The Ria of Vigo, one of the classical rias of SW Europe, is an environment of high production of organic matter naturally induced by the Galician upwelling. The organic matter is partly supplied by small rivers but mainly by sewage plants along the ria shoreline; jointly they contribute 725 t y−1 of POC, of which 72% is of anthropogenic origin. The freshwater flux is equivalent to a supply of 5 g m−2 yr−1 of allochtonous POC to the ria floor. However the rate of accumulation of POC is dominated by the order of magnitude higher supply of autochthonous material from the net primary production. The present accumulation rate of organic matter (49–58 gPOC m−2 yr−1) is lower than the average supply, estimated from the sedimentary record, to the ria since the middle of the nineteen Century (>60 gPOC m−2 yr−1). This difference may be due to anthropogenic activity or changes in the upwelling pattern. The composition of the organic matter in the sediment reflects the relative importance of the various sources (terrestrial-marine). While terrestrial woody materials dominate the inner ria, phytoplankton remains dominate the remainder of the ria. Rock-Evaluation analysis indicates the inner ria is the site of deposition of gas-prone material and it is inferred that the outer ria of oil-prone organic matter. The controls on the accumulation of POC in the rias show many differences to those found in estuaries affected by anthropogenic activities e.g. agriculture and increasing human population.

The writers would like to record their gratitude to members of the former Department of ‘Recursos Naturales y Medio Ambiente (Univ. of Vigo)’ for helping in collecting the samples with the support of CICYT projects NAT 89-1075 and AMB 93-0300; to METRIA project participants and, particularly, to Dr. Santos-Echeandía and Mr. Otxotorena, for their help with the sampling; to Dr. Z. Ali and Mr. M.A. Ahmad of the former Department of Geology (Imperial College of London) for the carbon and nitrogen analyses; to Dr Ettienn Drosse (Institut Français du Pétrole) for the rock-evaluation values; to Mr. Portela for the gauging station measurements in the Oitavén River and to Dr. Alvarez Eijo (USC) for the gauging station calibration and river flow quantification; to Ms. Rodríguez-Riveiros for her technical assistance; and to the sewage plant companies ‘Pridesa’, ‘Aqualia’, ‘Espina y Delfín’ and ‘Fergó Galicia’ for their kindness in cooperating in the sampling and providing sewage flow and population equivalent data. This work is a contribution to the Spanish LOICZ program and was partially supported by CICYT under the project ‘Biogeochemical budget and 3D transport model of trace metals in a Galician Ria’ (reference REN2003-04106-C03).

Peer reviewed