The biogeochemical cycling of dissolved Silicate in a Galician Ria

Abstract

Abstract The annual variation of the dissolved silica contributions to the ria of Vigo and its spatial distributions on the silicate sediment is considered using a box model. During winter, the silicate freshwater flux is high, about 10 mol Si s-l. This mixes with the ria water and is exported 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 these seasons, the redissolution of biogenic silicate is the second source and causes up to 46% of silicate inputs to the photic zone. The utilisation of dissolved silicate by the diatoms was 1 mol Si s-1, but increased 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 consequence of the cited fluxes. The bottom of the inner middle zone in the 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 sediment stirring during winter. The external mid-zone of the ria is richer in remains of diatoms and opal, there being no resuspension and sedimentation is greater than in other areas during upwelling events (172 mg Si m-2 d-1), as indicated by the relative abundance of Chaetoceros spp. spores.