Nutrient recycling by fish versus zooplankton grazing as drivers of the trophic cascade in alpine lakes

Sarnelle, Orlando, Roland A. Knapp

Limnol. Oceanogr., 50(6), 2005, 2032-2042 | DOI: 10.4319/lo.2005.50.6.2032

ABSTRACT: In a multilake experiment, we found little effect of nutrient excretion by zooplanktivorous fish, but a large effect of herbivorous zooplankton, on phytoplankton biomass and phosphorus (P) limitation. Whole-lake removal of fish from small alpine lakes resulted in little change in phytoplankton biomass or the intensity of P limitation during 1-6 postremoval years, over which herbivorous zooplankton did not change. In contrast, significant decreases in phytoplankton biomass and the intensity of phytoplankton P limitation were observed after Daphnia became reestablished. Fish removal also caused large increases in the biomass of benthic macroinvertebrates, but the timing of these increases suggested that P recycling by the benthos did not confound our attempt to measure the effects of P recycling by fish. Estimates of the amount of P recycled by fish and zooplankton and estimates of P demand by the phytoplankton also supported the conclusion that fish recycling was not a major source of P to the phytoplankton in these lakes. Relative to Daphnia grazing, fish recycling of P appears to be relatively unimportant as a driver of trophic cascades in these alpine lakes.

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