Hypoxia (dissolved oxygen concentrations less than 1.4 ml/l) is a recurrent seasonal phenomenon on the Louisiana Shelf, caused by the combined effects of nutrient loading by the Mississippi and Atchafalaya River System (MARS), and density stratification. In 2004, three shelf wide cruises (in April, June and August) were conducted on the Louisiana Shelf to understand the mechanisms controlling hypoxia on the shelf, and examine the relationship between dissolved oxygen and nutrient concentrations during the hypoxic periods. The shelf was divided into three geographically separate zones: A (off the mouth of the Mississippi River), B (off the Terrebonne Bay) and C (off the mouth of the Atchafalaya). Each zone was different in terms of the physical and biochemical processes occurring there. In April, no hypoxia was observed on the shelf because of water column mixing by winds, even though high discharge occurred from the MARS. Nutrients were abundant in the surface waters but present only in little amounts at the bottom. In June, the water column was highly stratified. Because of the presence of upwelling favorable winds no vertical mixing occurred and caused extensive hypoxia on the shelf. Dissolved oxygen concentrations were negatively correlated with nutrients at the bottom of the water column. Nutrients were considerably higher at the bottom than at the surface (except for zone A, where high nutrients were seen even at the bottom), indicating remineralization below the pycnocline. Resuspension of organic material and remineralized nitrogen were sustaining hypoxia far from the river sources. In August, hypoxia was patchy on the Louisiana Shelf. Correlations between dissolved oxygen and nutrient concentration varied seasonally with highest correlations occurring during hypoxic conditions in June and August. The spatial distribution of nutrients and other oceanographic parameters, such as light transmission, fluorescence, and dissolved oxygen concentrations, indicate seasonal variability of biochemical processes that are related to physical processes that affect stratification.

Hypoxia (dissolved oxygen concentrations less than 1.4 ml/l) is a recurrent seasonal phenomenon on the Louisiana Shelf, caused by the combined effects of nutrient loading by the Mississippi and Atchafalaya River System (MARS), and density stratification. In 2004, three shelf wide cruises (in April, June and August) were conducted on the Louisiana Shelf to understand the mechanisms controlling hypoxia on the shelf, and examine the relationship between dissolved oxygen and nutrient concentrations during the hypoxic periods. The shelf was divided into three geographically separate zones: A (off the mouth of the Mississippi River), B (off the Terrebonne Bay) and C (off the mouth of the Atchafalaya). Each zone was different in terms of the physical and biochemical processes occurring there. In April, no hypoxia was observed on the shelf because of water column mixing by winds, even though high discharge occurred from the MARS. Nutrients were abundant in the surface waters but present only in little amounts at the bottom. In June, the water column was highly stratified. Because of the presence of upwelling favorable winds no vertical mixing occurred and caused extensive hypoxia on the shelf. Dissolved oxygen concentrations were negatively correlated with nutrients at the bottom of the water column. Nutrients were considerably higher at the bottom than at the surface (except for zone A, where high nutrients were seen even at the bottom), indicating remineralization below the pycnocline. Resuspension of organic material and remineralized nitrogen were sustaining hypoxia far from the river sources. In August, hypoxia was patchy on the Louisiana Shelf. Correlations between dissolved oxygen and nutrient concentration varied seasonally with highest correlations occurring during hypoxic conditions in June and August. The spatial distribution of nutrients and other oceanographic parameters, such as light transmission, fluorescence, and dissolved oxygen concentrations, indicate seasonal variability of biochemical processes that are related to physical processes that affect stratification.

Numerical experiments are used to study the wind- and buoyancy-modulated along-shore circulation over the Texas-Louisiana continental shelf inshore of 50-m water depth. Most attention is given to circulation in the non-summer flow regime. A major focus of this study is on a unique along-shore flow phenomenon – convergent along- shore flows, which is controlled jointly by wind forcing and buoyancy fluxes from the Mississippi-Atchafalaya river plume. The second problem addresses the forcing effect of buoyancy on the general along-shore circulation pattern over the shelf in non-summer. The convergent along-shore flows are characterized by down-coast flow from the northern shelf encountering up-coast flow from the southern shelf. This phenomenon is explored for both weather band and seasonal timescales. For the weather band, investigations are focused on non-summer convergent events. The formation of convergent flows is primarily caused by along-coast variation in the along-shore component of wind forcing, which in turn is due to the curvature of the Texas-Louisiana coastline. In general, along-shore currents are well correlated with along-shore winds. However, the points of convergence of currents and winds are not co-located; but rather, points of convergence of currents typically occur down-coast of points of convergence of wind. This offset is mainly caused by buoyancy forcing that forces down-coast currents and drives the point of convergence of currents further down-coast. No specific temporal shift pattern is found for the weather-band convergence, whereas monthly

