Effects of sewage-derived nutrients on an estuarine macrobenthic community

Abstract

Eutrophication is one of the foremost threats to coastal ecosystems today. Enhanced sewage treatment has often been implemented as a means of counteracting eutrophication by reduc- ing point source inputs of nutrients to estuaries. A 23 yr record (1972 to 1994) of macrobenthic com- munities in Himmerfjarden bay, Sweden, a coastal embayment of the Baltic proper, was evaluated in relation to nutrient discharges and physicochemical parameters. The study period started before the onset of sewage discharge and covers the introduction of improved (50%) nitrogen (N) removal from the local sewage treatment plant, which by 1994 served 240 000 people. Analysis of temporal vari- ability in species abundance and biomass showed that macrobenthos conformed to general models of organic enrichment. Common species (Macoma balthica, Monoporeia affinis) increased during ini- tial, moderate nutrient loading, but both density and biomass were reduced when nutrient inputs increased further, especially in the deeper basins (>35 m). Redundancy analysis (RDA) of both whole bay and basin-specific community changes identified sewage-derived N as the variable that best explained macrobenthic variability. The abundance of most taxa was negatively correlated to annual loads of sewage N. Comparison among inner, middle and outer reaches of the bay showed that the impacts of sewage N on species abundance declined with distance from the outfall. Overall, RDA using abundance data was more sensitive than biomass-based analyses and suggested that full com- munity recovery was not complete 5 yr after introduction of a N removal step in the sewage treat- ment. Finally, although sewage-derived nutrient impacts were evident even 20 km distant from the point source, comparisons among local basins showed that local physical features (e.g. sill depth) can greatly ameliorate or enhance impacts and highlighted the need for spatially explicit studies to detect human impacts.