Zooplankton predators and prey: trophic roles and seasonality of C,N isotopic signatures

Crustacean zooplankton biomass seasonal dynamics in the open water of large, deep lakes is characterized by two main phases: a spring-summer peak, during which primary consumers prevail; and an autumn-winter phase, during which contribution of secondary consumers to zooplankton standing stock biomass is far from negligible. The two phases are traced by different ?15N signatures of mesozooplankton >=450µm size: lower, under prevalence of primary, and higher, under prevalence of secondary, consumers. Differences in contribution of the two groups are largely underestimated in e.g. models dealing with biomagnification and the role of zooplankton in transfer of pollutants to fish. Based on a three-year study on Lake Maggiore, we provide evidence for a need to incorporate quantitative estimates of trophic role within crustacean zooplankton in view of gaining reliable estimates of energy, and pollutants, fluxes through the pelagic food web.