, as they may lose a large proportion of one of their most important food sources during their larval stage. Should we, however, manage to reduce emissions and hence reduce ocean acidification, we would be able to maintain copepod populations as they exist today," says Peter Thor.

The results have been published in the scientific journal Global Change Biology.