Nutrient dynamics in the Galician coastal area (Northwestern Iberian Peninsula): Do the Rias Bajas receive more nutrient salts than the Rias Altas?

Abstract

The Rias of Galicia are commonly divided into the Rias Altas and the Rias Bajas, the former being north, and the latter south of Cape Finisterre (NW Iberian Peninsula). The difference in their sediments and biological characteristics justifies a chemical study of the influence of nutrients in the Galician coastal zone. Data from 1977 to 1992, obtained from research cruises on the continental shelf and in the Rias are considered. The sources of nutrient salts, such as from land, remineralization, oceanic fluxes, and winter mixing have been taken into account. It is concluded that the Rias Bajas are richer in nutrient salts than the Rias Altas. This difference is not due to the river input since this is low and similar in both groups of Rias. Neither is it due to winter mixing, which is comparatively more beneficial to spring blooms in the Rias Altas than in the Rias Bajas. Instead, it is caused by summer processes, when nutrient salt remineralize and new nutrient salts become available in the euphotic zone, due to upwelling: these processes exert a far greater influence in the Rias Bajas than in the Rias Altas. Also the presence of depleted, nutrient-poor coastal waters in front of the Rias Altas impedes the penetration of upwelled seawater into these Rias, whereas south of Cape Finisterre, upwelled seawater can penetrate directly into the Rias Bajas. Hence the Rias Bajas receives more new nutrients in summer than the Rias Altas.