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Current CSCOR-Funded Research in Northern Gulf of Mexico
Continuing FY 2003 Projects
Seasonal Mapping of Productivity and Nutrient Limitation
James W. Ammerman - Rutgers University
The Mississippi River is the largest river in the U.S., it drains 40% of the continental U.S. land area and accounts for up to 90% of the freshwater runoff into the Gulf of Mexico . The Mississippi River plume and the Louisiana coast have a clear anthropogenic nutrient signal resulting in significant nutrient-enhanced productivity that has a clear impact on the coastal environmental quality. Nitrogen (N), phosphorus (P), and silica (Si) have all been implicated as limiting nutrients in this environment, though most of the focus has been on N-limitation. This project will map surface primary productivity and nutrient limitation in the Mississippi Plume and the Louisiana shelf several times during the high flow period in the spring and early summer of 2004. The high flow period during the spring and early summer is the period of greatest productivity in the Mississippi Plume and Louisiana coastal ecosystem and it is this productivity that is responsible for the seasonal hypoxia that is monitored every year in late July. Though there is annual variation in the extent of the hypoxic area based on river flow and weather patterns, the general trend is for a larger area of low oxygen nearly every year. Primary productivity will be mapped using a Fast Repetition Rate (FRR) Fluorometer and nutrient limitation will be mapped by measurement of cell-surface enzyme activities and nutrients. Based on recent evidence, phosphorus limitation is probable during this time period so the major enzyme activity to be measured will be alkaline phosphatase, though enzymes responsive to nitrogen limitation will also be measured. This study will provide a better understanding of the magnitude and cause of the increased primary productivity that produces this hypoxia. It will also produce extensive chemical data and biological rate measurements for assimilation into the next generation of predictive models.
Hypoxia, Fish, & Fisheries in the Northern Gulf of Mexico : An Ecosystem-Based Approach
J. Kevin Craig, Larry B. Crowder and Andrew J. Read - Duke University
Similar to many coastal and estuarine systems, there is evidence that the northwestern Gulf of Mexico continental shelf is under increasing stress due in part to anthropogenic activities in its adjoining watershed. Nutrient loading from the Mississippi River system, which drains ~40% of the conterminous United States, contributes to the formation of one of the largest seasonal hypoxic zones in the northern hemisphere (~22,000 km2 during 2002). Despite evidence that hypoxia is becoming more frequent and widespread in coastal and estuarine systems, its effects on fish and fisheries are not well understood. While hypoxia may clearly have a direct mortality effect on mobile species resulting in fish kills, there is growing recognition that the indirect effects of low dissolved oxygen, such as habitat loss, declines in food availability and growth, physiological stress, and increased exposure to predators are more likely to have population and community-level consequences. A potentially important, though rarely studied, effect of hypoxia and associated habitat loss is changes in aggregation and spatial overlap with natural (i.e., predators) and anthropogenic (fisheries) sources of mortality. We hypothesize that hypoxia and associated habitat loss result in shifts in community structure and the spatial distribution of component species that modify trophic relationships and spatial overlap between prey and predator (including fisheries) species with ultimate consequences for the productive capacity of the Gulf continental shelf ecosystem. The objectives of this proposal are to:
- Characterize habitat use of upper trophic levels including commercially, recreationally and ecologically important fishes, bycatch species and species of special concern (sea turtles, marine mammals), and shrimpers relative to fine-scale spatial structure in bottom dissolved oxygen,
- Develop indices of spatial overlap for target and nontarget fishery species, and predator (including shrimpers) and prey species,
- Assess trophic relationships among key components of the upper trophic level community,
- Develop statistical models relating distribution, feeding success, and spatial overlap to important environmental variables including bottom dissolved oxygen, and
- Compare community structure and associated impacts of hypoxia at small and regional spatial scales. Statistical models will be used to infer whether the location of areas of high relative abundance, feeding success, and overlap vary as a function of low dissolved oxygen, distance from the edge of the hypoxic zone, or between regions characterized by different oxygen conditions. These empirical models will provide information on ecological responses to hypoxia at the spatial scale at which such responses occur. They will also provide critical information for parameterizing and validating more process-oriented models of food web and fishery interactions, and how they may be impacted by hypoxia.
Mechanisms Controlling Hypoxia on the Louisiana Shelf
Steven F. DiMarco, Robert D. Hetland, Gilbert T. Rowe, Norman L. Guinasso,
Jr., Mahlon C. Kennicutt, Steven K. Baum and Matthew K. Howard - Texas
A&M University
Piers Chapman - Louisiana State University
The hypoxic region of the northern Gulf of Mexico , known popularly as "the dead zone" has increased in size during the 1990s. Its growth has been attributed to increased nitrate usage by farmers in the Mid-west, and an expensive mitigation strategy has been proposed. While many researchers accept this hypothesis, good arguments can be made for local stratification being as much of a control on the area affected, and this implies that reducing the nitrate input may have little effect on the affected area.
