Nutrient distribution in sediments from two estuarine environments: The Schelde estuary (Belgium, The Netherlands) and the Fraser estuary (Canada)

The nutrients (nitrite, nitrate, ammonia) and organic matter distributions in the sediments of two temperate estuaries: the Schelde Estuary (Belgium, The Netherlands) and the Fraser Estuary (British Columbia, Canada) were investigated. Three representative stations varying in salinity, and representing areas of continuous sedimentation with minimal erosion were selected. Samples were taken during periods of high and low river discharge. Organic matter content and concentrations of each DIN form in the Schelde Estuary were very high, with up to 14% organic matter and a range from 157 µM (in overlying water) to 5.904 µM (in sediment) for DIN, comparThe nutrient (nitrite, nitrate, ammonia) and organic matter distributions in the sediments of two temperate estuaries: the Schelde Estuary (Belgium, the Netherlands) and the Fraser Estuary (British Columbia, Canada) ed with those in the Fraser Estuary, where the organic matter content was up to 4% and DIN ranged from 4 µM (in overlying water) to 712 µM (in sediment). NH4 was the dominant form of nitrogen in the estuarine sediments examined as well as in the overlying water in the Fraser Estuary. In the Schelde Estuary the nitrogen dynamics in the estuarine water was dominated by NO3. In general, nutrient concentrations were salinity dependent, a strong seaward decrease in nutrient concentration was observed in the oligohaline part of the estuary. Further seaward, the increase in salinity caused little change in nutrient concentration. The organic matter distribution in the sediment varied with sediment composition (increased with lutum content) and did not vary with river discharge. The distribution of nitrite, nitrate and ammonia up to a 10 cm depth in the sediment column of both estuaries showed a very comparable pattern. Often a maximum value occurred at the uppermost 3 cm of the sediment layer. This study shows that most nutrients are delivered during periods of high runoff. In both estuaries nutrient transport increased with increasing river discharge. The ratio of the nutrient concentration in the sediment to its overlying water and the observed changes in concentration between low and high runoff showed than during the high runoff period, an increase in nutrient concentration occurred in the Fraser estuarine sediments, while an release of nutrient from sediments was more important in the Schelde. Thus sediments are a sink for nutrients but at the same time also act as a nutrient source for the overlying water. In the Schelde Estuary sediment resuspension led to nutrient redistribution followe by a seaward transport. While in the Fraser estuary a higher sedimentation rate resulted in a decrease of nutrient concentration in the overlying water.