PCR-Based Assessment of Freshwater Zooplankton Feeding on Edible and “Inedible” Prey In Situ.

Microbiota in pelagic ecosystems can affect zooplankton nutrition in several ways that are not readily assessable in situ, using classical approaches. In contrast to classical food web models identifying phytoplankton as the dominant food source for crustacean zooplankton, recent findings increasingly suggest that zooplankton may derive a significant part of the diet from a wide variety of taxa including ciliates, aquatic fungi, bacteria and small metazoan zooplankton (e.g. rotifers), in both marine and freshwaters. Direct quantification of soft-bodied and non-pigmented prey in zooplankton guts as well as symbionts and parasites on the prey and zooplankton itself has so far been impeded by the lack of appropriate methodology. We aim to establish molecular approaches to quantify these yet-understudied interactions in lake food webs. As a first step we have validated the qPCR detection method in laboratory experiments with cladoceran, calanoid and cyclopoid predators and algal prey species (Cryptomonas sp.). We plan to apply the method to study the dietary contribution of aquatic fungi – chytrids, which are parasites on inedible phytoplankton species, thus aiming to provide insights into the Mycoloop – energy transfer from inedible phytoplankton to zooplankton via fungal parasites. The quantitative PCR method, when validated for key zooplankton species and specific prey or parasite groups, has a potential for a broad range of applications in food web research.