The seasonal pattern of the phytoplankton-aloricate ciliate community is an adequate indicator of trophic coupling and nutrient cycling in coastal environments. The size-fractionated, abundance, biomass and composition of a phytoplankton-aloricate ciliate community were ascertained in the Bahía Blanca Estuary from February 1994 to February 1995. In addition, ciliate production and excretion rates by size class were estimated. A winter-early spring diatom bloom was the most important event in the annual cycle. To evaluate the significance of phytoplankton size in the food web, <25 µm and > 25 µm fractions of chlorophyll a were determined; the < 25 µm fraction was dominant. To classify the ciliate community in trophic groups, it was divided into volume-size classes, from <10³ µm³to > 10(5) µm³; this wide range of size indicated the potential prey. The smaller ciliates dominated abundance but the larger ones dominated biomass. Values of total ciliates, ranging from 0.2 to 5.2 x 10³ cells L-1 and 0.55 to 83.9 µg C L-1, and levels of total chlorophyll a reached 40.4 µg L-1 and were higher than those recorded for other temperate estuarine systems. The smaller ciliates were potentially important as nitrogen regenerators in summer and spring, when their estimated production was highest. The annual potential production of 3 mg C L-1 represents a reference parameter from which possible changes in the trophic status of this environment could be predicted.
Aloricate ciliates; Phytoplankton; Size-fractioned chlorophyll a; Trophic status; Estuary; Argentina