Transfer of pollutants in lacustrine food webs: crustacean zooplankton as solely primary consumer?

Crustacean zooplankton have an important role in transfer of matter, energy, and pollutants through lacustrine food webs. In the open-water, crustacean zooplankters are a major link between phytoplankton primary producers and zooplanktivorous fish. The latter selectively feed on them, at least at the young stage. Despite being well-known as a non-homogeneous functional compartment, including both, primary and secondary consumers, zooplankters are often regarded, at least in ecotoxicological studies, as solely primary consumers. Regarding them as a single step in transfer of pollutants, however, may lead to temporary anomalies in estimates of concentration of pollutants. Such anomalies are often overlooked when annual means are applied resulting evident when seasonal changes in concentration of pollutants are measured. We provide results of a study on a deep subalpine lake, Lake Maggiore, in which distinction between zooplankton primary and secondary consumers was obtained by applying carbon and nitrogen stable isotopes analysis. In this lake, as in most deep, temperate, thermally stratified lakes, two phases in open water zooplankton seasonal succession can be identified: a first phase during which primary consumers prevail; a second phase in which secondary consumers prevail. Including the two steps of seasonal succession into conceptual ecotoxicological models is of crucial importance for reliable estimates of paths and mechanisms responsible in transferring persistent organic pollutants along the pelagic food web.