Total lipid and fatty acid composition of mesozooplankton functional group members in the NE Pacific over a range of productivity regimes

Abstract

Fish, whales, and seabirds along the productive west coast of Vancouver Island (WCVI), NE Pacific, rely on copepod prey that are assumed to be rich in lipid. However, the total lipid (TL) and fatty acid content of most copepod species and other mesozooplankton in this region have not been measured. To assess the diets and quality of zooplankton prey off the WCVI, we investigated differences in lipid content and composition of mesozooplankton according to their taxonomic versus functional group identities. Copepods, chaetognaths, euphausiids, and amphipods, belonging to 3 different functional groups, were sampled on the WCVI during pre-, mid-, and post-bloom conditions. Compared to taxonomic classifications, the functional group approach was superior in its ability to discern statistical differences in TL among zooplankton and between seasons. Furthermore, multivariate analyses using zooplankton fatty acid composition from all oceanographic regimes produced 3 to 4 statistically different clusters of species that corroborated functional group designations. However, some trophic flexibility was observed in several copepod species, particularly Calanus marshallae, that may relate to food scarcity or the presence of potentially noxious prey. The omnivorous-herbivorous functional group frequently contained higher amounts of TL and masses of essential fatty acids than the carnivorous group. Euphausiids and C. marshallae contributed the most lipid (mg m-3) to the pelagic ‘lipid pool’, due to very high lipid content and high abundance, respectively. However, on the continental slope and shelf break, chaetognaths and Neocalanus spp. copepods contributed substantially to the lipid pool.