Three microalgal communities were studied between August 1996 and September 1997 in a brackish lagoon, Laguna de Rocha (Uruguay), located on the south-eastern coast of South America. The system is shallow and periodically connected to the Atlantic Ocean and the anthropogenic influence is scarce. The freshwater and marine water inflows cause an extreme horizontal gradient in water level (range = 0.25-1.2 m), conductivity (range = 0.2-48.9 mS cm(-1)), nutrient concentration and sediment characteristics (sandy to muddy). At the northern station, dominated by freshwater, and at the southern one, dominated by the marine influence, the chlorophyll a amount of phytoplankton, microphytobenthos and epiphyton was measured on a monthly basis. Relating to lake surface, chlorophyll a ranged from 0.7 to 9.0 mg m(-2) for phytoplankton, and from 2.7 to 162 mg m(-2) for microphytobenthos. The epiphyton reached chlorophyll amounts from 2.7 to 536 mg m(-2) relating to the surface of the colonized macrophytes. The contribution of the epiphyton to the total algal biomass of the system was negligible however, since the development of the macrophyte substrate was scarce during the whole study period. Phytoplankton biomass was similar at both stations whereas microphytobenthos biomass was significantly higher (p < 0.05) at the freshwater station, probably due to better nutrient availability. Considering the relative microalgal biomass estimated through the chlorophyll a content of the three communities, the system was dominated by the epipelon in the freshwater area and by epipsammon in the marine one. The benthic dominance classifies the system in a 'dry state' (sensu Goldsborough & Robinson, 1996), favoured by the extreme shallowness, the high nutrient content and light availability at the bottom.
