Interactions between hydrodynamics and pelagic ecosystems: relevance to resource exploitation and climate change

Abstract

A proposal is made to classify the pools of biogenic carbon in the oceans by reference to their turnover times. Break points in the continuum of turnover times, located at 10−2 and 102 years, distinguish between short-lived organic carbon, long-lived organic carbon and sequestered biogenic carbon. The three pools of biogenic carbon are discussed by reference to the more usual oceanographic concepts of new v. regenerated production, and export v. recycled production. Short-lived organic carbon is mainly associated with the microbial food-loop, long-lived organic carbon is relevant to renewable marine resources, and sequestered carbon is pertinent to global climate change (greenhouse effect). Recycling, export and sequestration are controlled by the proportions of primary production effected by small (<5 μm) and large (>5 μm) phytoplankton respectively, and by the selective grazing pressure experienced by cells in the various size-classes. These ecosystem processes are in turn governed by hydrodynamics. At low levels of auxiliary (mechanical) energy, the physical environment is stable, so that pelagic ecosystems are dominated by the microbial food-loop and biogenic carbon is mainly short-lived. Transitions from high levels of auxiliary energy to more stable conditions favour the production of large phytoplankton. When this takes place along predictable regular modes, primary production may be channelled into the long-lived pool, whereas stochastic transitions may lead to carbon sequestration. This theoretical framework may help the modern approaches of fisheries oceanography and biogeochemical oceanography to converge.