Primary production and physiological state of phytoplankton at the Ushant tidal front (west coast of Brittany, France

Abstract

Physical, chemical, and biological parameters relevant to phytoplankton were measured in late summer 1982 across the southern part of the Ushant front. Daily primary production rates (g C m-' d-') increased and assimilation number decreased from the open sea to the frontal zone. However, productivity (mg C [mg Chl a]-' h-') in the frontal upper layers (> 10 O/ O light surface intensity) was significantly higher than in stratified and transitional waters. Offshore, no dinoflagellates but only nanoflagellates were observed, with diatoms (mainly Nitzschia seriata) in more coastal waters. On spring tides, diatoms were widespread over the transitional area, even on the stratified side. Physiological state of the phytoplankton was assessed on the basis of photosynthetic properties, adenylate energy charge, ATP: Chl a and POC: Chl a ratios, and specific growth rates. Physiological activity of nanoflagellates in well-stratified, nutrient-deplete waters was mainly influenced by light level. Physiological state of the diatom populations was not constant over the transitional area, but in the stratified frontal zone was greater in the upper layer. On the mixed side, by contrast, the physiological state of the phytoplankton was lower, particularly as regards light adaptation.