Distribution and Retention of Effluent Nitrogen in Surface Sediments of a Coastal Bay

• Published in: Limnology and oceanography 2004-09, Vol.49 (5), p.1503-1511
• Main Authors: Savage, Candida, Leavitt, Peter R., Elmgren, Ragnar
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Bacillariophyceae; Biological and medical sciences; Cyanobacteria; Diatoms; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Estuaries; Eutrophication; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; Isotopes; Marine; Marine and continental quaternary; Nitrogen; Phytoplankton; Pigments; Pollution, environment geology; Sea water ecosystems; Sediments; Sewage effluent; Surficial geology; Synecology
• ISSN: 0024-3590; 1939-5590
• DOI: 10.4319/lo.2004.49.5.1503

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 δ 15N, 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 (δ 15N ∼ 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 $(23-26\times 10^{4}\ \text{kg}\ \text{N}\ \text{yr}^{-1})$ were less than those estimated for denitrification $(30-60\times 10^{4}\ \text{kg}\ \text{N}\ \text{yr}^{-1})$, but they were substantially greater than biological uptake by macroalgae $(\sim 2\times 10^{4}\ \text{Kg}\ \text{N}\ \text{yr}^{-1})$. Taken together, these patterns demonstrate the idea that coastal sediments can be effective sinks of wastewater N, even after 30 yr of effluent input.