2000 Progress Report: Fisheries-induced changes in the structure and function of shallow water "nursery habitats": an experimental assessment

EPA Grant Number: R827072C009
Subproject: this is subproject number 009 , established and managed by the Center Director under grant R827072
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: Alabama Center For Estuarine Studies (ACES)
Center Director: Shipp, Robert L.
Title: Fisheries-induced changes in the structure and function of shallow water "nursery habitats": an experimental assessment
Investigators: Heck, Kenneth L. , Cowan, James H. , DeVries, Dennis R , Valentine, John F.
Institution: Dauphin Island Sea Lab
Current Institution: University of South Alabama
EPA Project Officer: Packard, Benjamin H
Project Period: October 1, 1999 through September 30, 2000
Project Period Covered by this Report: October 1, 1999 through September 30, 2000
RFA: Alabama Center For Estuarine Studies (ACES) (1999) RFA Text |  Recipients Lists
Research Category: Ecological Restoration , Targeted Research


Nutrient enrichment and overfishing are two of the most common man-induced perturbations of coastal systems. Eutrophication can produce many undesirable effects in coastal systems including: 1) a decline in submerged aquatic vegetation (SAV) through increased light attenuation and algal overgrowth, and 2) reduction in primary and secondary production (including commercially important finfish and shellfish that rely on SAV as a "nursery" ground) in near-coastal waters. Losses of top predators by overfishing could indirectly lead to the disappearance of SAV in both freshwater and marine ecosystems, producing a decline similar to that associated with eutrophication. Removing top, or apex, predators might result in the following "trophic cascade": 1) increased small fish densities, with a subsequent decrease in their prey (i.e. epibenthic algal grazers such as amphipods and snails); 2) increased fouling on SAV after decreases in grazer populations; and 3) loss of macrophytes due to overgrowth by algal epiphytes. Important differences in ecosystem responses resulting from the loss of predators to overfishing are, however, to be expected among riverine, estuarine, marine and freshwater ponds and lakes since cumulative effects are likely to be more profound in small, "closed" systems and less important in large, "open" systems. It has also been suggested that marine communities, with many omnivorous taxa and high levels of food-web redundancy may be less susceptible to "trophic cascades" than simpler fresh water communities. This "top-down" alternative to the "bottom-up" nutrient enrichment hypothesis may explain reductions in SAV biomass in heavily fished areas, but to date remains inadequately tested. We sought to remedy this by carrying out field experiments over a multi- year period. Specific objectives for this project are to: 1) develop a mechanistic understanding of the indirect effects resulting from overharvesting large predators in two different but common types of SAV-dominated aquatic ecosystems, 2) evaluate the degree to which "openness" influences the susceptibility to top/down effects, and 3) evaluate the degree to which "omnivory" influences the susceptibility to top/down effects.

Progress Summary:

Experiments were run concurrently at two separate sites during 2000: Big Lagoon, Florida and Mobile Bay Delta, Alabama. The dominant SAV species are Thalassia testudinum and Vallisneria americana, respectively. The experimental design is a two by two factorial with one factor being the degree of openness ("open" and "closed") and the second factor being amount of omnivory (omnivorous fish and strictly carnivorous fish), resulting in four treatments: 1) open/omnivorous, 2) open/ carnivorous, 3) closed/omnivorous and 4) closed/ carnivorous. Thirty 2m2 enclosures (6 replicates/ treatment plus 3 no fish/open; 3 no fish/ closed and 3 controls) were established at each of the study sites within the SAV beds. "Open" enclosures were constructed of 1" PVC frames covered with ?" bird netting, while the "closed" enclosures were constructed of a similar PVC frame covered with 1mm mesh. Controls consisted of 4 pieces of 1" PVC marking a 2m2 area. To ensure that effects were not missed, if they existed, enclosures were stocked with ten times the natural density of fish found within the respective systems (Perdido: omnivore = pinfish, carnivore = redfish; Delta: omnivore = bluegill, carnivore = bass). The experiments were conducted twice in Big Lagoon, Florida from June 8, 2000- August 9, 2000 and August 30, 2000 - October 30, 2000 during year 2 and in Mobile Bay from June 28, 2000 - August 25, 2000. During the second experimental run in Mobile Bay, a drought caused salinity to rise at our study site and exceed the tolerance levels of both the plants and fish, preventing any usable data from being obtained.

We had some difficulty last year in maintaining desired fish densities, due to escape and faulty enclosure design. However, by redesigning the enclosure (a slightly smaller mesh for the "open" enclosures and better reinforcement for the "closed" enclosures), we were able to recover " 70 % of the stocked fish in both the open and closed enclosures during year 2.

