Abstract

In the seasonally hypoxic semi-enclosed Omura Bay, Japan, regression equations estimating year-round nutrient [NO3 + NO2, NH4, PO4, Si(OH)4] concentrations in pore water were successfully constructed based on water temperature and the concentration of biogenic elements in surface sediments, except for NH4 and PO4 concentrations in summer when the hypoxic water mass develops. Core incubation studies revealed that pore water NH4 and PO4 concentrations under anaerobic conditions increased 1.4–2.2 and 1.8–2.1 times, respectively, above levels estimated with the regression equations. These were in good agreement with the values observed in the field during summer. The gradient in nutrient concentrations between the measured values in the overlying water column and the estimated values in the pore water revealed that spatial fluctuations in nutrient fluxes at the sediment–water interface across the Omura Bay ranged from 1.5 to 2 times, reaching up to 3–4 times in the summer. Annual nutrient fluxes from the sediment (690–780 ton-N year−1 for DIN, 77–85 ton-P year−1 for DIP, and c.a. 4700 ton-Si year−1 for DSi) were comparable to those from terrestrial areas. Accurate estimation of internal loadings is essential to properly evaluate residual nutrients and organic matter in the water column and their effect on aquatic ecosystems.

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