MEPS 251:1-14 (2003)  -  doi:10.3354/meps251001

Convection and primary production in winter

Jan O. Backhaus1,*, Else Nøst Hegseth2, Henning Wehde1, Xabier Irigoien3, Kerstin Hatten1, Kai Logemann1

1Institute of Oceanography, University of Hamburg, Troplowitzstr.7, 22529 Hamburg, Germany
2The Norwegian College of Fisheries Science, University of Tromsø, 9037 Tromsø, Norway
3Technological Institute for Fisheries and Food AZTI, Herrera Kaia, Portualde z/g, 20110 Pasaia, Spain

ABSTRACT: We present observations of temperature, salinity and phytoplankton from late winter of 1999 obtained along a meridional transect running from 52 to 72°N through the northern North Atlantic and Norwegian Sea. The field work was conducted with the intention to find evidence for a postulated relationship between convection and primary production in the open ocean in winter. In the investigated area, convection ventilates water masses in a deep mixed layer which may cover several hundreds of meters, depending on atmospheric forcing and stratification. Inspired by the results of a coupled phytoplankton convection model, we postulated that sinking phytoplankton from the seasonal thermocline in autumn is dispersed within the mixed layer by vertical orbital motions and turbulence induced by convection. Orbital motions allow intermittent visits of plankton to the euphotic layer from much deeper depths. Hence, convection distributes the chance of plankton receiving light over the entire mixed layer. This supports production and allows compensation of losses due to sinking. We hypothesised that a winter stock of living phytoplankton in the open ocean would always coincide with the deep mixed layer formed by convection, irrespective of its depth. Indeed, along the entire meridional transect, living phytoplankton was found within the layer, whereas only marginal amounts or virtually no phytoplankton existed beneath it. The layer extent varied between 300 and 800 m. In the northern North Atlantic, average concentrations of chloro- phyll a (as a measure of phytoplankton biomass) in the mixed layer were about 0.3 mg m-3, and 0.6 mg m-3 in the Norwegian Sea. We concluded that this winter stock forms the inoculum for a spring production in the open ocean.

KEY WORDS: Primary production · Winter · Convection · Vertical orbital motions

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