Conde, D., S. Bonilla, L. Aubriot, R. de León & W. Pintos 
Abstract
An annual study of the primary productivity of three microalgal communities was carried out in a brackish coastal lagoon (34° S - 54° W). Laguna de Rocha is a shallow water body periodically connected to the Atlantic Ocean, which presents few anthropogenic influence. The freshwater and marine water loads force extreme horizontal gradients of the water level (0.25 - 1.2 m), salinity (0.2-48.9 mS cm-1 from the freshwater end to the marine one), nutrient concentration and sediment characteristics (mud to sand). In two stations at both ends of the system, chlorophyll a and gross primary production of phytoplankton, microphytobenthos and epiphyton were measured on a monthly basis, from August 1996 to September 1997. The relation of the annual phytoplankton chlorophyll and total phosphorus values classified the system as eutrophic to hypertrophic. Clorophyll a (in mg m-2) was 0.7 to 9.0 for phytoplankton, 2.7 to 162.7 for microphytobenthos and 2.7 to 536 for epiphyton. Primary production (in mg C h-1 m-2) were 1.5 to 45.7 for phytoplankton, 2.7 to 20.6 for microphytobenthos and 1.2 to 12.8 for epiphyton. Generally, microphytobenthos showed the highest biomass, except at the end of the period when epiphyton ranged higher. Phytoplankton was the most productive community along almost all the study period. Nevertheless, the spatial development of the macrophyte substrate available for the epiphyton was extremely scarce during the whole study, making the contribution of this community negligible to the total primary production of the system. According to the relative algal biomass contribution, the system was epipelon dominated (“Dry state”) from early spring to late summer, momentary shifting to slight and short phytoplankton dominance at the marine influence area. The hydrologic, physical and chemical factors related to this pattern are discussed.