PHYTOPLANKTON BIOMASS AND NUTRIENT DISTRIBUTIONS IN THE AMAZON RIVER PLUME - ENVIRONMENTAL CORRELATES

The distribution of phytoplankton biomass in the plume of the Amazon R iver over the Brazilian continental shelf is analyzed by the use of mu ltiple regression. Previous attempts to assess how different parameter s control phytoplankton biomass have used pairwise correlations. A mul tiple regression approach, however, allows the elucidation of collinea rity between these parameters. This approach reveals that phytoplankto n biomass may be predicted largely by the following three groups of co llinear variables that resemble the ''factors'' of factor analysis: su spended-sediment concentration and transparency (which generally descr ibe irradiance availability), salinity and temperature (which describe vertical stratification, a measure of water-column stability), and th e ambient concentrations of nutrients (phosphate, nitrate, silicic aci d, and nitrite). The effects of water clarity and nutrients have been previously described, but the importance of vertical stability has nev er been separated from the other two. Additional important single vari ables were oxygen, ammonia, and urea. The strength of the contribution of particular variables to a regression model depends on the season o f the cruise and hence on the volume of riverine discharge.