Quantifying the Relative Importance of Riverine and Open‐Ocean Nitrogen Sources for Hypoxia Formation in the Northern Gulf of Mexico

Abstract

The Mississippi and Atchafalaya River System discharges large amounts of freshwater and nutrients into the northern Gulf of Mexico (NGoM). These lead to increased stratification and elevate primary production in the outflow region. Consequently, hypoxia (oxygen <62.5 mmol/m3), extending over an area of roughly 15,000 km2, forms every summer in bottom waters. High‐resolution models have significantly improved our understanding of the processes controlling hypoxia formation in the NGoM and have strongly implicated riverine nutrients as the dominant nutrient source. However, the relative importance of different nutrient sources (i.e., the Mississippi and Atchafalaya Rivers and offshore) has not been assessed before now. Here, we combine a high‐resolution model with an element tracing method to directly quantify the relative contributions of nitrogen from the two rivers and the open ocean to primary production and sediment oxygen consumption, which is the main oxygen sink contributing to hypoxia in the NGoM. Our results indicate that, averaged over 2001–2011, Mississippi and Atchafalaya nitrogen support 51 ± 9% and 33 ± 9% of summer sediment oxygen consumption, respectively, while open‐ocean nitrogen supports 16 ± 2%. The higher relative impact of Mississippi inputs results from longer transit times compared to those of Atchafalaya inputs. We also analyze the effect of riverine nitrogen load reductions and a larger diversion of discharge to the Atchafalaya River. These scenario simulations show that nutrient load reductions are most effective in mitigating hypoxia.

Statistical Analysis of Nutrient Loads from the Mississippi-Atchafalaya River Basin (MARB) to the Gulf of Mexico

“…The seasonal cycle that mostly reflects freshwater discharge of NO2 − + NO3 − loads in Mississippi river is summer. Highest loads of nitrogen usually occur in May/June, while loads are lowest in September/October [39].…”

Quantifying the contributions of riverine vs. oceanic nitrogen to hypoxia in the East China Sea

“…Passive dye tracers were used to track FW inputs from the Changjiang River similar to previous ROMS applications (Große et al, 2019;Hetland and Zhang, 2017;Rutherford and Fennel, 2018;Zhang et al, 2010Zhang et al, , 2012. The rate of change of the 90 concentration of a passive dye tracer from the i-th source (C i p ) is described as:…”

“…(2) is applied using the post-processing software of Große et al (2017), which has been adapted for ROMS (Große et al, 2019). N tracing is only applied in the region without NO − 3 nudging (see 'tracing region' in Fig.…”

“…However, we further found that a 50% reduction of the Changjiang River N loads more than compensates for this increase in hypoxia. In the following, we compare the hypoxic zones of the ECS and the NGoM with respect to their sizes and timing, based on observations, and the main sources of O 2 consumption, based on the results of this study and an analogous nutrient tracing study for the NGoM (Große et al, 2019). The key messages of this comparison are summarized in Table 2.…”

“…By combining a high-resolution biogeochemical model with this active element tracing method expanded for the quantification of the contributions to O 2 processes (Große et al, 2017(Große et al, , 2019, we provide such an analysis here. We apply the element tracing method to an implementation of the Regional Ocean Modeling System (ROMS; Fennel et al, 2006;Haidvogel et al, 45 2008) configured for the ECS (Zhang et al, same issue).…”