Premium
Quantifying the Relative Importance of Riverine and Open‐Ocean Nitrogen Sources for Hypoxia Formation in the Northern Gulf of Mexico
Author(s) -
Große Fabian,
Fennel Katja,
Laurent Arnaud
Publication year - 2019
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1029/2019jc015230
Subject(s) - hypoxia (environmental) , nutrient , environmental science , sink (geography) , oceanography , sediment , hydrology (agriculture) , discharge , nitrogen , pelagic zone , submarine pipeline , geology , drainage basin , oxygen , ecology , geography , chemistry , biology , paleontology , cartography , geotechnical engineering , organic chemistry
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/m 3 ), extending over an area of roughly 15,000 km 2 , 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.