Hypoxia formation triggered by the organic matter from subsurface chlorophyll maximum in a large estuary-shelf system.

This study reports, for the first time, the role of shoreward transport of organic matter (OM) from subsurface chlorophyll maximum (SCM) in triggering hypoxia off the Pearl River Estuary (PRE, an outstanding example of typical estuary-shelf systems) based on field measurements. Compared to frequently observed hypoxia driven by surface eutrophication and terrestrial OM during large river discharge, we demonstrate that the upslope-transported SCM played a critical role in forming offshore hypoxia during low river discharge. Together with the plume-sourced OM trapped below the surface plume front, upslope-transported OM originating from the SCM accumulated underneath the pycnocline and consumed dissolved oxygen (DO), enhancing the bottom hypoxia. The DO consumption induced by the SCM-associated OM was estimated to contribute ∼ 26% (±23%) of the DO depletion under the pycnocline. Based on coherent and consistent physical and biogeochemical evidence and reasoning, this study reveals the contribution of SCM to bottom hypoxia off the PRE, which is unreported and likely occurs in other coastal hypoxic systems.

Evaluation of the total maximum allocated load of dissolved inorganic nitrogen using a watershed-coastal ocean coupled model.

Due to the recent rapid increase in human activity and economic development, many coastal areas have recently experienced a high degree of land-based pollution. Evaluating the total maximum allocated load (TMAL) of dissolved inorganic nitrogen (DIN) nutrients and the remaining capacity is of importance for improving water quality. A considerable amount of nutrients derived from the coastal watershed can be found in wet seasons, which is non-negligible for the estimation of remaining capacity. Therefore, we use a watershed-coastal ocean coupled model combined with an optimization algorithm to tackle this issue. In contrast with previous studies, this study provides a method to estimate the spatiotemporal variations in TMALs and we then compare it to the current DIN nutrient load, including both point sources and non-point sources. Our results suggest that the TMAL of Daya Bay (DB), which is located in the northern part of the South China Sea, is about 7976 metric tons per year (t/yr) and ranges from 191 metric tons per month (t/month) to 1072 t/month. The increase of non-point source (NPS) DIN input also plays an important role in daily overload events during wet seasons. Moreover, the TMALs show an inverse exponential correlation with the water age, but only about 65% of the variance is explained. This suggests that the variations from the optimization algorithm and from local water function zoning plans are also important. According to our prediction of the DIN input, the TMAL of DB will soon be exhausted in the next several years. Consequently, prompt actions are necessary to consider the distribution of TMALs in urban developments and to decelerate the rapid growth of DIN input. Therefore, the results of this study will be helpful for both local pollution control and future urban planning.