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The influence of global climate and hydrography on microbial activity in the ocean : results of a N-S Atlantic transect

Hans-Georg Hoppe, Klaus Gocke Institut of Marine Science, Kiel, Germany The Atlantic in his longitudinal dimension, stretches over most of the dominating global climatic and hydrographic zones. The aim of this study was to characterize zones of biological activity along a N-S transect through the Atlantic and to see whether these corresponded to well known physical oceanographic and climatic zones. During this transect measurements of chlorophyll, cyanobacteria, actively metabolizing bacteria, protease activities, V m and turnover rate of easily degradable substances were made in 4 to 24 h intervals. The cruise took place from November 1991 to January 1992, thus an autumn situation in the North could be compared to a spring situation in the South. The results demonstrate, that there is a distinct zonation of the Atlantic with respect to biological activity, which clearly reflects the large scale environmental conditions. All variables showed their highest values in the corresponding northern and southern moderate climate zones. Low values were measured in the two corresponding subtropical zones. In the tropical zone (somewhat smaller than geographically defined), activities increased in comparison to the subtropical zones. Lowest values were nearly always measured in the Antarctic region. Normalized to chlorophyll (representing the food source for hetero­ trophic organisms), it turned out that most of the relative activities reached their maximum in the tropical zone, whereas in the adjacent northern and southern zones they decreased considerably. This suggests a temperature dependency of heterotrophic activity. However, if the activities are related to water temperature it turns out, that moderate climate zones with higher nutrient supply show high values, whereas activities in the tropical/­ subtropical zones are relatively low and more similar to those found in Antarctic waters. This is surprising because these zones cover a temperature range of about 30 ·c. It is concluded, that especially in Antarctic waters, nutrient supply and probably enzyme adaptation, are compensating low temperature effects to a certain extent. In the warm region low nutrient supply may be responsible for relative low activity rates.