THE BIOGEOCHEMICAL CYCLING OF DISSOLVED SILICATE IN A GALICIAN RIA

The annual variation of the dissolved silica contributions to the ria of Vigo and its spatial distributions on the silicate sediment is cons idered using a box model. During winter, the silicate freshwater flux is high, about 10 mol Si s(-1). This mixes with the ria water and is e xported to the ocean. During the rest of the year, the most important source of dissolved silicate is the subsurface incoming flux of marine water to the ria which provides from 52% to 65% of the total. In thes e seasons, the redissolution of biogenic silicate is the second source and causes up to 46% of silicate inputs to the photic zone. The utili sation of dissolved silicate by the diatoms was 1 mol Si s(-1), but in creased to 11 mol Si s(-1) in the event of upwelling. The abundance of silicate in sediments in different parts of the ria of Vigo is a cons equence of the cited fluxes. The bottom of the inner middle zone in th e ria receives diatoms in spring and summer when no upwelling occurs ( from 50 to 120 mg Si m(-2) d(-1)), and silicate is resupended due to s ediment stirring during winter. The external mid-zone of the ria is ri cher in remains of diatoms and opal, there being no resuspension and s edimentation is greater than in other areas during upwelling events (1 72 mg Si m(-2) d(-1)), as indicated by the relative abundance of Chaet oceros spp. spores.