Comparison of heterotrophic bacterial production rates in early spring in the turbid estuaries of the Scheldt and the Elbe

Abstract

In spring bacterial production rates were estimated by tritiated thymidine incorporation in the turbid estuaries of the rivers Scheldt and Elbe. Bacterial production rates in the Scheldt were 5 times higher than in the Elbe. In the Scheldt bacterial production rates correlated better with the DOC concentration than in the Elbe. Organic matter concentrations in the marine part of the estuaries were the same while in the brackish part concentrations in the Scheldt were much more higher. In the Scheldt, but not in the Elbe, oxygen depletion occurred in the maximum turbidity zone caused by bacterial growth and respiration. The water in the Scheldt was well-mixed while in the turbidity maximum of the Elbe salinity and bacterial production was higher near the bottom than at the surface. Nutrient concentrations in the Scheldt were higher than in the Elbe. Bacterial production rate values in the Scheldt are among the highest reported in the literature. The relatively high bacterial production rates in both estuaries are caused by a high load of waste water. Comparison of bacterial growth rates and water residence time suggests an intensive grazing by probably protozoa. Production rates showed a tidal dynamic. In the Elbe high current velocities caused resuspension of sediment and increased bacterial production rates near the bottom. The high production rates in the turbidity maximum and freshwater part of both estuaries show that a large amount of organic matter is degraded in this region.