ABSTRACT: Dilution experiments were conducted from May to September 1998 in the epilimnion of a recently flooded reservoir (Sep Reservoir, Puy-de-Dôme, France: 46°2'N, 3°1'E), to estimate growth and microzooplankton (20 to 200 µm) grazing mortality of 2 size classes of the phytoplankton community. This community was dominated by <25 µm cells, which averaged 77% (range 41 to 98%) of GF/F-collected chlorophyll a and 67% (27 to 99%) of total counts from inverted light microscopy. Total particulate DNA content was also significantly higher for the <25 µm size class, compared to the >25 to 200 µm size fraction. Micrograzers were largely dominated by ciliated protozoa (86 to 96% of total abundance), and also comprised rotifers and copepod nauplii. Experiments with and without added nutrients (N and P) indicated, together with changes in particulate protein, RNA and DNA, that phosphorus is a limiting element in the Sep Reservoir. Grazing activity of microzooplankton was significant on nanoalgae, averaging 0.38 ± 0.19 d^-1 (range 0.16 to 0.66 d^-1). The grazing activity balanced the daily production of the target algal community by 71 ± 11% (range 54 to 88%). Microzooplankton herbivory and the growth rates of nanoalgae were strongly correlated, suggesting the existence of an operating homeostatic interaction between the prey and the predators. The high mortality:growth ratio indicated that a substantial fraction of phytoplankton carbon is recycled in surface waters through microbial grazing. We conclude that microzooplankton herbivory provides an effective and substantial link to higher trophic levels in the Sep Reservoir, and might contribute to fueling planktonic communities with the limiting nutrient through regeneration.

KEY WORDS: Phytoplankton · Microzooplankton · Growth · Grazing · Nutrient limitation · Dilution experiments · Reservoirs

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Published in AME Vol. 22, No. 3 (2000) on October 26
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 2000