Quarterdeck Volume 6, Number 1, May 1998
The influence of Loop Current
eddies on Texas-Louisiana slope circulation
Surface geological characteristics
of Alaminos Canyon, Gulf of Mexico
Determining dining habits of
bottom- dwelling fish on the continental shelf
Ocean circulation over the Texas-Louisiana shelf and slope can respond to the clockwise-circulating Loop Current eddies which often are located over the slope off the Texas shelf. My dissertation research considered how Loop Current eddies influence this circulation.
I examined several types of oceanic and satellite data collected between April 1992 and December 1994. Hydro-graphic data included temperature, salinity, and depth measurements taken through the water column at many stations during 12 cruises. These data were used to compute a measure of the sea-surface elevation called "geopotential anomaly."
Current velocity data consisted of records of current speed and direction, sampled at regular intervals, in a time series, at 31 locations in the gulf. These data and the hydrographic data were collected during programs sponsored by the Minerals Management Service.
Sea-surface height anomaly (SSHA) data derived from satellite altimeters provide another measure of sea-surface elevation. Data from altimeters attached to two satellites-TOPEX/Poseidon and ERS-1-are processed by Dr. Robert Leben at the University of Colorado.
Other scientists had shown altimeter data could be used to track large Loop Current eddies in the deep waters of the gulf, but the reliability of SSHA over shallower waters of the slope and shelf had not been determined. To investigate the usefulness of SSHA over these shallower waters, I first qualitatively compared SSHA maps with hydrographic, current velocity, and other oceanic data. I looked for similarities and differences in Loop Current eddy locations and movements over the slope and at the shelf edge indicated by each different data type. I found the movements over the slope indicated by the SSHA data were consistent with the oceanic measurements examined.
Since two data types provide a measure of sea-surface elevation, they should be directly related; so I used statistics to quantitatively correlate the SSHA data with geopotential anomaly data over the slope and the shelf. SSHA and geopotential anomaly were significantly correlated in water of depth 200 meters or more. The SSHA fields can be used with confidence to analyze circulation over the Texas-Louisiana shelf edge and slope.
Correlations in shallower water over the shelf, however, were poor, and the SSHA fields should not be used until the reasons are determined. The poor correlations may occur because of a mismatch in the time and space scales used to process the SSHA data (12 days and 100 kilometers) and the scales of shelf circulation (a few hours and 15-35 kilometers). Another reason could be problems with computing geopotential anomaly in shallow water.
Next, I developed a mathematical formula for combining the sea-surface elevation data with the current velocity data to improve mapping of sea-surface elevation and current velocity fields. I applied it to the data for the time periods of the 12 hydrographic cruises. Loop Current eddies and their related cyclonic eddies dominated shelf edge and slope circulation. The largest water elevations and strongest currents occurred with these eddies.
Flows over the slope were anti-cyclonic when associated with a Loop Current eddy and cyclonic when associated with a cyclonic eddy. In regions without these eddies, currents were weaker and less organized. When the Loop Current eddies were close to the shelf edge, they moved the saltier deep waters onto the shelf and pulled the fresher shelf waters off the shelf. Through the 12 cases, I demonstrated that the evolution of Loop Current eddies can be studied in detail with a time series of water elevation and current velocity fields.
Author's note: My dissertation is dedicated to the memory of two great pioneers of the physical oceanography of the Gulf of Mexico, Professor John D. Cochrane and Dr. Takashi Ichiye. Both touched and enriched my life; both are sorely missed.
Author's note: My dissertation is dedicated to the memory of two
great pioneers of the physical oceanography of the Gulf of Mexico, Professor
John D. Cochrane and Dr. Takashi Ichiye. Both touched and enriched my life;
both are sorely missed.
Editor's note: Ann Jochens is an associate research scientist in the Department of Oceanography at Texas A&M and the Deputy Program Manager of the Louisiana-Texas Shelf Physical Oceanography program and the NEGOM hydrography project.
[41K] A map of sea-surface height anomaly shows a blend of TOPEX/Poseidon and ERS-1 satellite data from May 9-19, 1993. (Data courtesy of Robert Leben, Colorado Center for Astrodynamics Research)
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Understanding the surface morphology and sediment in the deep water environment is important for constructing platforms and pipelines, estimating potential geological hazards, and developing physical oceanography and sediment transport models.
My research uses Sea Beam bathymetry data, GLORIA II sidescan sonar images, core samples, and other supporting geophysical data to examine and interpret the geological characteristics of the Alaminos Canyon in the Gulf of Mexico.
The Sea Beam bathymetry data were collected in 1990 using R/V Atlantis II. The raw data were depth corrected, erroneous depth soundings caused by machine errors were rejected, and the data were geographically registered. I tested four popular interpolation methods to grid the depth soundings to a 50-meter spaced lattice. The depth difference caused by different interpolation methods can be more than 35 meters, which is greater than the inaccuracy of the machine and seven times the depth correction in the study area. I selected an interpolation method that minimized errors, and used the results to generate other seafloor features like the slope gradient, roughness, slope direction, and drainage network.