This proposal comprises an integrated, multidisciplinary, numerical and observational study of the competing controlling mechanisms of hypoxia, which will investigate the relative importance of both nitrate inputs and physical factors, such as winds, river flow, and local circulation patterns. The central hypothesis that will be tested is that the region affected by hypoxia may be separated into three zones, controlled by chemical, biological, and physical processes respectively.
The objectives are to investigate:
- How stratification changes with forcing by freshwater and local winds;
- How respiration rates across the region are related to stratification; and
- How the combination of the physical controls and in situ respiration rates interact to maintain the "dead zone".
The work will include a combination of a high-resolution physical model, a model of the biogeochemical regime, and in situ sampling. The latter will include moored measurements above and below they pycnocline to monitor the oxygen and nitrate concentrations at three sites, along with the associated currents. Sedimentation rates of organic and inorganic material will be followed with suspended traps, while in situ benthic respiration will be measured using a benthic lander. Additionally, a series of hydrographic stations will be occupied around each mooring site to examine the local scales of interaction.
Initially, the models will be tested using historic data in a series of process experiments. The field measurements will be used both to improve the models and as a test of their ability to predict how the system changes. It is anticipated that the results from this work will enable researchers to differentiate regions controlled by biological or chemical activity from those where physical processes such as stratification are more important. This work has a direct bearing on the likely effectiveness of the proposed remediation process for the region.
Hypoxia Studies in the Northern Gulf of Mexico
Nancy N. Rabalais - Louisiana Universities Marine Consortium
R. Eugene Turner, William J. Wiseman, Jr. and Greg W. Stone - Louisiana State
University
Michael R. Roman and William C. Boicourt - University of Maryland
Hypoxia occurs in many parts of the world's aquatic environments. Hypoxic and anoxic (no oxygen) waters have existed through geologic time, but their occurrence in shallow coastal and estuarine areas appears to be increasing, most likely accelerated by human activities. The largest zone of oxygen-depleted coastal waters in the United States , indeed the entire western Atlantic Ocean, is in the northern Gulf of Mexico on the Louisiana continental shelf. The inability of trawlers to capture any shrimp or demersal fish in hypoxic waters illustrates the implications to fisheries yield. The Gulf zone now ranks equal in areal size with the northwestern shelf of the Black Sea (20,000 km2) but smaller than the Baltic basins (84,000 km2). As a result of Mississippi River flooding in 1993, the increase in the size of the hypoxic zone and its severity that year, a petition to the US E.P.A. for a 319(b) General Management Conference, and increased public awareness of the extent of hypoxia on the Louisiana shelf and its linkage with nutrient changes in the Mississippi River, the issue of hypoxia has been elevated to a high national priority for watershed management and nonpoint source nutrient control. Without exception, several reports and documents call for further monitoring and research on Gulf of Mexico hypoxia and its linkage with nutrient concentration and transport within the Mississippi River basin . The objectives of our proposed work are to:
- Provide a consistent and sequential series of long-term data that document the temporal and spatial extent of hypoxia,
- Continue the collection of the hydrographic, chemical, and biological data related to the development and maintenance of hypoxia over seasonal cycles,
- Enhance surveys of the hypoxic zone with a high resolution undulating sensor package,
- Support modeling components of NGOMEX 2002 with relevant data from observations and process studies, and
- Continue public outreach and add a web site for ready access to information developed from this research program and past hypoxia studies.
We propose a continuation of the mid-summer shelfwide cruise, the continuation of transect C and F cruises on a monthly and bimonthly basis, and an instrument mooring in the core of the hypoxic zone at station C6B. These activities continue and expand long-term data sets from which we have garnered our best and basic understanding of hypoxia on the Louisiana shelf. In addition we will conduct a continuous, towed, undulating sensor package (Scanfish MkII) cruise each summer for hydrographic and biological variables.
Our group of collaborators has conducted research in an interdisciplinary way that promotes excellent science and that we have communicated in diverse ways to various societal sectors. We have participated in many agency, public, and private discourses to increase the understanding of these developments, including the usual scientific talks, but also an amazing variety of newspaper, radio, television, magazine and public. We will continue these activities with the addition of a hypoxia web site to be developed, housed, and maintained by the LUMCON research team.