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

Abstract

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 14 C-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 .

Zooplankton structure in the Loosdrecht lakes in relation to trophic status and recent restoration measures

“…Our observations confirm that the two species can coexist, even though for short intervals, in a range of lakes varying from highly eutrophic ones like the Loosdrecht lakes (Gulati, 1983 ;Gulati et al, 1985) and Friesian lakes (Beattie et al, 1978) to mesotrophic lakes like Lake Vechten (Gulati et al, 1982) .…”

“…The cladocerans are dominated by Bosmina spp . for only short periods in spring and early summer, rather than also by one or two species of Daphnia as is often the case in deep stratifying lakes (Gulati et al, 1982). Second, the cyclopoids, with their 4 species, and nauplii persist almost throughout the year .…”

“…zooplankton (> 150 µm) and seston fraction (< 150 µm), was measured using the COD method (Golterman, 1969), with some modifications (Gulati et al, 1982). Oxygen consumed is a measure of carbon oxidized which is obtained by multiplying the oxygen consumed by 0 .364 (Winberg & collaborators, 1971) .…”

“…Also D . hyalina, an important inhabitant of other eutrophic lakes like Tjeukemeer (Vijverberg & Richter, 1982) as well as mesotrophic lakes (Gulati et al, 1982), is sparse in the Loosdrecht lakes . In Tjeukemeer this species has been shown to utilize blue-green algae (Bloem & Vijverberg, 1984), and even though it is an important prey of planktivore fish, it has not disappeared .…”

A mathematical model of the phosphorus cycle in Lake Loosdrecht and simulation of additional measures

“…Grazing is described as a Monodlike function of the seston concentration, the specific filtering rate decreasing hyperbolically with increasing seston concentration (Gulati et al, 1982;Gulati et al, 1985). A selectivity constant is used for each food species to account for preference of the zooplankton for certain kinds of food: 1.0 for green algae, 0.5 for diatoms, cu.…”