Original Articles

Physical influences on biological processes: how important are they?

Published in: South African Journal of Marine Science
Volume 12, issue 1, 1992 , pages: 107–121
DOI: 10.2989/02577619209504695
Author(s): K. H. Mann


Physical processes in the ocean influence biological processes at all scales from microns to thousands of kilometres. The viscous boundary layer around phytoplankton limits nutrient uptake; partial relief is obtained by sinking or swimming. During the progress of an upwelling event, diatoms, which sink, tend to be replaced by flagellates, which swim. The formation of a dense layer of flagellates in a stratified water column may be critical to the survival of first-feeding fish larvae. At the scale of 10–100 m, vertical mixing brings nutrients to the surface, but stratification holds phytoplankton in the euphotic zone. Alternation of these two processes in an upwelling system is critical to high productivity. On an interannual scale, the productivity of upwelling systems may be limited by the intrusion of anomalous warm surface waters. This process is well documented for the Peruvian upwelling system (El Niño-Southern Oscillation) and a similar process has been observed in the Benguela system. There are tentative correlations between these events and changes in the population dynamics of pilchard, anchovy and hake. In general, physical factors tend to dominate marine ecosystem function, and prediction of fish productivity requires an ability to predict physical events.

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