Estimation of inward nutrient flux from offshore into semi-enclosed sea (Tokyo Bay, Japan) based on in-situ data.

In previous studies, to address the problem caused by eutrophication and oligotrophication in coastal waters, nutrient cycles have been estimated using bio-geophysical models. These studies strictly evaluated land-derived nutrient loading but not the nutrient loading from offshore. This study demonstrated the quantitation of the offshore-derived nutrient-loading based on in-situ data not based on a bio-geochemical model in the semi-enclosed Tokyo Bay, Japan affected by oligotrophication and discusses about the contribution of a decadal change in the inward nutrient flux to oligotrophication. The inward water flux from offshore was calculated using Ferry-mounted current profiler data. The dissolved inorganic nutrient concentrations of the inward water were estimated using the in-situ sea temperature following the negative correlation relationship between the sea temperature and nutrient concentration. Estimated inward nutrient flux from the Pacific Ocean peaks in March (spring) and the amount of the inward nitrogen has the same order of magnitude as the land-derived input. Notable variability of the inward nutrient flux was confirmed during 2010–2019. The significant decadal change of the inward water mass flux was not confirmed because of the lack of significant decadal changes in the in-situ controlling factors identified by the hydrodynamic model. The decadal decreasing trend in the total nitrogen of inward water suggests a decrease in the inward nutrient flux which contributes to the oligotrophication in Tokyo Bay. • Inward nutrient flux from offshore was estimated by in-situ data. • Maximum inward nutrient flux is the same order as land-derived input in Tokyo Bay. • Notable variability of inward nutrient fluxes was confirmed. • Decadal decrease of offshore-derived nutrient contributes to oligotrophication. [ABSTRACT FROM AUTHOR]