Anthropogenic nitrogen (N) from sewage can be a major cause of coastal eutrophication, especially in areas of limited water exchange. This thesis examines the fate and effects of sewage N in a Baltic coastal ecosystem, the Himmerfjärden bay system.

The impact of sewage N was evident throughout the bay system, but most pronounced in the inner basins. Elevated nutrient levels stimulated primary production, which was recorded in surface sediments as a gradient of decreasing pigment concentration with distance from the outfall (Paper III). Similarly, elevated N content and del-15N values in macroalgae (Paper II) and surface sediments (Paper III) near the outfall indicated a greater influence of sewage N in the inner basins. Secondary effects of eutrophication were apparent in the benthos, with macrofaunal communities initially stimulated by moderate nutrient loading, but later declining as nutrient loads became excessive (Paper I). Sewage N was the variable that best explained change in macrobenthos, in particular in the vicinity of the outfall. Local topographic features such as sills and depressions were shown to enhance the impact of sewage loads on benthos.

Ecological responses to enhanced N removal in the sewage treatment plant were most clearly expressed in the inner basins. Comparison of macroalgal del-15N values before and after enhanced treatment indicated a clear lessening of sewage impacts, with macroalgal del-15N 2.5-6 per mil lower within 24 km of the outfall, and approximating background del-15N values within 12 km of the outfall after treatment (Paper II). In contrast, surface sediments were elevated in del-15N relative to outer coastal areas throughout the bay system (Paper III).

Coastal sediments can moderate marine eutrophication by removing N through denitrification and N burial. Calculations estimated that about 5-11 percent of annual sewage N loads were sequestered in sediments, presumably by uptake in phytoplankton followed by sedimentation, especially of diatoms in spring (Paper III). In contrast, long-term N retention in macroalgae was small, with short-term assimilation of at most 3 percent of total annual N loads, and little or no long-term accumulation (paper II).

Anthropogenic nutrients have had pervasive effects on the Himmerfjärden bay system, starting already in the 19th century, but with greatest impact in the last half-century (Paper IV). The loss of macrobenthos, resulting in the formation of laminated sediments, an increase in sedimentary del-13C and pigments, and a two-fold increase in sediment organic matter since the mid-1950s coincide with increased fertilizer use and population increase in the watershed after WWII. However, the most dramatic shifts in sedimentary del-15N (3 per mil enrichment in 15 yrs) and maxima in diatom abundance followed initiation of treated sewage discharge in 1974.

As human populations continue to move into urban centers and expand along coastal margins, coastal eutrophication will assume even greater importance on environmental agendas. Understanding the fate, retention and effects of anthropogenic nutrients in aquatic ecosystems should facilitate water quality management and mitigation efforts in coastal areas.