Arnaud Laurent

A modeling study of physical controls on hypoxia generation in the northern Gulf of Mexico

The Louisiana shelf (LA shelf) in the northern Gulf of Mexico experiences hypoxic conditions every summer due to the combination of eutrophication and strong water column stratification. Here we use a three-dimensional circulation model coupled with a simple oxygen model to examine the physical controls on hypoxia generation on the LA shelf. The model assumes a constant oxygen utilization rate in the water column and a sediment oxygen consumption rate that depends on the bottom water oxygen concentration and temperature. Despite its simplicity, the model reproduces the observed variability of dissolved oxygen and hypoxia on the LA shelf, highlighting the importance of physical processes. Model results demonstrate that both river discharge and wind forcing have a strong effect on the distribution of the river plume and stratification, and thereby on bottom dissolved oxygen concentrations and hypoxia formation on the LA shelf. The seasonal cycle of hypoxia is relatively insensitive to the seasonal variability in river discharge, but the time-integrated hypoxic area is very sensitive to the overall magnitude of river discharge. Changes in wind speed have the greatest effect on the simulated seasonal cycle of hypoxia and hypoxic duration, while changes in wind direction strongly influence the geographic distribution of hypoxia. Given that our simple oxygen model essentially reproduces the evolution of hypoxia simulated with a full biogeochemical model and that physical processes largely determine the magnitude and distribution of hypoxia, a full biogeochemical model might not be necessary for short-term hypoxia forecasting.

Effects of model physics on hypoxia simulations for the northern Gulf of Mexico: A model intercomparison

A large hypoxic zone forms every summer on the Texas-Louisiana Shelf in the northern Gulf of Mexico due to nutrient and freshwater inputs from the Mississippi/Atchafalaya River System. Efforts are underway to reduce the extent of hypoxic conditions through reductions in river nutrient inputs, but the response of hypoxia to such nutrient load reductions is difficult to predict because biological responses are confounded by variability in physical processes. The objective of this study is to identify the major physical model aspects that matter for hypoxia simulation and prediction. In order to do so, we compare three different circulation models (ROMS, FVCOM, and NCOM) implemented for the northern Gulf of Mexico, all coupled to the same simple oxygen model, with observations and against each other. By using a highly simplified oxygen model, we eliminate the potentially confounding effects of a full biogeochemical model and can isolate the effects of physical features. In a systematic assessment, we found that (1) model-to-model differences in bottom water temperatures result in differences in simulated hypoxia because temperature influences the uptake rate of oxygen by the sediments (an important oxygen sink in this system), (2) vertical stratification does not explain model-to-model differences in hypoxic conditions in a straightforward way, and (3) the thickness of the bottom boundary layer, which sets the thickness of the hypoxic layer in all three models, is key to determining the likelihood of a model to generate hypoxic conditions. These results imply that hypoxic area, the commonly used metric in the northern Gulf which ignores hypoxic layer thickness, is insufficient for assessing a models ability to accurately simulate hypoxia, and that hypoxic volume needs to be considered as well.

Eutrophication‐induced acidification of coastal waters in the northern Gulf of Mexico: Insights into origin and processes from a coupled physical‐biogeochemical model

Nutrient inputs from the Mississippi/Atchafalaya River system into the northern Gulf of Mexico promote high phytoplankton production and lead to high respiration rates. Respiration coupled with water column stratification results in seasonal summer hypoxia in bottom waters on the shelf. In addition to consuming oxygen, respiration produces carbon dioxide (CO2), thus lowering the pH and acidifying bottom waters. Here we present a high-resolution biogeochemical model simulating this eutrophication-driven acidification and investigate the dominant underlying processes. The model shows the recurring development of an extended area of acidified bottom waters in summer on the northern Gulf of Mexico shelf that coincides with hypoxic waters. Not reported before, acidified waters are confined to a thin bottom boundary layer where the production of CO2 by benthic metabolic processes is dominant. Despite a reduced saturation state, acidified waters remain supersaturated with respect to aragonite.

Climate Change Projected to Exacerbate Impacts of Coastal Eutrophication in the Northern Gulf of Mexico

