Robert D. Hetland

The science of hypoxia in the Northern Gulf of Mexico: a review.

The Mississippi River is one of the worlds 10 largest rivers, with average freshwater discharge into the northern Gulf of Mexico (GOM) of 380km(3) year(-1). In the northern GOM, anthropogenic nitrogen is primarily derived from agricultural fertilizer and delivered via the Mississippi River. The general consensus is that hypoxia in the northern Gulf of Mexico is caused primarily by algal production stimulated by excess nitrogen delivered from the Mississippi-Atchafalaya River Basin and seasonal vertical stratification of incoming stream flow and Gulf waters, which restricts replenishment of oxygen from the atmosphere. In this paper, we review the controversial aspects of the largely nutrient-centric view of the hypoxic region, and introduce the role of non-riverine organic matter inputs as other oxygen-consuming mechanisms. Similarly, we discuss non-nutrient physically-controlled impacts of freshwater stratification as an alternative mechanism for controlling in part, the seasonality of hypoxia. We then explore why hypoxia in this dynamic river-dominated margin (RiOMar) is not comparable to many of the other traditional estuarine systems (e.g., Chesapeake Bay, Baltic Sea, and Long Island Sound). The presence of mobile muds and the proximity of the Mississippi Canyon are discussed as possible reasons for the amelioration of hypoxia (e.g., healthy fisheries) in this region. The most recent prediction of hypoxia area for 2009, using the current nutrient-centric models, failed due to the limited scope of these simple models and the complexity of this system. Predictive models should not be the main driver for management decisions. We postulate that a better management plan for this region can only be reached through a more comprehensive understanding of this RiOMar system-not just more information on river fluxes (e.g., nutrients) and coastal hypoxia monitoring programs.

Relating River Plume Structure to Vertical Mixing

The structure of a river plume is related to the vertical mixing using an isohaline-based coordinate system. Salinity coordinates offer the advantage of translating with the plume as it moves or expanding as the plume grows. This coordinate system is used to compare the relative importance of different dynamical processes acting within the plume and to describe the effect each process has on the structure of the plume. Vertical mixing due to inertial shear in the outflow of a narrow estuary and wind mixing are examined using a numerical model of a wind-forced river plume. Vertical mixing, and the corresponding entrainment of background waters, is greatest near the estuary mouth where inertial shear mixing is large. This region is defined as the near field, with the more saline, far-field plume beyond. Wind mixing increases the mixing throughout the plume but has the greatest effect on plume structure at salinity ranges just beyond the near field. Wind mixing is weaker at high salinity classes that have already been mixed to a critical thickness, a point where turbulent mixing of the upper layer by the wind is reduced, protecting these portions of the plume from further wind mixing. The work done by mixing on the plume is of similar magnitude in both the near and far fields.

Quantifying the Contributions of Tidal Straining and Gravitational Circulation to Residual Circulation in Periodically Stratified Tidal Estuaries

Abstract This numerical modeling study quantifies for the first time the contribution of various processes to estuarine circulation in periodically stratified tidal flow under the impact of a constant horizontal buoyancy gradient. The one-dimensional water column equations with periodic forcing are first cast into nondimensional form, resulting in a multidimensional parameter space spanned by the modified inverse Strouhal number and the modified horizontal Richardson number, as well as relative wind speed and wind direction and the residual runoff. The along-tide momentum equation is then solved for the tidal-mean velocity profile in such a way that it is equated to the sum of the contributions of tidal straining (resulting from the temporal correlation between eddy viscosity and vertical shear), gravitational circulation (resulting from the depth-varying forcing by a constant horizontal buoyancy gradient), wind straining, and depth-mean residual flow (resulting from net freshwater runoff). This definitio...

Forecasting Gulf’s hypoxia: The next 50 years?

