The zooplankton: its community structure,food and feeding,and role in the ecosystem of Lake Vechten

The structure, feeding and metabolism of the filterfeeders community of Lake Vechten (The Netherlands) were investigated for seven years in relation to the functioning of the lake's ecosystem. The ¹⁴C-technique used in the grazing and assimilation study is discussed in detail with a critical analysis of the methodological errors.The three major species which contributed to the annual density, biomass and grazing maxima in spring are: Bosmina longirostris, Daphnia spp. and Eudiaptomus gracilis. The rise in grazing pressure in recent years, particularly in May, was accompanied by a corresponding decrease in the seston (<33 µm) biomass, and in increase of inedible algae, especially Ceratium hirudinella, in late summer. The means of daily grazing ranged from 3% in March to 34% in June. The mean annual ratio ingestion: phytoplankton production varied from 70 to 230%. The specific filtering rate, SFR (ml · day^–1 · mg^–1 · zoop · C), was related directly to water temperature but inversely to the food concentrations.The main errors in the ingestion and assimilation rates were related to the leaching of the isotope from the animals in the preservation fluid. The loss of tracer was 42 and 26%, respectively, for the two rates.In spring, the food removed by the grazers per day was equivalent to 125–400% of the daily primary production. This caused a sharp decrease in the seston concentrations and a recurring clear water phase because of a sharp increase in the Secchi depth. The zooplankton assimilatory removal of carbon and the sedimentation loss rates to the hypolimnion exceeded the primary production rates. The inconsistencies in the carbon budget are possibly due to our lack of knowledge of the horizontal transport of material from the littoral, bacteria as an alternative food source for zooplankton, and the DOC dynamics.The grazers' activity as SFR in deep, stratifying lakes like Vechten is 3 to 4 times that in the shallow, mixed and more eutrophic Dutch lakes. In the former category of lakes the crustacean herbivores serve as an important link in the food chain in the limnetic region.