Grantee Research Project Results
2000 Progress Report: Effects of Low Dissolved Oxygen on Trophic Interactions, Distributions, and Survival Within the Dominant Planktonic Food Web of a Eutrophic Temperate Estuary
EPA Grant Number: R827097Title: Effects of Low Dissolved Oxygen on Trophic Interactions, Distributions, and Survival Within the Dominant Planktonic Food Web of a Eutrophic Temperate Estuary
Investigators: Breitburg, Denise L. , Rose, Kenneth A. , Decker, Mary Beth , Purcell, Jennifer E.
Institution: Academy of Natural Sciences
EPA Project Officer: Hahn, Intaek
Project Period: May 1, 1999 through April 30, 2002
Project Period Covered by this Report: May 1, 1999 through April 30, 2000
Project Amount: $374,559
RFA: Exploratory Research - Environmental Biology (1998) RFA Text | Recipients Lists
Research Category: Biology/Life Sciences , Human Health , Aquatic Ecosystems
Objective:
High nutrient loadings and the resultant low dissolved oxygen concentrations are an increasing problem in estuaries and other coastal waters. In this research program, we seek to improve understanding and develop a predictive model of how low dissolved oxygen may alter trophic interactions, distributions, and larval fish survival in a eutrophic temperate estuary, the Chesapeake Bay. Specifically, we are examining: (1) how low dissolved oxygen affects predation on zooplankton, fish eggs, and fish larvae by the two dominant gelatinous predators in the Chesapeake Bay; (2) whether summer oxygen depletion mediates interactions between these two gelatinous species; and (3) whether and how the combination of increased zooplankton production and bottom-layer oxygen depletion (both resulting from excess nutrient loadings) favors dominance of the system by gelatinous predators.Our work includes a balanced program of field studies, laboratory experiments, and modeling. Vertical distributions of gelatinous zooplankton, mesozooplankton, and fish eggs and larvae will be determined through a series of cruises at sites with relatively stable and more variable low dissolved oxygen in the mainstem Chesapeake Bay and one of its tributaries, the Patuxent River. Avoidance behavior in response to low dissolved oxygen also will be tested in the laboratory in tanks that maintain a stratified water column. Effects of low dissolved oxygen on predation rates will be tested in 100 L and 1 m3 mesocosms and with video. A spatially explicit individual-based model will be used to synthesize data and predict effects of bottom-layer oxygen depletion in a stratified water column on plankton dynamics and survival of larval fishes. 3-D maps of habitat quality for medusa and ctenophores will be generated for current, improved, and worsened nutrient-loading scenarios.
Progress Summary:
Shifts in Vertical Distributions Within an Oxygen-Stratified Water Column
During summer 2000, we conducted three 24-hour cruises in the mainstem Chesapeake Bay to examine how the vertical distributions of animals change with variation in bottom dissolved oxygen concentrations. Samples were taken during day and night at two stations in the mesohaline portion of the bay. Tucker trawl samples (224- m mesh net with a 1-m2 opening) in the surface, pycnocline, and bottom layer were used to determine the coarse-scale vertical distribution of ctenophores, medusae, and fish eggs and larvae. Samples from approximately 2-m depth intervals in the bottom layer and 1-m intervals in the pycnocline and surface layers were pumped through a 30- m mesh net to provide detailed vertical distributions of zooplankton and fish eggs. Niskin sampler collections in the surface, pycnocline, and bottom layer will provide coarser-scale information on vertical distributions of microzooplankton. A video camera was used to determine fine-scale vertical distributions of ctenophores. Diver collections of ctenophores in the surface, pycnocline, and bottom layers will be used to calculate feeding rates in relation to dissolved oxygen and prey abundance. Sample and data analyses currently are in progress. Underwater video analyses, fine-scale zooplankton counts, and ctenophore gut contents have been completed for all cruises conducted in 1999 and 2000.
Summer 2000 was a highly unusual year in both the abundance and timing of organisms. We obtained important information on ctenophores and crustacean zooplankton. However, fish eggs and larvae, which typically peak in July and are abundant during June-August, were virtually absent in our July and August samples. Furthermore, the sea nettle (Chrysaora quinquecirrha), which is considered the dominant invertebrate planktivore in the system and the most important predator of early life stages of fishes, was extremely rare. Nevertheless, examination of Tucker trawl samples indicated that at the strongly stratified stations in the mainstem Chesapeake Bay, bay anchovy Anchoa mitchilli eggs were retained within the surface layer, but scieanid eggs primarily were found in the bottom layer at oxygen concentrations that were presumably lethal. Thus, the effect of low oxygen on fish egg mortality appears to vary both between the strongly stratified mainstem and the more weakly stratified Patuxent River, as well as between fish species. As in the Patuxent River, fish larvae, juvenile fishes, and ctenophores were virtually absent from the bottom layer in mainstem Chesapeake Bay samples with bottom dissolved oxygen concentrations <1 mg l-1.
Effects of Low Dissolved Oxygen on Predator-Prey Interactions
Laboratory and Mesocosm Experiments: Laboratory experiments are being used to test the effects of low dissolved oxygen on the behavior of ctenophores, sea nettles, fish larvae, and zooplankton. Swimming speeds of ctenophores and naked goby larvae were measured at a variety of dissolved oxygen concentrations for inclusion in the individual-based model. Sea nettles were maintained in the laboratory, and were subjected to four dissolved oxygen treatments (air saturated, 0.5, 1.5, and 2.5 mg l-1). Survival was documented over a period of several days. Our results indicate that sea nettles can survive prolonged exposure to hypoxic conditions.
