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Current CSCOR-Funded Research in Northern Gulf of Mexico
Continuing FY 2002 Projects
Which Nutrients Really Determine C Flux and Hypoxia
Quay Dortch and Nina de Luca - Louisiana Universities Marine Consortium
The large area of hypoxic bottom water on the Louisiana shelf is one of the worst cases of nutrient enhanced coastal eutrophication in the US. There has been considerable debate about the need to reduce nutrients as a management strategy. Central to that debate is determining which nutrient should be reduced. This is a particularly difficult question because at present nutrient inputs are nearly balanced with regard to phytoplankton requirements. Consequently, small seasonal changes in nutrient input ratios cause silicon (Si), phosphorus (P), or nitrogen (N) to be limiting at different times of the year. There is general agreement that N availability determines the overall productivity of the system. However, recent evidence suggests that Si or P availability may, at least in part, regulate carbon flux, and, thus, hypoxia, through their effect on diatoms. Diatoms are often the dominant phytoplankton group, may account for most of the sinking C flux leading to hypoxia, and, may be uniquely sensitive to Si and P limitation in comparison with other phytoplankton. Thus, the effect of nutrient availability on C flux, and, thus, hypoxia, may be much more complex than previously realized.
A large data set is now available which can be used to test hypotheses about the relationship between nutrient availability, phytoplankton species composition, and carbon flux. The data have been collected since 1989 and include moored and floating sediment trap data, a 10+ year time series of phytoplankton counts at one station, and phytoplankton counts on 24 cruises, covering various parts of the Louisiana shelf, along with nutrient and other environmental data. Parts of this data have been analyzed for specific purposes, but it has never been used as a whole to evaluate larger questions. We propose to answer the following questions.
- Are certain groups of phytoplankton associated with high carbon flux, either by direct sinking or fecal pellet production?
- What environmental conditions, particularly with regard to nutrients, lead to the growth and C flux of the phytoplankton identified in (1)?
- Can variations in the abundance of phytoplankton groups or the nutrient conditions that stimulate their growth or sinking be related to differences in hypoxia from year to year or seasonally?
This is a cost-effective approach to answer the question about what nutrients really determine C flux and hypoxia, since it involves retrospective analysis of data that are already available. Answering this question will provide guidance for future research efforts and help focus the debate about management options.
An Integrated Monitoring and Modeling Assessment of the Oxygen Sources and Sinks in the Gulf's Hypoxic Zone.
Dubravko Justic' and Brian Fry - Louisiana State University
Problem: The northern Gulf of Mexico is presently the site of the largest (up to 20,000 km2) and most severe hypoxic zone in the western Atlantic Ocean. Increased primary production in offshore Mississippi River plume and strong water column stratification are the primary biological and physical agents whose synergistic action leads to hypoxia.
There is a general consensus among scientists that large-scale reductions in the nitrogen and phosphorus fluxes of the Mississippi River would eventually lead to a decrease in areal extent and severity of hypoxia. This is supported by model scenarios that demonstrate a general increase in the bottom oxygen content in response to a decreased freshwater and nutrient loading from the Mississippi River. However, the degree of nutrient reduction needed to produce temporal or spatial diminishment in the hypoxic zone cannot be determined from the existing models. This is largely due to poor model parameterization arising from the lack of detailed understanding of the partitioning of oxygen dynamics amongst the key biological and physical processes. Thus, in spite of the considerable monitoring efforts carried out since 1985, imprecise filed oxygen budgets strongly limit the accuracy of model forecasts and hindcasts about timing and extent of hypoxia.
Objectives and approach: Overall project objectives are to: (1) use a dual budget, cross-checking approach to partition oxygen dynamics amongst the key biological and physical processes for the Gulf's hypoxic zone, and, (2) extend ongoing modeling efforts to predict probable changes in oxygen budgets and severity of hypoxia under hypothetical altered hydrologic and climate change scenarios. A dual budget, field measurement approach is proposed to quantify key physical and biological processes leading to hypoxia. Budget 1 is based on conventional oxygen concentration measurements, while budget 2 will use isotopic measurements of this same oxygen as a second, independent, and novel way to assess oxygen dynamics. Oxygen concentration measurements are already being made as part of ongoing monitoring programs, but proposed additional oxygen isotope measurements will greatly enhance our understanding of oxygen dynamics in the hypoxic regions. Proposed oxygen isotope studies will particularly help identify natural variations in P:R ratios and sources of benthic oxygen demand. The overall dual budget approach should considerably enhance and cross-check field estimates of key processes to be incorporated into ongoing modeling efforts that use our published 1-dimensional coupled physical-biological oxygen model.