The continental shelf in the northern Gulf of Mexico experiences expansive seasonal hypoxic conditions and eutrophication-driven acidification in bottom waters. Rising surface ocean temperatures, freshwater and nutrient inputs, and atmospheric CO2 will further exacerbate these conditions. Using a high-resolution, regional circulation-biogeochemical model, we simulated the spatiotemporal dynamics of oxygen and inorganic carbon in the northern Gulf of Mexico under present and a projected future (2100) climate state. Results indicate a modest expansion of the hypoxic zone, but more severe hypoxia and greater exposure to prolonged hypoxic conditions. The main drivers underlying these changes are a reduction in oxygen solubility (accounting for 60–74% of the change) and increased stratification (accounting for less than 40%). pH is projected to decrease across the shelf with lowest values in hypoxic waters where aragonite saturation will approach the saturation limit. In the model simulations, acidification is primarily driven by atmospheric and offshore CO2 levels, while the enhancement in stratification only accounts for 7% or less of the total change in pH. Decreased buffering capacity and increased stratification in the future will enhance respiration-induced acidification (i.e., a decrease in bottom water pH by respired CO2), which will amplify the climate-induced acidification. According to the model, the magnitude of future changes varies significantly from year to year. The largest effects are simulated in years with large freshwater discharge and upwelling-favorable winds. Plain Language Summary The continental shelf in the northern Gulf of Mexico experiences eutrophication-driven seasonal low-oxygen conditions (hypoxia) and acidification (a decrease in bottom water pH by respired CO2). Rising surface ocean temperatures, freshwater and nutrient inputs, and atmospheric CO2 will further exacerbate these conditions. We simulated the variations of oxygen and inorganic carbon in the northern Gulf of Mexico at present and under a projected future (2100) climate state. Results indicate more severe and prolonged periods of hypoxia in the future due to reduced oxygen solubility in warmer waters and increased stratification. pH is projected to decrease significantly with lowest values in low-oxygen waters. Future acidification is primarily driven by rising atmospheric and offshore CO2 levels. A decreased buffering capacity of seawater and increased stratification will enhance respiration-induced acidification, which will further amplify the climate-induced acidification. The magnitude of projected changes varies significantly from year to year, with the largest effects in years with large freshwater discharge and upwelling-favorable winds.

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

The occurrence of hypoxia in coastal oceans is a long-standing and growing problem worldwide, and clearly linked to anthropogenic nutrient inputs. While the need for reducing anthropogenic nutrient loads is generally accepted, it is costly and thus requires scientifically sound nutrient5 reduction strategies. Issues under debate include the relative importance of nitrogen (N) and phosphorus (P), and the magnitude of reduction requirements. The largest anthropogenically induced hypoxic area in North American coastal waters (of 15,000+/-5,000 km) forms every summer in the north10 ern Gulf of Mexico where the Mississippi and Atchafalaya Rivers deliver large amounts of freshwater and nutrients to the shelf. A 2001 plan for reducing this hypoxic area by nutrient management in the watershed called for a reduction of N loads. Since then evidence of P limitation during the time 15 of hypoxia formation has arisen, and a dual nutrient reduction strategy for this system has been endorsed. Here we report the first systematic analysis of the effects of single and dual nutrient load reductions from a spatially explicit physical-biogeochemical model for the northern Gulf 20 of Mexico. The model has been shown previously to skillfully represent the processes important for hypoxic formation. Our analysis of an ensemble of simulations with stepwise reductions in N, P and N&P loads provides insight into the effects of both nutrients on primary production and hy25 poxia, and allows us to estimate what nutrient reductions would be required for single and dual nutrient reduction strategies to reach the hypoxia target. Our results show that, despite temporary P limitation, N is the ultimate limiting nutrient for primary production in this system. Nevertheless, a 30 reduction in P load would reduce hypoxia because primary production in the region where density stratification is conducive to hypoxia development, but reductions in N load have a bigger effect. Our simulations show that, at present loads, the system is almost saturated with N, in the sense that 35 the sensitivity of primary production and hypoxia to N load is much lower than it would be at lower N loads. We estimate that reduction of 63% +/18% in total N load or 48% +/21% in total N&P load are necessary to reach a hypoxic area of 5,000 km, which is consistent with previous estimates from 40 statistical regression models and highly simplified mechanistic models.

Modeling River-Induced Phosphorus Limitation in the Context of Coastal Hypoxia

The urban development of coastal areas and the increased use of chemical fertilizers over the last century have led to a worldwide expansion of coastal eutrophication and a significant increase in the occurrence and intensity of human-induced coastal hypoxia. Proportionally, nitrogen load has often increased more severely than phosphorus load and phosphorus limitation became a common seasonal phenomenon in many eutrophic coastal systems. Phosphorus limitation may alter the magnitude, timing, and location of phytoplankton production with potential effects on hypoxia. Yet, because of the difficulty in observing these effects, limited work has been carried out to assess the influence of P limitation on hypoxia. Models are thus useful tools for simulating the effects of river-induced phosphorus limitation on coastal hypoxic systems. Modeling P limitation is important to better understand the processes controlling hypoxia, to improve the predictive skill of hypoxia prediction models, and to design and evaluate nutrient management strategies for hypoxia mitigation. Here, we review the effects of phosphorus limitation on a continuum of coastal hypoxic systems, contrasting the effects of P limitation on systems that are primarily one-dimensional (or “flow-through”) like the Neuse River Estuary versus more dispersive open systems like the Mississippi River plume. We discuss modeling frameworks and techniques that are relevant in this context and summarize recent modeling work that quantitatively assesses the effect of phosphorus limitation on hypoxia development in the Mississippi River plume.