Katja Fennel, Dalhousie University
Numerical simulation of hypoxia in the northern Gulf of Mexico and implications for Mississippi River nutrient management
A large hypoxic area (15,000 km2 on average) forms every summer over the Texas-Louisiana shelf in the northern Gulf of Mexico due to decay of organic matter that is primarily derived from nutrient inputs from the Mississippi/Atchafalaya River System. Efforts are underway to reduce the extent of hypoxic conditions through nutrient management in the watershed, but open questions are how far nutrient loads would have to be decreased in order to produce the desired reductions in hypoxia and what role physical processes play in determining hypoxia and interannual and short-term variability. Several coupled circulation-hypoxia models are under development to address these questions and improve mechanistic understanding of hypoxia generation in this region. In my presentation I will first report results of an intercomparison of these hypoxia models, which is being undertaken within the NOAA-funded Coastal & Ocean Modeling Testbed (COMT) and has, up to now, focused on the effects of model physics. Four circulation models are included: two implementations of the Regional Ocean Modeling System (ROMS), one implementation of the Finite Volume Coastal Ocean model (FVCOM), and one implementation of the U.S. Navy’s coastal ocean model (NCOM). In order to elucidate the effects of model physics, all circulation models were run with the same, highly simplified hypoxia model, which parameterizes oxygen sinks in water column and sediment, and includes air-sea gas exchange. Then, I will present results from a large number of multi-year nutrient load reduction scenarios with our biogeochemical ROMS model, which explicitly includes nitrogen (N) and phosphorus (P) species as inorganic nutrients, and has been shown to realistically reproduce the key processes responsible for hypoxia generation. I will discuss the effects of differential reductions in river N and P loads on hypoxic extent. An assessment of the effects of N versus P reductions is important because, thus far, nutrient management efforts have focused on N, yet P is known to limit primary production in spring and early summer. A debate is ongoing as to whether targets for P reductions should be set and whether nutrient reduction efforts should focus solely on P, which results primarily from urban and industrial point sources and is uncoupled from agricultural fertilizer application.