Hydrodynamic Control of Marine Phytoplankton Production: The Paradox of Stability

Abstract

Abstract Phytoplankton studies in a given environment are generally conducted on a small range of scales, so that no clear picture emerges as to the factors fundamentally critical, to the dynamics of phytoplankton. In the Estuary of the St Lawrence, where a wide range of observation scales are covered, stability of the water column is the only hydrodynamic factor significant to phytoplankton that is observed on all spatiotemporal scales. In various environments (estuarine, coastal, oceanic, frontal regions) vertical stability is reported to influence phytoplankton on scales from 6.2 h to one year, the forcing mechanisms being climatic, river runoff, tides or winds. An apparent paradox is that neither stabilization nor destabilization of the water column favours phytoplankton production: indeed, at any spatio-temporal scale, only the alternation of stabilization and destabilization is an hydrodynamic mechanism conducive to enhance primary production. A simple conceptual model of phytoplankton dynamics accounts for instances of the temporal succession of stratification and destratification of the water column, observed on a wide range of scales, and it may be applied to such structures as fronts and intermittent upwellings. It is therefore proposed to characterize the phytoplankton production potential of marine ecosystems by their frequency of stabilization-destabilization, since the resulting input of mechanical energy does enhance the primary production. Other factors such as temperature, turbidity, nutrient background, and so on, may limit the production potential built up by the alternation of stabilization and destabilization. Contrary to marine ecosystems driven by nutrient regeneration, which are vertically closed systems, those dominated by stabilization-destabilization vertically exchange nutrients regenerated at depth and energy stored by the phytoplankton at surface, which is one of the basic mechanisms of marine phytoplankton production.