Biotic Effects of Grazing by Tilapia (Oreochromis aureus) on Phytoplankton Populations in Solar Silos

Abstract

ABSTRACT The role of grazing fish in the phytoplankton dynamics of solar silo microcosms was examined by following the quantitative effects of grazing as measured by chlorophyll a (chl a), total protein (TP), and phytoplankton cell counts for 15 weeks. Qualitative effects during this period were measured by recording phytoplankton community composition changes. Three treatments were used: a control silo with no fish, two “low” density silos containing Oreochromis aureus at 8 m−3, and two “high” density silos containing O. aureus at 17 m−3. Phytoplankton cell numbers reached 5.1×108 cells ml−1 in the silos and were actively grazed. Grazing reduced cell numbers by 1–2 orders of magnitude in the high density silos but increased cell numbers in the low density silos. Grazing in the silos reduced chl a by 45–76% and TP by 61–72% during the last 4 weeks of the experimental period. A successional trend in the phytoplankton community to Chlorella spp., Golenkinia paucispina and Gloeocystis major in the “low” density silos, and to two gelatinous species, Dictyosphaerium pulchellum and Sphaerocystis schroeteri, in the “high” density silos was observed. Similar phytoplankton community shifts to gelatinous algae have also been reported as a result of intense grazing by herbivorous zooplankton in eutrophic lakes.