Miño River dams discharge on neighbor Galician Rias Baixas (NW Iberian Peninsula): Hydrological, chemical and biological changes in water column

An unusual intrusion of low-salinity water coming from the Miño River (40 km south of the Ria of Pontevedra) was observed in the Ria of Pontevedra (NW Spain) on 12–13 May, 1998. The abnormally high discharge was due to the sudden release of water from the dams that control the Miño River run-off. This release results in a strong cross-axis salinity gradient: salinity decreases seawards, giving rise to a two-layered circulation pattern, with water near the riverbed moving seawards and water near the surface moving landwards. In addition, a clear gradient of chemical and biological variables was observed in the ria, with decreasing values being evident towards the middle zone. The nutrient salts, the oxygen, the chlorophyll and the phytoplankton patterns suggest the existence of a bloom that penetrates the ria, embedded in a water mass that is fresher than the estuarine one. The high concentration of the phytoplankton Skeletonema costatum was observed as late as May, which also supports the idea that this bloom exhibits the features of a typical early spring bloom in the Rias Baixas and that this is linked to haline stratification that is caused by an important supply of continental water.