William W. Schroeder

The Role of Stratification in the Deoxygenation of Mobile Bay and Adjacent Shelf Bottom Waters

Oxygen depletion in the shallow bottom waters of Mobile Bay, Alabama, and in adjacent nearshore and continental shelf waters, is shown to be directly related to the intensity of water column stratification. Low winds speeds are coincidental with the onset of water column stratification and the occurrence of hypoxic events. Hourly, daily, and seasonal changes in the relationship between percent oxygen saturation or oxygen concentration in the bottom waters and surface-bottom density differences indicate that the oxidized materials are recently formed, and not relic or overwintering carbon sources. The influence of density structure (water column stratification) in other oxygen-depleted coastal water masses is compared to Mobile Bay.

Salinity stratification in a river-dominated estuary

Analysis of salinity data from Mobile Bay indicates that stratification-destratification events within this broad, shallow estuary are not uncommon. These events are related to the strength of the winds, through their influence on wave generation and subsequent bottom drag coefficient increases, and to the strength of river discharge. They do not appear to be due to the strength of tidal currents, as has been observed elsewhere. Furthermore, river flow appears to be the dominant control, the winds being important only in the absence of large freshwater discharges. The annual spring freshet can flush most of the salt from the bay. During other times of the year the relative strengths of river discharge and wind stress change the bay from highly stratified to nearly homogeneous and back on a variety of time scales ranging from daily to seasonal.

Temporal Variability in Summertime Bottom Hypoxia in Shallow Areas of Mobile Bay, Alabama

This paper addresses temporal variability in bottom hypoxia in broad shallow areas of Mobile Bay, Alabama. Time-series data collected in the summer of 2004 from one station (mean depth of 4 m) exhibit bottom dissolved oxygen (DO) variations associated with various time scales of hours to days. Despite a large velocity shear, stratification was strong enough to suppress vertical mixing most of the time. Bottom DO was closely related to the vertical salinity gradient (ΔS). Hypoxia seldom occurred when ΔS (over 2.5 m) was <2 psu and occurred almost all the time when ΔS was >8 psu in the absence of extreme events like hurricanes. Oxygen balance between vertical mixing and total oxygen demand was considered for bottom water from which oxygen demand and diffusive oxygen flux were estimated. The estimated decay rates at 20°C ranging between 0.175–0.322 d−1 and the corresponding oxygen consumption as large as 7.4 g O2 m−2 d−1 fall at the upper limit of previously reported ranges. The diffusive oxygen flux and the corresponding vertical diffusivity estimated for well mixed conditions range between 8.6–9.5 g O2 m−2 d−1 and 2.6–2.9 m2 d−1, respectively. Mobile Bay hypoxia is likely to be associated with a large oxygen demand, supported by both water column and sediment oxygen demands, so that oxygen supply from surface water during destratification events would be quickly exhausted to return to hypoxic conditions within a few hours to days after destratification events are terminated.

Subtidal circulation patterns in a shallow, highly stratified estuary: Mobile Bay, Alabama

Mobile Bay is a wide (25-50 km), shallow (3 m), highly stratified estuary on the Gulf coast of the United States. In May 1991 a series of instruments that measure near-surface and near-bed current, temperature, salinity, and middepth pressure were deployed for a year-long study of the bay. A full set of measurements were obtained at one site in the lower bay ; all but current measurements were obtained at a midbay site. These observations show that the subtidal currents in the lower bay are highly sheared, despite the shallow depth of the estuary. The sheared flow patterns are partly caused by differential forcing from wind stress and river discharge. Two wind-driven flow patterns actually exist in lower Mobile Bay. A barotropic response develops when the difference between near-surface and near-bottom salinity is less than 5 parts per thousand. For stronger salinity gradients the wind-driven currents are larger and the response resembles a baroclinic flow pattern. Currents driven by river flows are sheared and also have a nonlinear response pattern. Only near-surface currents are driven seaward by discharges below 3000 m 3 /s. At higher discharge rates, surface current variability uncouples from the river flow and the increased discharge rates drive near-bed current seaward. This change in the river-forced flow pattern may be associated with a hydraulic jump in the mouth of the estuary.

LOW-FREQUENCY SHELF-ESTUARINE EXCHANGE PROCESSES IN MOBILE BAY AND OTHER ESTUARINE SYSTEMS ON THE NORTHERN GULF OF MEXICO

Two years of data from Mobile Bay show that subtidal shelf-estuarine exchanges at periods of 2 to 4 days are driven by the cross-shelf along-estuary wind stress. At periods of 3 to 20 days, coastal Ekman convergence/divergence, driven by the alongshore wind stress, becomes important in driving the exchanges. Such a situation is common to many estuaries around the northern rim of the Gulf of Mexico. At seasonal scales, river runoff becomes important to both the barotropic and baroclinic exchanges.

