Diatom-lipid-copepod nexus under threat by global change

The central role of lipids in the ecology of high-latitude and temperate seas is under threat by global change. This hinges on the tight ecological links between diatoms and copepods of the genus Calanus and similar (e.g. Neocalanus, Calanoides) that produce and accumulate energy-rich lipids. Strongly coupled through seasonal water-column stability, nutrient supply, spring-bloom succession, and overwintering life strategies, this nexus is foundational to the highly productive food webs of these regions. We experimentally tested how a change in phytoplankton type from diatoms to flagellates, along with 2°C warming, affects lipid accumulation of the subarctic copepod Calanus finmarchicus. We report that the ability of C. finmarchicus to accumulate lipids is seriously impaired by a changing diet, being threefold higher on diatoms than flagellate-based diets, whereas within diet types, up to a sevenfold reduction in lipids is associated with a 2°C temperature rise. This, together with higher specific respiration rates, smaller size, and reduced diatom productivity, suggests the ability of Calanus to overwinter may be compromised, threatening their contribution to regional productivity and to sequestered carbon via the lipid pump. Similar lipid-mediated couplings are prevalent throughout seasonal environments, casting uncertainty on their future provision of ecosystem services.