A combination of in situ sampling and numerical modeling was used to investigate the effects of mesoscale (<50 km) circulation patterns and stratification on the evolution of hypoxia on the Louisiana Shelf. Temperature, salinity, and dissolved oxygen concentrations records reveal the presence of an alongshelf meander, which is manifested vertically and horizontally as a wave-like distribution of the properties in the water column. The observations suggest the meander is a ubiquitous characteristic of the shelf with alongshore spatial scale approximately 50 km and less, which is consistent with the locations of sandy shoals along the coast and the local deformation radius. Twelve numerical experiments using an idealized three-dimensional shelf circulation model were performed to evaluate the relative importance of the variable bottom topography and freshwater forcing on the development, evolution, and scales of the dynamic instabilities. The inclusion of the shoals into the bottom topography showed the development of the dynamic instabilities as the flow passed over the shoals and downstream. Introduction of fresh water onto the shelf resulted in greater salinity differences, and, as a consequence in the formation of the dynamically unstable salinity fronts along the plume edge. The combination of the freshwater forcing and shoaling topography produced competing and complex interactions. Six numerical experiments were analyzed in order to investigate the effect of dynamic instabilities on spatial and temporal patterns of dissolved oxygen concentrations along the shelf. Although a linear relationship between Brunt-Väisälä frequency and dissolved oxygen deficit was expected, a nonlinear loop-like relationship was discovered that reflects the response of biochemical properties to the alongshelf variability of the density field. Comparison of the numerical modeling runs to observations of density and dissolved oxygen concentrations on the Louisiana Shelf reinforces the importance of physical processes such as topographic steering and/or freshwater forcing on the alongshore distribution of physical and biochemical properties. It suggests that the time scales of respiration (~3 days) and buoyancy transfer processes (~5-7 days), associated with the physical processes that are responsible for water column stability and ventilation, are similar to the time scales associated with the benthic respiration rates.

The Texas-Louisiana Shelf Circulation and Transport Study featured moorings that covered the shelf during 1992 to 1994, and captured the oceanic response on the shelf to category 4 Hurricane Andrew in August of 1992. Eighty-one current meters distributed over 31 moorings along several contours of isobaths provided excellent spatial and temporal coverage over the shelf. The low-frequency variability (2 days and longer) of current observations and tide gauges to the West of the storm are analyzed after the passage of Andrew, focusing on the region outside of direct hurricane forcing. Wavelet analyses are utilized to investigate the dominant periods excited by the storm over the shelf and their temporal evolution after forcing has subsided. Subsequent to the storm's passage, the observations and wavelet transforms show a two-to-four day period coastal trapped wave that propagate westward at speeds near 6 m/s and then around the Texas bend along the bathymetry. The signal remains detectable in observations as far south as Port Isabel, Texas. The prominent frequencies determined from wavelet analysis are compared with predicted coastal trapped wave dispersion modes and show good agreement in the predicted group speed and cross-shelf structure of the first mode. The energies calculated from the data indicate a largely barotropic shelf wave response which is corroborated in the observed currents and by theory.

Commonly referred to as “red tide”, harmful algal blooms (HABs) formed by Karenia brevis occur frequently in the Gulf of Mexico (GOM). A bloom is defined as cell abundances >105 cells L-1. This thesis will focus primarily on Karenia brevis, formerly known as Gymnodinium breve, in the Gulf of Mexico. K. brevis is harmful because it produces brevetoxin, a ladder-frame polyether that acts as a potent neurotoxin in vertebrates. K. brevis commonly causes fish kills, respiratory irritation in humans, and Neurotoxic Shellfish Poisoning (NSP) if ingested. Blooms of K. brevis occur almost annually along the West Florida Shelf (WFS) in the late summer and early fall, when the coastal current is favorable for bloom initiation. Along the Texas-Louisiana shelf (TLS) however, blooms of K. brevis are infrequent and sporadic. While much is known of the blooms along the WFS due to their frequent presence, little is known of the mechanisms driving the blooms along the TLS due to their inconsistent presence. To understand the stochastic nature of HABs along the TLS, historical data of bloom occurrences from 1996 to present were compared with NOAA station PTAT2 wind, sea-level pressure, air and water temperature data and NCEP NARR-A sea-level pressure data. The difference in the monthly-mean along-shore component of the wind was statistically significant between bloom and non-bloom years in September (p<<0.001) and April (p=0.0015), with bloom years having a strong downcoast current. Monthly mean water temperature values yielded similar results between bloom and non-bloom years. Both March and September monthly-mean water temperature values were lower during non-bloom years with p-values of 0.01 and 0.048, respectively. These results suggest the possibly of forecasting for HABs along the TLS with currently measured, publicly available data.