We examined the possibility that Acartia tonsa (A. tonsa) copepods are locally adapted to hypoxia by distinguishing between phenotypic and genetic differences between populations. Both field-collected individuals and offspring of Chesapeake Bay A. tonsa, which have been historically exposed to oxygen gradients, avoided hypoxic bottom waters. In contrast, copepods from an area not typically characterized by oxygen depletion did not avoid lethal oxygen concentrations.
We also completed mesocosm experiments testing the effect of dissolved oxygen concentration on predation by sea nettles feeding on ctenophores. Although predation rates were highly variable, our data indicate that sea nettle predation rates are not affected by hypoxia at oxygen concentrations at which sea nettles and ctenophores commonly co-occur.
Progress in Model Development
We expanded the individual-based larval model code to include bioenergetics growth of the larvae based on zooplankton densities and temperature. In the previous versions of the larval model, larval growth rates were simply assigned. The enhanced model will be used to generate spatial maps of larval growth and mortality rates for the Chesapeake Bay using June and July bay-wide surveys. These surveys measured the density and lengths of larvae, densities of zooplankton prey for the larvae, and densities and lengths of invertebrate predators. We will use these data as input to the larval model and perform short-term (3-day) simulations to predict larval growth rates and survival rates at each station. From these maps of survival and mortality, we will identify areas that are good habitat for larval fish (bay anchovy in this case). Simulations under assumed dissolved oxygen scenarios will then be simulated to determine how different dissolved oxygen conditions affect the location and magnitude of good larval habitat.
We also initiated the coupling of a watershed water quality model to the larval model for the lower Patuxent River. The predicted daily output of dissolved oxygen from the water quality model is read into the individual-based larval model. Assuming a predator field, the larval model then predicts larval survival associated with the dissolved oxygen conditions. The water quality model will be run under a variety of assumed land-use scenarios; the output from water quality model read into the larval model, and the larval model used to predict larval survival at series of locations in the river. This coupling of the models allows a direct linkage for quantifying how land-use changes in the watershed can affect larval survival in the river.
A Monte Carlo uncertainty analysis program (called PRISM) has been linked to the individual-based larval fish model. The Monte Carlo analysis program generates values of model inputs from specified probability distributions. The larval fish model is then run repeatedly using new sets of input values for each run. Model predictions of larval survival are then analyzed statistically to determine which inputs most affect the predictions of survival. The Monte Carlo analysis will be used to determine which model parameters are important and need to be measured better, and used to compare how parameter importance may change depending on the environmental conditions (e.g., tributary versus mainstem bay).
Coding, data synthesis, and preliminary simulations of the spatial maps, linkage
to the water quality model, and Monte Carlo uncertainty analyses are underway
and will be completed during the next year.
Future Activities:
p> During Year 3, our primary efforts will be on conducting cruises in the mainstem Chesapeake Bay, continuing laboratory predation and behavior experiments, completing the expansion of the individual-based model, and manuscript preparation. Supplemental EPA funds were granted to conduct two cruises during summer 2001 to complete the data needed to assess our hypotheses. We are confident that by repeating the two unsuccessful Year 2000 cruises during 2001, we will obtain sufficient data to fully meet the project's objectives. Predation experiments between ctenophores and bay anchovy yolk sac larvae will be conducted in 100-L tanks at a variety of dissolved oxygen concentrations, and additional trials with bay anchovy eggs and naked goby feeding larvae at intermediate dissolved oxygen concentrations will be performed. The behavioral response of ctenophores to bottom-layer oxygen depletion, and its potential effect on their vertical distributions, will be tested in the laboratory in the absence of prey and predators, whose presence may confound interpretation of vertical distributions in the field. Swimming speeds of ctenophores and naked goby larvae at a variety of dissolved oxygen concentrations will be analyzed. Small-scale interactions between ctenophores and fish larvae at high and low dissolved oxygen concentrations will be videotaped to assess the behaviors of both species during a predation encounter. Our experiments will provide information on effects of low oxygen on encounter rates and provide parameters for the model. Modeling efforts will expand the current scope of the model to provide feedback among the model compartments. Manuscripts prepared during Year 3 will include: the effects of hypoxia on feeding and fine-scale vertical distributions of ctenophores, the behavior of copepods and ctenophores in response to bottom-layer oxygen depletion, and a comparison of trophic interactions in systems having relatively stable and more variable low dissolved oxygen.Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 2 publications | 1 publications in selected types | All 1 journal articles |
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Purcell J, Decker M, Breitburg D, Broughton K. Fine-scale vertical distributions of Mnemiopsis leidyi ctenophores:predation on copepods relative to stratification and hypoxia. Marine Ecology Progress Series 2014;500:103-120. |
R827097 (2000) |
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Supplemental Keywords:
anoxia, Chesapeake Bay, ecological effects, estuary, eutrophication, fish, plankton, hypoxia, individual-based model, water quality., RFA, Scientific Discipline, ECOSYSTEMS, Ecosystem Protection/Environmental Exposure & Risk, Aquatic Ecosystems & Estuarine Research, Aquatic Ecosystem, Aquatic Ecosystems, Environmental Monitoring, Ecological Risk Assessment, Ecology and Ecosystems, eutrophication, nutrient dynamics, ecosystem monitoring, estuaries, fish, aquatic food web, dissolved oxygen status, plankton, water quality, estuarine food web, trophic interactions, land useRelevant Websites:
http://www.anserc.org/research/projects/coastes/coastes.html Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.