Relevance: Proposed research presents a novel, integrated measuring-modeling approach to the study of hypoxia in the northern Gulf of Mexico. This research focuses on specifically improving our understanding of oxygen dynamics, a vital focus because it is these oxygen dynamics that are key endpoints in future management decisions about hypoxia.
Hypoxia, Nekton and Habitat in the Northern Gulf of Mexico : Modeling and Retrospective Analyses
Donald M. Baltz and Kenneth A. Rose - Louisiana State University
Edward J. Chesney - Louisiana Universities Marine Consortium
Hiram W. Li - Oregon State University
Introduction/Rational: The northern Gulf of Mexico (nGOM) is one of the most valuable coastal ecosystems in the world. The seasonal development of a large zone of hypoxic bottom water on the Louisiana-Texas shelf is a consequence of the substantial primary production of the area coupled with strong stratification along the coast. Nutrients discharged from the Mississippi River are the primary stimulus for the prolific primary and secondary production. Low-oxygen bottom water (hypoxia) has been a recurrent seasonal feature of this highly productive area for many years, but recent monitoring and paleo-environmental indicators suggest that the hypoxia in the nGOM may be worsening, yet the fisheries yields to the area remain substantial. Because of current and potential future effects on fish habitat, foo-web structure and other issues, hypoxia is considered a threat to the sustainability of high fisheries production in the region.
Scientific Objectives: We will undertake retrospective statistical analyses and qualitative modeling of the recent and historical fishery and fishery-independent data for the nGOM. The goal of these analyses will be to identify those species of nekton that are most likely to suffer habitat effects from hypoxia, document potential population or community structure changes that may be a response to hypoxic conditions and then explore qualitative linkages that can explain the observed effects.
Summary of Work: We will undertake a comprehensive analysis of existing fisheries and fishery-independent surveys for the nGOM system. We will compile relevant fisheries related data (bycatch, fishery and fishery-independent surveys), including data sets not in electronic format and not previously analyzed. We will: 1) compile the data sets and check them for errors, 2) develop life history matrices for key species of nekton, 3) analyze the community structure of nekton in the coastal zone of the nGOM with non-parametric methods, 4) apply qualitative models (Loop0 Analyses) to the ecosystem, 5) review results of Loop modeling and statistical analyses in the context of hypoxia and environmental factors affecting nekton, and 6) develop Habitat Suitability Indices for key species and life history stages to describe essential fish habitat and relate retrospective analyses and models to observed changes in fish assemblages.
Population-Level Effects of Large-Scale Hypoxia on Shrimp and Fish in the Northwestern Gulf of Mexico
Larry B. Crowder - Duke University
Coastal ecosystems dominated by major river systems are important areas of biological productivity but are under increasing stress due to anthropogenic activities in adjacent coastal watersheds. The northwestern Gulf of Mexico currently experiences the largest seasonal hypoxic zone in the northern hemisphere with an areal extent of up to ~20,000 km 2 or about one-fourth of the Louisiana continental shelf. Hypoxia in the Gulf is a result of nutrient loading from the Mississippi River system which drains ~40% of the conterminous United States. Areas of the Louisiana shelf now subject to hypoxia and unavailable to demersal species historically supported the highest levels of shrimp and fish in the region. Though the Gulf contributes disproportionately to the landings and value of U.S. commercial fisheries, little is known about how hypoxia impacts these upper trophic levels. This proposal addresses the hypothesis that hypoxia in the northwestern Gulf of Mexico impacts the biological production of demersal species such as shrimp and fish. an integrated approach that includes analysis of existing, long-term datasets, modeling, and targeted field studies in conjunction with existing monitoring programs is proposed to address this hypothesis.
