Effects of water-column mixing on bacteria,phytoplankton, and rotifers under different levels of herbivory in a shallow eutrophic lake |
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Authors: | G Weithoff A Lorke N Walz |
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Institution: | (1) Institute of Freshwater Ecology and Inland Fisheries, Dept. Limnology of Shallow Lakes and Lowland Rivers, P.O. Box 850 205, Müggelseedamm 301, 12587 Berlin, Germany, DE |
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Abstract: | Water-column mixing is known to have a decisive impact on plankton communities. The underlying mechanisms depend on the size
and depth of the water body, nutrient status and the plankton community structure, and they are well understood for shallow
polymictic and deep stratified lakes. Two consecutive mixing events of similar intensity under different levels of herbivory
were performed in enclosures in a shallow, but periodically stratified, eutrophic lake, in order to investigate the effects
of water-column mixing on bacteria abundance, phytoplankton abundance and diversity, and rotifer abundance and fecundity.
When herbivory by filter-feeding zooplankton was low, water-column mixing that provoked a substantial nutrient input into
the euphotic zone led to a strong net increase of bacteria and phytoplankton biomass. Phytoplankton diversity was lower in
the mixed enclosures than in the undisturbed ones because of the greater contribution of a few fast-growing species. After
the second mixing event, at a high biomass of filter-feeding crustaceans, the increase of phytoplankton biomass was lower
than after the first mixing, and diversity remained unchanged because enhanced growth of small fast-growing phytoplankton
was prevented by zooplankton grazing. Bacterial abundance did not increase after the second mixing, when cladoceran biomass
was high. Changes in rotifer fecundity indicated a transmission of the phytoplankton response to the next trophic level. Our
results suggest that water-column mixing in shallow eutrophic lakes with periodic stratification has a strong effect on the
plankton community via enhanced nutrient availability rather than resuspension or reduced light availability. This fuels the
basis of the classic and microbial food chain via enhanced phytoplankton and bacterial growth, but the effects on biomass
may be damped by high levels of herbivory.
Received: 3 May 1999 / Accepted: 13 April 2000 |
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Keywords: | Diversity Enclosure Plant-herbivore interactions Intermediate disturbance hypothesis |
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