Disentangling the mechanisms behind climate effects on zooplankton

Abstract

Understanding how climate influences ecosystems is complicated by the many correlated and interrelated impacting factors. Here we quantify climate effects on Calanus finmarchicus in the northeastern Norwegian Sea and southwestern Barents Sea. By combining oceanographic drift models and statistical analyses of field data from 1959 to 1993 and investigating effects across trophic levels, we are able to elucidate pathways by which climate influences zooplankton. The results show that both chlorophyll biomass in spring and C. finmarchicus biomass in summer relate positively to a combination of shallow mixed layer depth and increased wind in spring, suggesting that C. finmarchicus biomass in summer is influenced by bottom-up effects of food availability. Furthermore, spatially resolved C. finmarchicus biomass in summer is linked to favorable transport from warmer, core areas to the south. However, increased mean temperature in spring does not lead to increased C. finmarchicus biomass in summer. Rather, spring biomass is generally higher, but population growth from spring to summer is lower, after a warm compared with a cold spring. Our study illustrates how improved understanding of climate effects can be obtained when different datasets and different methods are combined in a unified approach. responses in zooplankton phenology, distribution, abundance, and composition (1, 2), but the controlling mechanisms behind the associations are often elusive. For example, a change in temperature might directly affect zooplankton physiology (3) or indirectly influence zooplankton through effects on their prey (4) or ecosystem trophic structure (5). To make realistic projections of climate effects on marine ecosystems, there is a need for improved understanding of the mechanisms by which climate affects the different trophic levels.The Atlantic waters of the Norwegian Sea-Barents Sea (NS-BS) host a highly productive ecosystem including several large fish stocks of high socioeconomic importance (6). The area experienced increased water temperatures during the past decades (7) and is, like other high-latitude regions, predicted to warm substantially throughout the 21st century (8). Climate simulations further suggest globally increased ocean stratification, accompanied by decreased primary production in temperate regions but increased primary production in the subarctic (including the Barents Sea) (9, 10).Calanus finmarchicus dominates mesozooplankton biomass and is an important predator on phytoplankton throughout the North Atlantic (11, 12). In the NS-BS, young stages of C. finmarchicus are preyed upon by larvae of demersal fish, and older stages are preyed upon by various pelagic stocks (12, 13). Several studies have indicated that C. finmarchicus in the NS-BS is top-down controlled, particularly by Barents Sea capelin (14-16) and Norwegian spring spawning herring (17, 18). Consistent effects of climate or food availability have on the other hand rarely been demonstrated in situ (19-21).Investigating environmental effects on z...

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