Bacterial abundance and production in the southwest Atlantic Ocean

A bacterial dynamics study was conducted in the southwest Atlantic Ocean off the Brazilian coast from October to December 1998 with samples collected at the depth of chlorophyll a maximum. The bacteria were counted by flow cytometry with the SYTO13 DNA dye and bacterial activity was measured as carbon production by the uptake of H-3-leucine. The samples were also analysed for basic hydrographical variables. Bacterial counts varied from 2.1 to 9.7 x 10(5) cells ml(-1) and bacterial production varied from 4.6 to 126.6 ng C L-1 h(-1). Spatial distribution of nitrate, chlorophyll a, bacterial abundance and bacterial activity showed areas of water fertilization in the oligotrophic Atlantic Ocean. Statistical analysis of data characterized the division of the area in two by latitude 19degreesS. On the north fertilizations are related to river discharge and on the south to the input of nutrients from deep waters. Remobilisation of nutrients in those spots results among all in higher biological activity. However the relation between bacterial abundance and activity is top-down controlled characterizing most of the waters as oligotrophic. Eddies showed by bacterial data can probably sustain the fisheries resources for the whole area. The influence of gradients across eddies on biological activity and food chain structure should be an important question to be studied in oligotrophic oceans.