Distribution and retention of effluent nitrogen in surface sediments of a coastal bay

Abstract

Anthropogenic nitrogen (N) often causes coastal eutrophication, yet little is known about the fate and retention of effluent N in coastal waters and, hence, about the system’s ability to assimilate excess N loads. We used the spatial distribution of stable N isotope ratios and algal pigments in sedimentary organic matter from a Baltic bay receiving tertiary‐treated effluent to evaluate the extent of effects and the role of nearshore marine environments as sinks of anthropogenic N. Surface sediments (0–2 cm and 2–4 cm) exhibited a pronounced spatial gradient of δ15N, with the most elevated values (~8‰) near the outfall; values decreased linearly to values of ;4‰ outside the bay. Sedimentary pigment concentrations were consistent with water‐column data and showed that phytoplankton biomass was elevated in the inner reaches of the bay. In particular, diatoms were heavily labeled (δ15N ~ 10‰), reached maximum abundance near the effluent outfall, and were likely the main mechanism delivering effluent N to the sediments. Sediments within the bay removed ~5–11% of wastewater N inputs, with 50% of the sequestered effluent N buried in the basin nearest to the outfall. Magnitudes of N removal by sediments (23–26 x 104 kg N yr−1) were less than those estimated for denitrification (30–60 x 104 kg N yr−1), but they were substantially greater than biological uptake by macroalgae (~2 x 104 kg N yr−1). Taken together, these patterns demonstrate the idea that coastal sediments can be effective sinks of wastewater N, even after 30 yr of effluent input.