Seasonal and spatial variations in heterotrophic nanoflagellate and bacteria abundances in sediments of a freshwater littoral zone

We studied seasonal variation in heterotrophic nanoflagellates (HNAN) and bacterial, densities at different depths in the sediment of two freshwater littoral stations. Station 1 was in a reed bed of Phragmites australis; station 2 was outside the reed zone in open water. Benthic HNAN abundances ranged from 0.7 x 10(3) to 167.9 x 10(3) cells cm(-3), with small cells (<5 mu m) being the most abundant. Bacterial densities were high, ranging from 1.5 to 16.4 x 10(9) cells cm(-3) sediment. HNAN and bacterial densities showed significant time, depth, and station effects. At station 1, the bacterial standing stock increased during the course of the observation period, probably because of an increase in macrophyte biomass. At station 2, bacterial densities in the upper 0.5 cm of the sediment fluctuated little and without a clear seasonal pattern. In the top layer of both stations, HNAN densities were high in winter and lower in midsummer and early autumn. Density ratios between bacteria and HNAN were very high in this layer, ranging from 22.4 x 10(3) to 2.0 x 10(6) bacteria HNAN(-1). Contrary to our expectations, these ratios suggest that the impact of benthic HNAN on the bacterial dynamics is minimal in this littoral zone. We hypothesize that throughout the year, a limited access to bacteria prevented HNAN from reaching high abundances. Seasonal HNAN density fluctuations probably were caused by epi-meiobenthos grazing on HNAN in summer and early autumn.