Flows and budgets of nutrient salts and organic carbon in relation to a red tide in the Ria of Vigo (NW Spain)

Abstract

In late September 1986, a major red tide occurred in the Ria of Vigo. Hydrographic conditions led to water retention, with circulation in a semi-closed circuit. In this study nutrient salts and carbon fluxes were calculated, and their consumption quantified, by use of a box model for the ria. The dinoflagellate Gymnodinium catenatum, scarcely present on 4 September, increased in concentration to 10(up 4) cell dm(-3) by 21 September. This was favoured by chemical causes, such as the high availability of ammonium (6.2 mol s(-1) were delivered to the upper water layer) and organic carbon, and by physical causes, such as the rapid circulation (862 X 10 (up to 3) kg s(-l) ) with reducing mixing, almost half that of the preceding date. The nutrient salts came mostly from the remineralisation of organic matter. The amounts contributed by freshwater, 5 % of the total, did not have an appreciable influence on the formation of the red tide. Vertical migration of the dinoflagellate played an important role. It is estimated that this organism was responsible for 22% of the synthesis of nitrogenous organic matter. The red tide came to an end with the November rains, which created rapid circulation which washed out the ria.