Phytoplankton biomass and nutrient distributions in the Amazon River plume: Environmental correlates

The distribution of phytoplankton biomass in the plume of the Amazon River over the Brazilian continental shelf is analyzed by the use of multiple regression. Previous attempts to assess how different parameters control phytoplankton biomass have used pairwise correlations. A multiple regression approach, however, allows the elucidation of collinearity between these parameters. This approach reveals that phytoplankton biomass may be predicted largely by the following three groups of collinear variables that resemble the factors of factor analysis: suspended-sediment concentration and transparency (which generally describe irradiance availability), salinity and temperature (which describe vertical stratification, a measure of water-column stability), and the ambient concentrations of nutrients (phosphate, nitrate, silicic acid, and nitrite). The effects of water clarity and nutrients have been previously described, but the importance of vertical stability has never been separated from the other two. Additional important single variables were oxygen, ammonia, and urea. The strength of the contribution of particular variables to a regression model depends on the season of the cruise and hence on the volume of riverine discharge.