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 Utermöhl settling technique. Picophytoplankton densities ranged from 0.7 to 448.6 cells • 106 l-1. Nano- and microphytoplankton abundances varied between 0.2 and 7.9 cells • 106 l-1. The picoplankton fraction was represented mainly by cyanobacteria and the Utermöhl fraction by nano-sized 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 observation suggest that grazing exert an important influence on phytoplankton dynamics, mainly on the micro fraction, and the diatoms seem to play an important role in the food web dynamics of this coastal fishery.