Ensemble modeling informs hypoxia management in the northern Gulf of Mexico

Abstract

A large region of low-dissolved-oxygen bottom waters (hypoxia) forms nearly every summer in the northern Gulf of Mexico because of nutrient inputs from the Mississippi River Basin and water column stratification. Policymakers developed goals to reduce the area of hypoxic extent because of its ecological, economic, and commercial fisheries impacts. However, the goals remain elusive after 30 y of research and monitoring and 15 y of goal-setting and assessment because there has been little change in river nitrogen concentrations. An intergovernmental Task Force recently extended to 2035 the deadline for achieving the goal of a 5,000-km 2 5-y average hypoxic zone and set an interim load target of a 20% reduction of the spring nitrogen loading from the Mississippi River by 2025 as part of their adaptive management process. The Task Force has asked modelers to reassess the loading reduction required to achieve the 2035 goal and to determine the effect of the 20% interim load reduction. Here, we address both questions using a probabilistic ensemble of four substantially different hypoxia models. Our results indicate that, under typical weather conditions, a 59% reduction in Mississippi River nitrogen load is required to reduce hypoxic area to 5,000 km 2 . The interim goal of a 20% load reduction is expected to produce an 18% reduction in hypoxic area over the long term. However, due to substantial interannual variability, a 25% load reduction is required before there is 95% certainty of observing any hypoxic area reduction between consecutive 5-y assessment periods.ensemble modeling | hypoxia | Gulf of Mexico | nitrogen-loading targets A large region of low-dissolved-oxygen (DO) bottom waters (hypoxia; DO < 2 mg·L −1 ) has formed nearly every summer in the Gulf of Mexico for at least the last three decades (1-3). Hypoxia forms because of respiration of organic matter in bottom waters and a vertically stratified water column restricting reaeration (4-6). Both factors are related to the outflow of freshwater and nutrients from the Mississippi River Basin, which typically peaks in March through May. Nutrients stimulate phytoplankton production, much of which settles in early summer, and bottom water DO (BWDO) is consumed during its decomposition. River outflows create a fresher, warmer surface layer above a colder, saltier bottom layer, limiting the vertical diffusion of DO (2).Policymakers developed hypoxia reduction goals because of ecological, economic, and commercial fisheries impacts (7-10). However, the goals remain elusive after >30 y of research and monitoring (3, 11) and >15 y of assessment and goal-setting (5, 12-16). A Gulf Task Force recently agreed to retain the 5-y moving average goal of a 5,000-km 2 hypoxic zone, but extended the deadline from 2015 to 2035 (17). The timeframe was reset because the 2015 hypoxic zone was 16,760 km 2 and the most recent 5-y average was 14,024 km 2 -greater than the long-term average of 13,751 km 2 (1). Missing the goal was not unexpected because the 5-y average load of late sp...

N and P as ultimate and proximate limiting nutrients in the northern Gulf of Mexico: Implications for hypoxia reduction strategies

“…Our TN reduction estimate is very close to 5 those of Scavia et al (2013Scavia et al ( , 2017. Our model suggests that a proportional reduction of P loads would reduce the necessary load reduction by 15% (from 63% to 48%) about twice the 8% reduction (from 50% to 42%) that Greene et al (2009) predicted with their model 11.…”

“…In the recent estimates by Scavia et al (2017), three of the four different models (UM, NCSU and LSU) are remarkably consistent; however, all assume reductions to different pools of nitrogen load.…”

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

“…An increase in spatial extent and severity of hypoxia is predicted to occur in the future due to reduced O 2 solubility and increased stratification in a warmer climate (Laurent et al, ; Lehrter et al, ). Modeling studies on the effect of riverine N load reductions on hypoxia suggest that a reduction on the order of 60% would be sufficient to achieve the 5,000 km 2 management target (Fennel & Laurent, ) in agreement with results from statistical models (Scavia et al, ).…”

“…The 20% reduction corresponds to the interim target defined by the Task Force (), whereas the 60% reduction is the estimated reduction necessary to reach the 5,000 km 2 target for the hypoxic zone (Fennel & Laurent, ; Scavia et al, ). In both cases, river forcing is as in the control simulation, except that the riverine concentrations of the N‐related state variables are multiplied by 0.8 or 0.4.…”

“…Significant progress has been made in quantifying the relative importance of different factors contributing to hypoxia formation and its temporal variability using a range of modeling approaches from purely statistical to high‐resolution mechanistic models. Statistical models have linked the spatial extent of hypoxia to the riverine nutrient loads and have been used to predict hypoxic spatial extent (Scavia et al, and references therein). These models show that riverine May N load is a predictor of year‐to‐year variability in hypoxic area; however, because FW discharge and nutrient loads are highly correlated (Obenour et al, ) these analyses cannot differentiate between the effects of FW on stratification and nutrient inputs on O 2 consumption.…”