The GLORIA II side-scan sonar data used in the study were collected by the U.S. Geological Survey in 1985. The side-scan sonar records or sonographs are usually displayed in grayscale and represent the energy reflected and scattered back to the sidescan sonar. The sonographs are functions of the seafloor's relief, microtopography and roughness, and physical properties. In my research I focused on improving the accuracy of backscattered pixels' locations and on differentiating true backscattering from machine caused errors, or artifacts.
Four core samples were collected in the Alaminos Canyon during a 1996 R/V Gyre cruise. A multi-sensor core logger was used to measure physical properties of the sediment in these piston core samples at one-centimeter intervals. The physical properties are used to infer a geological scenario and offer parameters for modeling how the sonar energy changes with sediment depth. I calculated the sediment accumulation rate based on two core samples, and found periodic sediment input in the canyon during the Holocene. The calculated acoustic transmission coefficient indicates homogeneous sediment, and the sonar energy model suggests side-scan sonar images represent the sediments' physical properties up to a few meters below the seafloor.
The intrinsic properties of the bathymetry indicate that 10°-15° slope gradients typify the canyon rim and gullies that dissect the canyon escarpment. Slopes in excess of 20° are mainly located in the northeast and the central southwest portions of the canyon. These high slope gradient areas also have the roughest bathymetry (greater than five meters), which may indicate active movement of salt beneath the seafloor. The paths through which sediment flows into the canyon indicate sediment sources to its north and west.
The U-shaped main drainage path exhibits fluctuating incisions, which suggests the main path is a mature conduit and not formed by a single event. Sediment compresses in a north-northeast to south-southwest direction, and contributes to formation of faults in the northern wall of the main drainage path. These serve as channel conduits for sediment entering the canyon. Y
Editor's note: Jia-Yuh Liu graduated with his Ph.D. in December 1997. He currently is an Assistant Research Scientist and works with Dr. William Bryant on a Texas A&M Gulf of Mexico/NOAA Bathymetry (TGMNB) CD-ROM and slope stability in the Gulf of Mexico.
[68K] A digital image of Alaminos Canyon bathymetry shows the seafloor topography in fine detail.
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The majority of the world's fisheries occur on the continental shelf, and unfortunately, humans overfish many of these areas. Fishery companies and individuals have historically fished for one particular species, like tuna, using equipment that harvests additional, unwanted species, like dolphins. In the single-species harvest model, large numbers of marine animals are discarded as "trash fish" when they are caught along with the intended species.
To better manage our continental shelf fisheries, scientists need to develop multi-species models for the fishing industry. One essential part of the models is information on the food of the fishes. My research focused on food habits of benthic fishes that live on or near the seafloor of the continental shelf of Mississippi and Alabama. The intent of the study was to use an ecosystem approach by analyzing the food habits of a large portion of the fishes collected.
My fellow Texas A&M University scientists and I participated in five oceanographic research cruises on the R/V Tommy Monroe (Gulf Coast Research Laboratory) from February 1987 to August 1989. We collected about 120 trawl samples from 12 different stations on cruises during the summer and winter. Fish were preserved aboard the ship, then returned to College Station where they were identified, measured, weighed and numbered, and then selected for food-habits analysis.
Close to 6,000 stomachs were dissected, and all food items were removed from the stomachs, identified, and enumerated. Food items were identified to the lowest possible classification, such as "family," "genus," or "species." In addition, the volume of major groupings of the food items was determined. To enable further analysis, I generalized the major grouping of food items into "ecological food categories," which were based on the size, living habits, and location in the water column of the food organisms.
The food of the fishes, by volume, consisted primarily of shrimp, with fishes, worms (polychaetes) and crabs also being very important food items. Small crustaceans (amphipods, copepods, mysids), jellyfish tentacles and mantis shrimp (stomatopods) were also important food items for selected fishes.
Using the ecological food categories, the 56 species of fishes studied were divided into "guilds." The guilds were based on groupings of fishes that consumed ecologically similar food items.
For example, some fishes, such as the anchovies, primarily consumed small planktonic organisms such as copepods and crab, shrimp, and barnacle larvae. These guilds may be likened to a bread bakers' guild, in which all members of the guild bake bread, yet they may use different grains. Guilds of plankton-consuming fish all feed on plankton, yet different species of fish consume different species of plankton.
In addition to the guild analysis, I placed the fish into groups, or assemblages, based on where they were caught on the continental shelf. Once the assemblages were established, I analyzed the fishes' diets to search for differences in diet by season, by guild, and by assemblage.
Finally, I combined all of the results to give an overview of the food web structure of the benthic fishes on the Mississippi-Alabama continental shelf. The guilds differ because the foods they consume are different sizes, have different living habits, and occur at different locations in the water column. Different guilds are linked to one another through the food they eat, with some guilds more closely linked than others. These connections are particularly strong for those guilds occurring in the benthic realm.
[37K] A diagram of the benthic guilds that live on the Mississippi -Alabama shelf.
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Last updated May 4, 1998