The contribution of picophytoplankton to community structure in a Mediterranean brackish environment

The seasonal composition of phytoplankton communities was investigated in a Mediterranean brackish area (Varano lagoon). Twelve stations were sampled monthly from March 1997 to February 1998. Numbers of prokaryotic and eukary otic picophytoplankton cells were estimated by epifluorescence microscopy, while larger phytoplankton (nano and micro fractions) were enumerated by th e Utermohl settling technique. Picophytoplankton densities ranged from 0.7 to 448.6 cells x 10(6) l(-1). Nano- and microphytoplankton abundances varie d between 0.2 and 7.9 cells x 10(6) l(-1). The picoplankton fraction was re presented mainly by cyanobacteria and the Utermohl fraction by nanosized ph ytoflagellates (56.2%) and diatoms (20.1%). The phytoflagellates had a grea ter abundance over time while diatoms reached the highest densities in summ er and fall. In Varano lagoon, phytoplankton development is related to 'nit rogen-poor' waters and to phosphorus availability. Suspension-feeding bival ves (Mytilus galloprovincialis) are sufficiently abundant to filter a volum e equivalent to the volume of Varano lagoon at least once daily. These obse rvations suggest that grazing exerts an important influence on phytoplankto n dynamics, mainly on the micro fraction, and that diatoms seem to play an important role in the food web dynamics of this coastal fishery.