Response of an estuarine ecosystem to reduced organic waste discharge

Since the mid-19(th) century large amounts of organic waste were sluiced out into the Ems Estuary on the border between Germany and The Netherlands. This waste originated from the regional potato flour and cardboard industries making the inland waterways completely anoxic. In the estuary serious oxygen depletion occurred, especially during autumn. Most of the organic waste entered the estuary in the Dollard, a brackish embayment of the Ems Estuary. An intensive sanitation scheme was started in the 1970s, leading to a stepwise reduction of the organic waste load on the estuary. In this paper, a review is given of the response of the benthos living at intertidal mudflats, viz. microphytobenthos (diatoms), meiofauna (nematodes) and macrozoobenthos. The benthos response is described mainly on the basis of data obtained under conditions of high (ca. 1980), intermediate (1987) and largely reduced (1993) organic waste loading. Reduction of organic loading caused significant changes in abundance, species composition and standing stock of diatoms and nematodes. Macrobenthic populations recovered from being severely reduced regularly during the autumnal waste discharges towards a more stable situation. Altogether, the intertidal mudflat benthos changed from organic waste loading stress towards a normal, estuarine environmental stress. This was especially the case at high and muddy flats in the southeast of the Dollard. At mixed sand/mud flats in the centre of the Dollard, a response of macrozoobenthos may have been obscured by the invasion of the polychaete Marenzelleria cf. wireni.