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

Savage, Candida, Peter R. Leavitt, Ragnar Elmgren

Limnol. Oceanogr., 49(5), 2004, 1503-1511 | DOI: 10.4319/lo.2004.49.5.1503

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¬ís 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.

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