Control of production of organic matter in the ocean on short and long terms by stratification and remineralization

Vertical stratification sets the rules of the game for the short-term production of organic matter in deep aquatic systems. Some examples of this are reviewed and discussed. Basic physical conditions for phytoplankton growth in well-mixed surface layers are discussed. For net growth of a phytoplankton population in a well-mixed surface layer the thickness of the layer must not exceed a certain maximum, the critical depth, which is determined by illumination and phytoplankton properties (Sverdrup, Journal du Conseil Permanent International pour lʹExploration de la Mer, 18 (1953) 287–295). Using observations from the Baltic Sea and Kattegat, it is demonstrated that this single condition is sufficient for predicting the time for onset and decline of phytoplankton growth in the sea. High-frequency cycles of stratification-destratification of surface waters may enhance plant production. Important examples of this are found at tidally forced shelf fronts that execute a fortnightly seaward-landward oscillation. It is also shown that thin, nutrient rich surface layers promote the growth of small or swimming phytoplankton. Factors influencing the widely occurring deep phytoplankton maxima in stratified water are briefly discussed. le source-sink nutrient balance shows that the long-term global rate of production of organic matter in the sea is determined by the magnitude of external sources and by the remineralization ability of the pelagic and benthic ecosystems. The latter may be influenced by the rate of oxygen supply to waters beneath the surface layer by vertical circulation. However, the rate of production is not globally uniform but strongly typed by the large-scale stratification and circulation in the upper ocean. Areas of horizontal divergence and upwelling are known to have particularly high production while areas of horizontal convergence and downwelling have low production.