Key biogeochemical processes evaluated by the stable nitrogen isotopes of dissolved inorganic nitrogen in the Yodo River estuary, Japan: significance of estuarine nutrient recycling as a possible source for coastal production

Abstract

Isotope analysis of dissolved inorganic nitrogen (DIN) and a mixing model along the salinity gradient were used to clarify the dominant biogeochemical processes controlling nutrient dynamics within a shallow eutrophic estuary in Japan. Although delivery of riverine DIN into the estuary is largely dominated by mixing with seawater during most months, internal biogeochemical processes of DIN assimilation by phytoplankton and DIN production by remineralization and subsequent nitrification within the estuary offer the most reasonable explanation for observed deviations in the isotopic compositions of $$ {\text{NO}}_{3}^{ - } $$ from mixing behavior. However, the balance of each process changed over time. During phytoplankton blooming in summer, co-occurrence of $$ {\text{NO}}_{3}^{ - } $$ assimilation and $$ {\text{NO}}_{3}^{ - } $$ regeneration inhibited the accumulation of $$ {\text{NO}}_{3}^{ - } $$ within the estuary. Moreover, assimilation of $$ {\text{NH}}_{4}^{ + } $$ as well as $$ {\text{NO}}_{3}^{ - } $$ by phytoplankton complicates the nutrient dynamics within the estuary. However, mostly conservative or productive behavior of DIN as well as $$ {\text{PO}}_{4}^{3 - } $$ showed that recycled nutrients are significant net source within the estuary. These results suggest recycled nutrients within the estuary could have a non-negligible impact on eutrophication in Osaka Bay.