Individual study: Response of esturine benthic mudflat taxa to reduced organic waste discharge in the Ems Estuary, Groningen, the Netherlands
Essink K. (2003) Response of an estuarine ecosystem to reduced organic waste discharge. Aquatic Ecology, 37, 65-76
Since the 1950s in northeast Netherlands, large quantities of organic waste from the potato-flour and cardboard industries was discharged into canals and small rivers making them anoxic. Due to the seasonality of potato processing, waste input was highest in autumn. The anoxic water entered the Dollard, a 90 kmÂ² brackish embayment in the Ems Estuary. A major sanitation scheme to reduce organic waste loading commenced in the 1970s. Summarised here are the responses to the reduced waste entering the estuary of selected benthos taxa living in the intertidal mudflats.
During survey during1973-1982, a zone of impoverished benthic fauna was evident on the intertidal mudflats in the southeast Dollard, due to low oxygen levels, especially in autumn. In the late 1970s, the National Institute for Coastal and Marine Management initiated a study to record the response of the benthos (microphytobenthos (diatoms), meiofauna (nematodes) and macrozoobenthos) to reduced organic waste loading.
Sampling: In 1976-1980 benthic diatoms were sampled monthly at three intertidal stations (0.8, 1.2 and 3.2 km from the mouth of the River Westerwoldsche Aa). In 1987 and 1993, small core samples were taken monthly at the same stations to record nematodes.
Macrozoobenthos was sampled (1978-1982 and 1985) in the southeast Dollard from August to January along transects using a corer at high tide from an inflatable boat. The central part of the Dollard had was monitored each year from 1977 along three transects sampled each February to April and August to September.
Reduction of organic loading over the years (high around 1980, intermediate by 1987 and much reduced by 1993) resulted in significant positive changes in abundance, species composition and standing stock of diatoms and nematodes. There was also a recovery of the macrobenthos from being severely reduced most autumns towards a more stable situation. Overall, the intertidal mudflat benthos recovered towards one of a normal estuarine environment. The changes are considered primarily attributable to a decrease of free sulphide, ammonia and oxygen depletion in water and sediments.
The unintended introduction (around the early 1980s) and subsequent invasion by the North American polychaete Marenzelleria cf. wireni brought about changes in the macrobenthic community and responses of native species may have been obscured by this invasion (e.g. the native polychaete Nereis diversicolor appeared reduced by the presence of high densities of Marenzelleria).
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