Abstract
We present evidence of denitrification on the continental slopes of the Ulleung Basin (UB) and the Eastern Japan Basin (EJB) near the Tatar Strait (TtS) in the East/Japan Sea (EJS), despite its high water column dissolved oxygen concentrations. Some nutrient concentration data deviate significantly from the fitted regression line of nitrate (N) vs. phosphate (P) in deep waters, indicating a loss of nitrate in the region. The EJS has a lower N/P ratio (ca. 12.4 below 300 dbar) than a traditional Redfield ratio (16). The N/P ratio and oxygen concentration are substantially lower at several locations whose depths are close to the sediment-water interface, near TtS (500-1100 dbar) and in UB (1100-2200 dbar). The decreased nitrate concentration is smaller than the expected nitrate level (a low N/P ratio of＜12.4), and a secondary nitrite peak near the bottom of these two regions: taken collectively, both indicate the presence of denitrification in the bottom layer. It is speculated that active re-mineralization and denitrification may occur simultaneously along the rich organic matter bottom layer on the slope environment. Denitrification rates are estimated at ~3-33 μmol N m^(-2) d^(-1). Current estimates do not support the previous idea of basin-wide denitrification in EJS, although the N/P ratio is low like in other hypoxic/anoxic seas. A better understanding of the denitrification process is necessary for predicting future changes of nitrogen cycle in the well-oxygenated EJS considering the decadal-scale physical and biogeochemical changes that have occurred.
Highlights
Denitrification is a microbiologically mediated process that reduces nitrate to nitrite and to nitrous oxide and results in the formation of dinitrogen ( NO "N2 O/N2 ) to decompose organic matters in the absence of oxygen
The nitrate to phosphate (N/P) ratio of East/Japan Sea (EJS) based on the 1999 data was estimated at 12.4 ± 0.1 (Fig. 3a), which is lower than the traditional Redfield ratio of 16
The low dissolved oxygen (DO) feature is well developed in the Eastern Japan Basin (EJB) near the Tatar Strait (TtS) between the 500 and 1100 dbar depth, and in Ulleung Basin (UB)
Summary
Denitrification is a microbiologically mediated process that reduces nitrate to nitrite and to nitrous oxide and results in the formation of dinitrogen ( NO "N2 O/N2 ) to decompose organic matters in the absence of oxygen. Denitrification is a microbiologically mediated process that reduces nitrate to nitrite and to nitrous oxide and results in the formation of dinitrogen Denitrification removes nitrate from the water deviating the nitrate to phosphate (N/P) ratio to below the traditional Redfield ratio of 16 (Hupe and Karstensen 2000). The global average N/P ratio of the ocean water column (14.5) lies below the traditional Redfield ratio (16), indicating a potential large-scale nitrogen removal via the denitrification process (Deutsch and Webber 2012). The EJS shows a low N/P ratio (9.8 to 14.7) compared to the traditional Redfield ratio (Table 1), but its driving mechanism is not yet clearly understood. Deep nitrite readings were observed near the bottom in the deep basins of EJS (Talley et al 2001; Lee et al 2007).
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