HYDRODYNAMIC AND CHEMICAL CONDITIONS DURING ONSET OF A RED-TIDE ASSEMBLAGE IN AN ESTUARINE UPWELLING ECOSYSTEM

The hydrodynamics and nitrogen/silicon biogeochemistry accompanying th e development of a red-tide assemblage were examined in the Ria de Vig o (northwest Spain), a coastal embayment affected by upwelling, during an in situ diel experiment in September 1991. Despite a low N:SI mola r ratio (0.5) of nutrients entering the surface layer, which was favou rable for diatom growth, the diatom population began to decline. Limit ed N-nutrient input, arising from moderate coastal upwelling in a stra tified water column, restricted net community production (NCP = 630 mg C m(-2) d(-1)). In addition, light-limitation of gross primary produc tion (GPP = 1525 mg C m(-2) d(-1)) was observed. The relatively high f -ratio (= NCP:GPP) recorded (0.41, characteristic of intense upwelling conditions) would have been as low as 0.15 had not GPP been limited b y light intensity. Temporal separation of carbohydrate synthesis durin g the photoperiod from protein synthesis in the dark could be inferred from the time-course of the C:N ratio of particulate organic matter. Severe light-limitation would lead to diatom collapse were the diatoms not able to meet all their energy requirements during the hours of da rkness. Under the hydrodynamic, nutrient and light conditions of the e xperiment, an assemblage of red-tide-forming species began to develop, aided by their ability to migrate vertically and to synthesize carboh ydrates during the light in surface waters and protein during the dark at the 4 m-deep pycnocline. Thermal stratification, reduced turbulenc e, intense nutrient mineralization, and the limited nitrogen input thr ough moderate upwelling were all favourable to the onset of a red-tide assemblage.