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 eukaryotic picophytoplankton cells were estimated by epifluorescence microscopy, while larger phytoplankton (nano and micro fractions) were enumerated by the Utermohl settling technique. Picophytoplankton densities ranged from 0.7 to 448.6 cells x 10(6) l(-1). Nano- and microphytoplankton abundances varied between 0.2 and 7.9 cells x 10(6) l(-1). The picoplankton fraction was represented mainly by cyanobacteria and the Utermohl fraction by nanosized phytoflagellates (56.2%) and diatoms (20.1%). The phytoflagellates had a greater abundance over time while diatoms reached the highest densities in summer and fall. In Varano lagoon, phytoplankton development is related to 'nitrogen-poor' waters and to phosphorus availability. Suspension-feeding bivalves (Mytilus galloprovincialis) are sufficiently abundant to filter a volume equivalent to the volume of Varano lagoon at least once daily. These observations suggest that grazing exerts an important influence on phytoplankton dynamics, mainly on the micro fraction, and that diatoms seem to play an important role in the food web dynamics of this coastal fishery.