Indirect zooplankton effects on water quality: grazing and nutrient remineralization

Funded by the Israel Ministry of Science and the German Federal Ministry of Education, Science, Research and Technology: DISUM 44, Grant no. GR 01565.
in cooperation with T. Zohary and H. Güde.

Objective:
Management of Lake Kinneret water quality means, in part, regulation of phytoplankton, which requires understanding of both positive and negative influences on phytoplankton growth, abundance and species composition. Among several such factors, grazing mortality and nutrient cycling by herbivorous zooplankton, including both micro-zooplankton (flagellated- and ciliated-protozoans and rotifers) and macro-zooplankton (cladocerans and copepods) are strong effectors of phytoplankton. Hence, better understanding of grazing and nutrient cycling and how they influence phytoplankton can better enable effective management of lake water quality.

Specific objectives of this BMBF-MOS-funded project were initially three-fold:
1) Quantification of both macro- and micro-zooplankton grazing rates on Lake Kinneret phytoplankton, protozoans, bacteria, and detritus;

2) quantification of excretion rates of nitrogen and phosphorus by zooplankton feeding on Lake Kinneret phytoplankton, protozoans, bacteria, and detritus; and

3) delineation of the specific role of Peridinium (both as live cells and as detritus) in zooplankton grazing and nutrient remineralization.

During the project two additional objectives were necessarily added:
4) verification of the theoretical model used for interpreting nutrient cycling rates by zooplankton and

5) quantification of the indirect confounding effects of predatory cyclopoid copepods.

Major findings:
1. Micro-zooplankton grazers (flagellated- and ciliated-protozoans, rotifers and copepod nauplii) dominated both grazing on phytoplankton and bacteria and subsequent nutrient recycling compared with macro-zooplankton grazers (cladocerans and cyclopoid copepodids) in Lake Kinneret.

2. We found no evidence to suggest that Peridinium-derived detritus (i.e., shed thecae) was consumed directly by macro-zooplankton, though consumption of smaller detrital particles by micro- and macro-zooplankton suggested indirect utilization of Peridinium-derived detritus via the microbial loop.

3. Top-down effects of grazers and predators were clearly important in the Lake Kinneret pelagic food web, even under conditions of high resource availabilities (as in our experiments). However, these effects were highly taxon-specific, making broad generalizations across producer and consumer groups difficult.

4. Results confirm current conceptual views (e.g., Hart et al. 2000) of the complexity of carbon and nutrient flows within the pelagia of Lake Kinneret.