Nutrient fluxes from the Ebro River and subsequent across-shelf dispersion

Coastal areas receive significant amounts of nutrients (nitrogen, phosphorus, silicon) mostly from land-based sources, which contribute to increased biological productivity often exceeding that naturally found in coastal and marine environments. Most coastal zones of southern Europe are naturally oligotrophic with relatively small freshwater discharges. Studies of circulation and mixing processes over the continental shelf off the Ebro River delta are motivated by the need to understand transport pathways of natural and anthropogenic discharges affecting the adjacent marine environment. This paper investigates the characteristics of the last 45 km of the Ebro River in NE Spain, in terms of physiographic conditions, river discharges and hydrochemical and biological environment at three different periods in the annual cycle. In spite of the high variability in water flow and ecological conditions, the study provides a reasonably good estimate of the overall amounts of nutrients discharged to the coastal environment. The largest nutrient load corresponds to nitrogen of which more than 10(4) tm yr(-1) are discharged. Nitrogen regeneration took place in the lower river waters during fall and spring and nitrogen uptake prevailed at all stations in summer. Phosphorus annual load of just 87 tm yr(-1) does not contribute in a significant way to the fertilization of the coastal zone unlike other rivers draining highly developed watersheds. This is due to the trapping effect of the dams existing in the middle course of the river. The sudden phosphorus contribution due to the wash out of the salt-water wedge whenever the river flow increases to pre-flood conditions (>400 m(3) s(-1)) may be significant (similar to1.5 tm or 2% of the overall P load). The area coverage to which the Ebro River would add a surplus primary production of about 50 g C m(-2) d(-1) (over a background of about 100 g C m(-2) d(-1)), may be estimated in terms of nitrogen, at about 1200 km(2) (40 km x 30 km). If, however, the figures were based on phosphorus, the limiting nutrient in the freshwater system, then this increase in 2 fertility would only affect an area of about 68 km(2). (C) 2002 Elsevier Science Ltd. All rights reserved.