NUTRIENTS AND PRIMARY PRODUCTION IN A SHALLOW STRATIFIED ECOSYSTEM IN THE IROISE SEA

Seasonal changes of nutrients and primary production were studied in a shallow stratified ecosystem in the Iroise Sea (Atlantic coast of France) in 1983-84. Changes of nutrients and primary production followed the pattern known for the temperate coastal waters that become seasonally stratified. Continued nitrification in the bottom waters, a rapid regeneration of silicon, and ammonium regeneration by microzooplankton are possibly more important to sustain production than the winter stocks of nitrate and silicon, and ammonium released by macrozooplankton. Light is the major environmental factor controlling primary production. Nutrient limitation of productivity occurs only in May-July. Production rates predicted by empirical models agreed well with measured rates. Dissolved nitrogen-containing organic substances trigger the Gyrodinium aureolum bloom. Prolonged stratification and an abundant nitrate supply maintain it and a total depletion of nitrate terminates it.