Phytoplankton dynamics in coastal lagoons are still poorly understood. Laguna de Rocha (34°33' S - 54°22' W) is a choked lagoon (mean depth = 0.5 m; area = 72 km^2) located on the Atlantic coast of Uruguay. The system is included in a Biosphere Reserve. Due to the natural water exchange with the ocean, several times per year, a spatial gradient of conductivity and nutrients is found from the marine influenced area (South) to the limnic one (North). In this study, the following hypothesis were considered: 1) the phytoplankton biomass and taxonomic composition of Laguna de Rocha is dominated by eurihaline and also benthic species, being nitrogen the limiting nutrient and 2) it is possible to identify a typical microalgal community, in terms of structure, functional groups and photosynthetic rate, for different hydrological conditions resumed in three conductivity levels (< 7.8, 7.8 – 30.0 and > 30.0 mS cm^-1). The main objective was to identify the principal abiotic factors that influence the spatial and temporal variation of the microalgal community that inhabit the pelagic. Monthly sampling (from 1996-1998 and 1998-2000) of abiotic and biological variables was performed at two stations: North (limnic influence) and South (marine influence). Functional groups were defined by combining the functional-morphological characteristics (resumed by the CSR life strategists) and the phyletic origin (Classes and Divisions) of the main species. An experimental approach was performed to determine the photosynthetic rate and its dependence on the nutrient availability (24 hours incubations).

The Zmix/Zeu relationship (< 3) indicated high PAR availability in the water column, while nutrient concentrations were variable (dissolved inorganic nitrogen, 0.64 - 18.4 micro-M; soluble reactive phosphorus, 0.10 - 2.69 micro-M and reactive silicate, 5 - 386 micro-M). The microalgal community included both planktonic and resuspended benthic species. Bacillariophyceae was the dominant group (Melosira moniliformis, Paralia sulcata, Chaetoceros subtilis var. abnormis f. simplex, Cylindrotheca closterium) followed by flagellates (Prorocentrum minimum, Rhodomonas spp., Chroomonas spp., Pyramimonas spp., Eutreptiella cf. gymnastica, and flagellates < 10 micro-m GALD). The total biovolume varied from 0.064 to 27.834 mm^3 ml^-1. Within the scheme of three life strategies (CSR), the community was represented mainly by species classified as C, R and CR strategists.

The temporal variation of the community was mainly explained by conductivity, dissolved oxygen, nitrogen and phosphorus concentrations. The twelve functional groups satisfactorily resumed the community variation, suggesting this is a useful approximation for ecological studies in coastal lagoons. Experiments indicated that nitrogen was always the limiting element for algal growth, while phosphorus was co-limiting. Under medium conductivity values, the highest primary production was detected (mean = 30.4 mg C m^3 h^-1), with the dominance of large planktonic eurihaline species. The conductivity levels explained the variation of the photosynthetic rate and the community (species and functional groups), while no significant differences were found between stations. Thus, the structure and function of the phytoplankton-benthic community varied according to the hydrology of the system, which can be inferred from the three conductivity levels approach.

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