Ocean biogeochemistry in the northern Gulf of Mexico, the East/Japan Sea, and the South Pacific with a focus on denitrification
Ocean nitrogen fixation and denitrification are crucial nitrogen source and sink mechanisms for the global ocean environment. While recent studies have reported that oceanic denitrification has increased over the last few decades, others have suggested that global ocean nitrogen fixation rates have been underestimated, and still others that anthropogenic perturbations have altered the global nitrogen cycle. This implies that the current estimates of the oceanic nitrogen inventory are incomplete and they need to be revised with more information. In addition, current denitrification estimates need to be reexamined due to their large associated uncertainties. Thus, I have conducted research estimating denitrification rates in three different locations: the northern Gulf of Mexico (GOM), the East/Japan Sea (EJS), and the South Pacific: from coastal to marginal to open ocean scale in different oceanographic conditions. Denitrification rates in the bottom layer (including bottom waters+sediments) at the shallow and often hypoxic northern GOM ranged from 103-544 [mu]mol N m⁻² d⁻¹ (=1.4 to 7.4 Gg N mon⁻¹ with area=3.24x10¹⁰m²), and were controlled not only by biogeochemical factors (i.e. organic matter supply and remineralization), but also by physical factors (i.e. stratification and relative contributions from different water masses). Despite high dissolved oxygen concentrations, the significant decrease in nitrate concentrations below the expected levels, low N/P ratio (<12.4), and deep nitrite peak in the bottom layer indicate a presence of denitrification in EJS, confined at the Tatar Strait and the Ulleung Basin areas. The estimated denitrification rates range from 0.3 to 33.2 [mu]mol N m⁻² d⁻¹, and was comparable to the directly measured denitrification rates from sediment samples. The high-quality repeat hydrographic datasets observed at 32°S of the South Pacific Ocean offer an opportunity to estimate water column denitrification rates on a basin-scale in the open ocean away from the Eastern Tropical Pacific oxygen minimum zones. The mean water column denitrification rates in the oxygen minimum layer of P06 line (32°S) were estimated to range between 7.1 and 18.5 [mu]mol N m⁻² d⁻¹. The results imply that, although very small at any particular site, once integrated over a basin-scale, the open ocean water column denitrification can be a significant component of the oceanic nitrogen budget. Denitrification is subject to seasonal, decadal and possibly climate scale variations. While it is commonly estimated at the oxygen minimum zones or sediments, denitrification is not merely confined to such regions only, and small amounts of denitrification occur in other oceanic parts. Once integrated, it may be quantitatively significant for the world's oceans. Denitrification is playing a significant role in local, regional, and global ocean scales. In the future, we need to consider variability of denitrification in coastal regions, and to investigate denitrification in unexpected and unexplored regions, in order to improve our knowledge on global oceanic mass balance.