Seasonal and daily changes in bacterivory in a coastal plankton community

Abstract

Fluorescently labeled bacteria (FLB) were used to examine the long-term changes (monthly for 8 mo) and short-term changes (daily for 10 d) of bacterial grazing in a natural plankton community. Two preliminary experiments also were performed to evaluate the effect of sample volume, incubation time, and light regime on the results. Experimental results indicate a 'bottle effect' for the 3 smallest sample volumes employed (100, 200, 500 ml); lower grazing rates were always observed during 24 h incubations than during 48 h incubations. The light regime (continuous darkness, continuous light, 12L:12D) did not affect grazing rates during a 48 h experimental period. Bacterivory in the seasonal study in Vineyard Sound, Massachusetts (USA) was highly responsive to water temperature and was the primary factor determining grazing pressure over the winter-summer period. Samples incubated at 20°C during winter consistently exhibited higher rates of consumption of bacteria than a triplicate set of samples incubated at ambient water temperature or 5°C. Daily fluctuations in the rates of bacterivory during a 10 d period in May were significant, but were ca 1/10 the magnitude of the seasonal fluctuation. Maximal rates of bacterivory removed up to 60 % of the bacterial assemblage daily.