Seasonal patterns of feeding by natural populations of Keratella, Polyarthra, and Bosmina: Clearance rates, selectivities, and contributions to community grazing1

Abstract

Seasonal changes in individual clearance rates, selective feeding behavior, and population grazing rates of Keratella cochlearis, Polyarthra vulgaris, Polyarthra dolichoptera, and Bosmina longirostris were investigated in a eutrophic lake using five tracer foods (Aerobacter, Rhodotorula, Chlorella, Chlamydomonas, and Euglena) and by in situ radioisotope techniques. Clearance rates ranged from <0.002 to 13 µ l·ind −1·h−i depending on type of cell and water temperature. Keratella and Bosmina ingested all foods; they preferred Chlamydomonas but differed in their preferences for other species. Polyarthra ingested only Chlamydomonas and Euglena. Significant direct relationships between clearance rates and temperature were found for Keratella, P. vulgaris, and Bosmina feeding on Chlamydomonas and for Keratella feeding on Rhodotorula. The clearance rates of Keratella and Bosmina were inversely related to the concentration of both tracer cells and natural particles (2.5–15‐µ m ESD). Temperature and food concentrations accounted for 87 and 89% of the seasonal variation in the clearance rates of Keratella and Bosmina. Although there were days and depths when Bosmina or Polyarthra populations dominated community grazing on Chlamydomonas‐like cells, Keratella populations accounted for about 80% of the grazing pressure on such cells during the year.