Colony morphology, age structure, and relative growth of two gorgonian corals, Leptogorgia hebes (Verrill) and Leptogorgia virgulata (Lamarck), from the northerm Gulf of Mexico

Along the northeastern Gulf of Mexico coast, Leptogorgia hebes is the predominant gorgonian at two inner shelf (22 and 27 m) sites, while Leptogorgia virgulata dominates a third shallow (1–2m), inshore site. The axial skeleton of both species is composed of concentric rings which appeared to exhibit annual periodicity. Although age structure of the populations differed between shelf sites, neither colony growth rates, estimated from direct measurements of ring widths plotted on Walford graphs, nor overall branching complexity, determined from bifurcation ratios (Rb), differed between the shelf populations. Larger individuals at the 27m shelf site were simply older than those at the shallower (22m) shelf site. Walford plots of growth increments revealed colony growth of the shallow inshore species to be similar to that of the offshore species.

Dynamics of the Ningaloo Current off Point Cloates, Western Australia

The Ningaloo Current (NC) is a wind-driven, northward-flowing current present during the summer months along the continental shelf between the latitudes of 22° and 24°S off the coastline of Western Australia. The southward flowing Leeuwin Current is located further offshore and flows along the continental shelf break and slope, transporting warm, relatively fresh, tropical water poleward. A recurrent feature, frequently observed in satellite images (both thermal and ocean colour), is an anti-clockwise circulation located offshore Point Cloates. Here, the seaward extension of the coastal promontory blocks off the broad, gradual southern shelf, leaving only a narrow, extremely steep shelf to the north. The reduction in the cross-sectional area, from the coast to the 50 m contour, between southward and northward of the promontory is ~80%. Here, a numerical model study is undertaken to simulate processes leading to the development of the recirculation feature offshore Point Cloates. The numerical model output reproduced the recirculation feature and indicated that a combination of southerly winds, and coastal and bottom topography, off Point Cloates is responsible for the recirculation. The results also demonstrated that stronger southerly winds generated a higher volume transport in the NC and that the recirculation feature was dependent on the wind speed, with stronger winds decreasing the relative strength of the recirculation.

Occurrence of deep-water Lophelia pertusa and Madrepora oculata in the Gulf of Mexico

One of the critical information needs identified at the 2003 Deep-Sea Corals Workshop in Galway, Ireland, was to locate and chart deep-sea corals in order to develop reliable estimates of their distribution and abundance. While reports of deep-sea corals from the Gulf of Mexico date back to the 1860s, relatively little is known about their distribution or abundance. This paper attempts to provide a current assessment of the occurrence of Lophelia pertusa and Madrepora oculata in water depths greater than 200 m in the Gulf of Mexico by summarizing records from (1) published material, (2) the 2003 National Museum of Natural History Taxonomic Database, (3) findings obtained during the September-October 2003 NOAA-OE RV Ronald H. Brown cruise RB-03-07-leg-2 in the northern Gulf, and (4) from various unpublished sources.

Circulation patterns inferred from the movement of detached buoys in the eastern Gulf of Mexico

Abstract The release times of five buoys, which broke free of moorings on the Alabama inner shelf, are known. The locations of subsequent sightings or recoveries of the buoys gave estimated trajectories of the buoys. These results show that the inner-shelf circulation is strongly wind-driven. When the Loop Current penetrates deeply into the northeastern Gulf of Mexico, outer shelf waters are often entrained by the Loop Current.

Temporal and spatial variations in sediment characteristics on the Mississippi-Alabama continental shelf

Abstract Sediment characteristics on the Mississippi-Alabama shelf are dynamic and change in time scales varying from less than 6 months to more than 2 years. Some sediment properties varied by more than an order of magnitude over the 2, years of study. Individual sediment components vary independently and can be described as cyclic, steadily increasing, random, or unchanging. Many of the variations are linked to influxes of terrestrial material associated with river discharge, non-point terrestrial run-off and/or outflow from coastal environments during storm events. Carbonate content and grain size varied from clay-rich fine-grained sediments associated with the Mississippi River delta complex to coarse-grained shell hash on the eastern shelf. Organic carbon content, extractable organic matter and hydrocarbons were elevated in sediments near the Mississippi River delta complex and in a band of sediments between the 100 and 200 m isobaths. TOC values in excess of 2% were observed, however, average TOC concentrations were 0.8%. Hydrocarbons in sediments were present at low concentrations and are a mixture of biological and petroleum hydrocarbons. Terrestrial plant biowaxes were ubiquitous and, when present, petroleum hydrocarbons were associated with elevated barium concentrations. Aromatic hydrocarbon compositions were indicative of unprocessed petroleum and were dissimilar to combustion PAH detected in adjacent bays. PAH concentrations were as much as six times lower than adjacent coastal sediment concentrations. Spatial and temporal heterogeneity in sediments is due to variations in inputs, preservation, diagenetic alteration and oceanographic setting.