The primary objectives are:
- Test the hypothesis that hypoxia alters movement, spatial distribution, growth, and condition of brown shrimp (Penaeus aztecus) and Atlantic croaker (Micropogonias undulatus),
- Estimate the mean and variation in immigration rates, growth rates, and mortality rates to structure and parameterize preliminary production models,
- Develop bioenergetics models to evaluate the effect of hypoxia on growth potential of shrimp and croaker,
- Conduct targeted field sampling to measure changes in diet, growth, and condition associated with hypoxia, and
- Develop preliminary production models to assess the population-level impacts of hypoxia.
A long-term, fishery-independent survey has been ongoing in the northwestern Gulf under the Southeast Area Monitoring and Assessment Program (SEAMAP) since 1982. The survey collects information on both environmental conditions in the Gulf, including bottom dissolved oxygen, and biological information on shrimp and fish from bottom trawls. Statistical analyses and modeling of these data will be used to address objectives (1) and (2). Bioenergetics models are an effective means of addressing the growth potential of organisms that inhabit particular environmental conditions and will be used to address objective (3). Objective (4) will be addressed by collecting information on diet, growth and conditions onboard the ongoing SEAMAP surveys and in a parallel sampling cruise with ongoing efforts to map the spatial extent of the hypoxic zone. Objective (5) will integrate this information into a modeling framework to estimate relative differences in production of shrimp and croaker for differing hypoxic conditions.
Considerable effort has been devoted to understanding the relationship between nutrients and hypoxia while much less is known regarding effects of hypoxia on upper trophic levels. The proposed work will provide insight on the population-level effects of hypoxia on biological resources of ecological and economic importance to the Gulf of Mexico.
Development of Larval Fish Vertical Distribution Models for use with Physical Transport Models, Biological Production Models and Hypoxia Effect models in the Northern Gulf of Mexico.
Jonathan A. Hare and John J. Govoni - Center for Coastal Fisheries and Habitat Research
The ability to model the vertical distribution of fish early life history stages is an integral step toward predicting how changes in the physical, chemical, and biological environment affect marine fish populations in the northern Gulf of Mexico . Fisheries in the northern Gulf of Mexico are ecologically important and economically productive. Changes in the northern Gulf of Mexico environment (e.g. river discharge, nutrient loading, hypoxia) will have effects on fisheries production: species composition, distribution, abundance, etc. Natural variability in fish population dynamics largely results from variability in the processes that affect survival of the early life history stages. Modeling offers an explicit approach to understanding how changes in the northern Gulf of Mexico affect survival of early life history stages of fishes and ultimately the dynamics of adult populations. Thus, to understand and predict changes in fish population dynamics resulting from changes in the northern Gulf of Mexico environment, it is necessary to determine and model the egg, larval, and juvenile fish survival as a function of environmental forcing. Environmental changes in the northern Gulf of Mexico affect horizontal and vertical ecosystem properties. In the vertical dimension, freshwater discharge forms a nutrient rich, buoyant plume, thereby influencing phytoplankton, zooplankton, and ichthyoplankton community processes and the physical transport of larval fishes. Nutrient loading and increased phytoplankton production, coupled with summertime hydrographic stratification leads to the development of a bottom hypoxic zone, which can affect larval survival directly (i.e. death), trophodynamically (e.g. changes in larval fish prey and predator fields), or physically (e.g. affecting vertical distributions resulting in altered transport pathways from spawning grounds to nursery habitats). Survival of egg, larval, and juvenile fishes in the northern Gulf of Mexico is likely to be directly linked to their vertical distributions in and around the expanding hypoxic zone. Models of early life history stage (ELHS) vertical distribution are required to link fish survival to environmental changes occurring in the northern Gulf of Mexico. The purpose of this proposal is to 1) conduct retrospective analyses of extant depth-discrete ichthyoplankton collections and develop preliminary, species-specific ELHS vertical distribution models; 2) conduct a field experiment to validate and calibrate these models to specific environmental variables important in the northern Gulf of Mexico; and 3) based on field validation and calibration, reformulate these ELHS vertical distribution models for inclusion in broader biological and physical models developed by other researchers in the N-GOMEX program.