Seasonal trends in phytoplankton biomass over the Uruguayan Shelf

Vertical and horizontal distributions of chlorophyll were analyzed during six cruises (three in autumn and three in spring), from 1991 to 1998, over the Uruguayan shelf. Differences in biomass were found between and within seasons, displaying great variability. Phytoplankton biomass was high even during the spring (max: 19.25mgm-3; mean ± SD: 2.82 ± mgm-3); autumn values were also relatively high (max: 7.62mgm-3; mean ± SD: 1.69 ± 1.45mgm-3) compared with other frontal regions. Maximum values were associated with Coastal Water (CW), Subtropical Water (STW) and the result of them mixing together in autumn. Maximum values were also observed in CW, Subantartic Water (SAW) and their mixing together during the spring. These features highlight the role of CW in fertilization of oligotrophic water masses (i.e. tropical water) and its action in adding stability to unstable SAW, thereby enhancing productivity. The highest values in both seasons were recorded in 1995 (ENSO cold year), emphasizing the high influence of SAW and defining it as a major nutrient source. Two well-defined zones with high chlorophyll concentration were observed in both seasons; one in the southern extreme of the study area and the other in the northern coastal region. Both zones were associated with frontal zones. This shelf convergence system is similar to the Brazil-Malvinas Confluence Zone, but with the CW as another important component, responsible for the surface density fronts and the main source of silicates.