This review discusses the use of hypoxia models in synthesizing the knowledge about the causes of Gulf of Mexico hypoxia, predicting the probable consequences of management actions, and building a consensus about the management of hypoxia. It also offers suggestions for future efforts related to simulating and forecasting Gulf hypoxia. The existing hypoxia models for the northern Gulf of Mexico range from simple regression models to complex three-dimensional simulation models, and they capture very different aspects of the physics, chemistry, and biology of this region. Several of these models were successfully calibrated to observations relevant for their process formulations and spatial-temporal scales. Available model results are compared to reach the consensus that large-scale hypoxia probably did not begin in the Gulf of Mexico until the mid 1970s, and that the 30% nitrogen load reduction that is called for by the Action Plan may not be sufficient to achieve its goal. The present models results suggest that a 40–45% reduction in riverine nitrogen load may be necessary to achieve the desired reduction in the areal extent of hypoxia. These model results underscore the importance of setting this goal as a running average because of significant interannual variability. Caution is raised for setting resource management goals without considering the long-term consequences of climate variability and change.

An Idealized Study of the Structure of Long, Partially Mixed Estuaries*

Classic models of estuarine circulation are reexamined using a three-dimensional, primitive equation numerical ocean model. The model is configured using an idealized estuary/shelf domain with rectangular cross section, constant vertical mixing, and steady riverine discharge. Tidal dispersion is neglected, so the analysis does apply to well-mixed estuaries and lagoons. Estuarine scales for the length of steady-state salt intrusion, vertical stratification, and estuarine exchange flow estimated from steady-state model results are found to have the same functional relationships to vertical mixing and riverine discharge as the classic analytic solutions. For example, for steady-state conditions, the stratification is found to be virtually independent of the strength of vertical mixing. The estuarine structure was controlled by the interior estuarine circulation, and not by limited exchange at the mouth. Thus, the numerical solutions were not ‘‘overmixed,’’ although the solutions showed a dependence on freshwater flux functionally similar to the overmixed solution. Estuarine adjustment time scales are also estimated from the simulations, and they are related to the steady-state estuarine scales. Two classes of nonsteady solutions are examined: the response to a step change in riverine discharge and estuarine response to changes in vertical mixing. Spring/neap tidal variations are examined by modulating the (spatially constant) vertical mixing with a fortnightly period. Unlike the steady solutions, there is a clear dependence of stratification on mixing rate in the time-dependent solutions. The simulations involving changes in riverine discharge show asymmetries between response to increasing and decreasing river flow that are attributed to quadratic bottom drag.

Multivariable statistical regression models of the areal extent of hypoxia over the Texas–Louisiana continental shelf

Observations of the areal extent of seasonal hypoxia over the Texas–Louisiana continental shelf from 1985 to 2010 are correlated with a variety of physical and biogeochemical forcing mechanisms. Significant correlation is found between hypoxic area and both nitrogen load (r 2 = 0.24) and east–west wind speed (r 2 = 0.16). There is also a significant increasing trend in the areal extent of hypoxia in time; a linearly increasing trend over the entire record (r 2 = 0.17), a step increase in area for the years 1994 and beyond (r 2 = 0.21), and a step increase for 1993 and beyond (r 2 = 0.29) were all found to be significantly correlated with area. The year 1988, often included in other studies, was found to be a statistical outlier, in that the statistical regression properties are strongly modified when this year is included. The exclusion of any other year does not have as great an effect as excluding 1988 from the record. The year 1989 is also excluded, as this year had no full shelf survey, for a total of 24 years of data for the record. Multivariable regression models using all possible combinations of the forcing variables considered were calculated. The best performing models included east–west wind, either a linear trend in time or step in time (1994 and beyond), and either nitrogen load or river discharge combined with nitrogen concentration. The range of adjusted correlation coefficients ranged from r 2 = 0.47 to 0.67. The best model (east–west wind, a step increase in time 1994 and beyond, river discharge, and nitrogen concentration) has a standard error of 3008 km 2 .

Near-Resonant Ocean Response to Sea Breeze on a Stratified Continental Shelf

Abstract The spatial structure and temporal characteristics of sea breeze and the associated coastal ocean response in the northwest Gulf of Mexico are investigated using moored instruments, hydrographic stations, and wind measurements. Near the study area of 30°N, motions in the diurnal–inertial band (DIB) may be significantly enhanced by a near-resonant condition between local inertial and diurnal forcing frequencies. Wavelet analysis is used to quantify the results. Results indicate that diurnal sea-breeze variability peaks in summer and extends at least 300 km offshore with continuous seaward phase propagation. The maximum DIB oceanic response occurs in June when there is a shallow mixed layer, strong stratification, and an approximately 10-day period of continuous sea-breeze forcing. DIB current variance decreases in July and August as the consequence of the deepening of the mixed layer and a more variable phase relationship between the wind and current. River discharge varies interannually and can s...

