Organic matter in ria sediments: Relevance of terrestial sources and temporal variations in rates of accumulation

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 g_POC 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 g_POC 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.