Biogeochemical Provinces of the Southwestern Atlantic Ocean between 5oN and 45oS

Adriana Gonzalez Silvera
e-mail: agglez@uabc.mx


Remote sensing of phytoplankton primary production requires the previous definition of “Biogeochemical Provinces”, where biological, physical and chemical parameters necessary to apply the mathematical models can be considered as known and stable both in adequate and relevant spatial and temporal scales. Their definition also requires a great quantity of data in order to consider such variability. Along the coasts of Brazil, Uruguay and Argentina, the Atlantic Ocean is influenced by several hydrographic processes that lead to great diversity in biological systems, and in situ observations are scarce and sparsely distributed.

In the present study, phytoplankton pigment concentration taken by the ocean color sensor CZCS (Nimbus-7/NASA) was used to propose a Biogeochemical Province classification for the region between 5oN and 45oS, and from the coastline to 25oW. Average pigment concentration from January 1982 to May 1986 was used to create a data matrix as input for a Principal Component Analysis (PCA). 152 locations were grouped in 14 sub-regions and most of them were associated to surface circulation patterns or hydrographic features over the study region. Sub-region R11 (Southern Brazil), where in situ data have been collected, was used as a case study.

Hydrographic data, chlorophyll-a concentration and the spectral coefficient of phytoplankton were measured in situ within the area between 30oS and 32oS. The tropical waters of the Brazil Current dominated the upper layers, where average chlorophyll-a concentrations were 0.51 mg/m3. Lower chlorophyll concentrations were associated with phytoplankton populations dominated by small flagellates (less than 5 micro-m). Two groups of stations were identified based on the shape of the absorption spectra of phytoplankton: Group 1, with chlorophyll-a concentrations greater than 1 mg/m3 and dominated by diatoms; and, Group 2, with pigment concentrations below 1 mg/m3 and dominated by flagellates less than 5 micro-m in size (ultra/picoplankton). These results were used to estimate an equation relating surface chlorophyll-a concentration with absorption coefficient of phytoplankton at 436 nm in the region. This information, together with the shape of the spectral absorption curve (Group 1 and 2), can be used to recover the spectral absorption curve of phytoplankton between 400 and 700 nm, as entry for primary production models. In addition, it is important information for validation of sub-region R11. These data indicate that R11 can be divided in at least two other and smaller regions, one associated with coastal waters, and the other with oceanic waters (tropical characteristics), which can be considered as part of the validation process of R11.