Efficient tools for marine operational forecast and oil spill tracking

Ocean forecasting and oil spill modelling and tracking are complex activities requiring specialised institutions. In this work we present a lighter solution based on the Operational Ocean Forecast Python Engine (OOFε) and the oil spill model General NOAA Operational Modelling Environment (GNOME). These two are robust relocatable and simple to implement and maintain. Implementations of the operational engine in three different regions with distinct oceanic systems, using the ocean model Regional Ocean Modelling System (ROMS), are described, namely the Galician region, the southeastern Brazilian waters and the Texas-Louisiana shelf. GNOME was able to simulate the fate of the Prestige oil spill (Galicia) and compared well with observations of the Krimsk accident (Texas). Scenarios of hypothetical spills in Campos Basin (Brazil) are illustrated, evidencing the sensitiveness to the dynamical system. OOFε and GNOME are proved to be valuable, efficient and low cost tools and can be seen as an intermediate stage towards more complex operational implementations of ocean forecasting and oil spill modelling strategies.

A Numerical Study of Sea-Breeze-Driven Ocean Poincare Wave Propagation and Mixing near the Critical Latitude

Abstract Near the vicinity of 30° latitude, the coincidence of the period of sea breeze and the inertial period of the ocean leads to a maximum near-inertial ocean response to sea breeze. This produces a propagating inertial internal (Poincare) wave response that transfers energy laterally away from the coast and provides significant vertical mixing. In this paper, the latitudinal dependence of this wave propagation and its associated vertical mixing are investigated primarily using a nonlinear numerical ocean model. Three-dimensional idealized simulations show that the coastal oceanic response to sea breeze is trapped poleward of 30° latitude; however, it can propagate offshore as Poincare waves equatorward of 30° latitude. Near 30° latitude, the maximum oceanic response to sea breeze moves offshore slowly because of the near-zero group speed of Poincare waves at this latitude. The lateral energy flux convergence plus the energy input from the wind is maximum near the critical latitude, leading to increa...

Impacts of diverted freshwater on dissolved organic matter and microbial communities in Barataria Bay, Louisiana, U.S.A.

Here we present results of an initial assessment of the impacts of a water diversion event on the concentrations and chemical composition of dissolved organic matter (DOM) and bacterioplankton community composition in Barataria Bay, Louisiana U.S.A, an important estuary within the Mississippi River Delta complex. Concentrations and spectral properties of DOM, as reflected by UV/visible absorbance and fluorescence, were strikingly similar at 26 sites sampled along transects near two western and two eastern areas of Barataria Bay in July and September 2010. In September 2010, dissolved organic carbon (DOC) was significantly higher (568.1-1043 μM C, x=755.6+/-117.7 μM C, n=14) than in July 2010 (249.1-577.1 μM C, x=383.7+/-98.31 μM C, n=14); conversely, Abs254 was consistently higher at every site in July (0.105-0.314) than in September (0.080-0.221), averaging 0.24±0.06 in July and 0.15±0.04 in September. Fluorescence data via the fluorescence index (FI450/500) revealed that only 30% (8 of 26) of the July samples had an FI450/500 above 1.36, compared to 96% (25 of 26) for the September samples. This indicates a more terrestrial origin for the July DOM. Bacterioplankton from eastern sites differed in composition from bacterioplankon in western sites in July. These differences appeared to result from reduced salinities caused by the freshwater diversion. Bacterioplankton communities in September differed from those in July, but no spatial structure was observed. Thus, the trends in bacterioplankton and DOM were likely due to changes in water masses (e.g., input of Mississippi River water in July and a return to estuarine waters in September). Discharge of water from the Davis Pond Freshwater Diversion (DPFD) through Barataria Bay may have partially mitigated some adverse effects of the oil spill, inasmuch as DOM is concerned.