Samples are still being processed although some preliminary data analysis has begun. In Perdido during Run #1, there was a significant (p=0.021) difference in epiphyte abundance between open/omnivore and open/carnivore treatments, with more epiphytes present in the open/carnivore treatment. These results may be explained by heavy redfish consumption of the grazers within the enclosure, which allowed epiphyte abundance to increase. There was also a decrease in epiphyte abundance between open/omnivore and closed/omnivore treatments, with epiphyte abundance greater, though not significantly so, in the closed/omnivore treatment. One possible explanation for these differences is that pinfish are consuming both grazers and epiphytes, permitting epiphytes to increase slightly due to invertebrate grazer reduction but not significantly due to direct epiphyte grazing by the pinfish. Closed/ carnivore treatments also showed an increase in epiphyte abundance, although this increase was not significantly different from the other treatments. A Two Way ANOVA conducted to determine which factor ("degree of openness" or "degree of omnivory") contributed to the differences seen in epiphyte abundance showed that omnivore treatments had significantly (p<0.05) less epiphytes than carnivore treatments; whereas, there were no significant differences in the open or closed treatments. This may indicate that the degree of omnivory plays a greater role in trophic cascades than the degree of openness in Big Lagoon, FL.

In Mobile Bay, there were no significant differences in epiphyte abundance among treatments; however, there was greater epiphyte abundance in the closed treatments than in the open treatments. Within the open treatments, a greater decrease in epiphyte abundance was found in the omnivore treatment than in the carnivore treatment, although this difference was not significant. Within the closed treatments, a greater increase in epiphyte abundance was found in the carnivore treatment compared to the omnivore treatment, although these differences were also not significant. The trends in epiphyte abundance suggest that in Mobile Bay the degree of openness may play a greater role in trophic cascades than the degree of omnivory.

The trends seen in our data suggest that the two experimental systems (Big Lagoon and Mobile Bay) may respond to the influences of the "degree of omnivory" and the "degree of openness" differently. Big Lagoon seems to be influenced more by the degree of omnivory, while Mobile Bay seems to be influenced more by the degree of openness. The results of year three will help us draw more definite conclusions about these two systems and their susceptibility to the effects of overfishing.

For year three we plan on starting experiments in late May or early June 2001.

Supplemental Keywords:

estuarine research, coastal ecosystem, eutrophication, algal blooms, human modifications, water use, watersheds, aquatic ecosystems, fisheries, nutrients, biomass., RFA, Scientific Discipline, Water, ECOSYSTEMS, Ecosystem Protection/Environmental Exposure & Risk, Ecology, Aquatic Ecosystems & Estuarine Research, estuarine research, Ecosystem/Assessment/Indicators, Ecosystem Protection, Restoration, Aquatic Ecosystem, Ecological Effects - Environmental Exposure & Risk, Aquatic Ecosystems, Ecological Monitoring, Ecology and Ecosystems, Ecological Risk Assessment, Aquatic Ecosystem Restoration, Ecological Indicators, coastal ecosystem, eutrophication, water use, nursery habitats, estuaries, fish, watersheds, nutrients, biomass, fisheries, aquatic plants, algal blooms, submerged aquatic vegetation, ecosystem, environmental indicators, water quality, estuarine waters, human modifications

Progress and Final Reports:

Original Abstract
  • Final

  • Main Center Abstract and Reports:

    R827072    Alabama Center For Estuarine Studies (ACES)

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
    R827072C001 Fluorescent Whitening Agents As Facile Pollution Markers In Shellfishing Waters
    R827072C002 Red Snapper Demographics on Artificial Reefs: The Effect of Nearest-Neighbor Dynamics
    R827072C003 Stabilization of Eroding Shorelines in Estuarine Wave Eliminates with Constructed Fringe Wetlands Incorporating Offshore Breakwaters
    R827072C004 Interaction Between Water Column Structure and Reproduction in Jellyfish Populations Of Mobile Bay (SGER)
    R827072C005 Effects of Variation in River Discharge and Wind-Driven Resuspension on Higher Trophic Levels in the Mobile Bay Ecosystem
    R827072C006 Results of Zooplankton Component
    R827072C007 Benthic Study Component
    R827072C008 A Preliminary Survey of Macroalgal and Aquatic Plant Distribution in the Mobile Tensaw Delta
    R827072C009 Fisheries-induced changes in the structure and function of shallow water "nursery habitats": an experimental assessment
    R827072C010 Effects Of Variation in River Discharge and Wind-Driven Resuspension on Lower Trophic Levels of the Mobile Bay Ecosystem
    R827072C011 Evaluation of Alabama Estuaries as Developmental Habitat for Juvenile Sea Turtles
    R827072C012 Effects of Salinity Stress on Natural and Anthropogenically-Derived Bacteria in Estuarine Environments
    R827072C013 The Role of Land-Use/Land-Cover and Sub-estuarine Ecosystem Nitrogen Cycling in the Regulation of Nitrogen Delivery to a River Dominated Estuary; Mobile Bay, Alabama
    R827072C014 Environmental Attitudes of Alabama Coastal Residents: Public Opinion Polls and Environmental Policy
    R827072C015 Synthesis and Characterization of an Electrochemical Peptide Nucleic Acid Probe
    R827072C016 Determinants of Small-Scale Variation in the Abundance of the Blue Crab Callinectes Sapidus
    R827072C017 Effects of Estrogen Pollution on the Reproductive Fitness of the Gulf Pipefish, Syngnathus scovelli
    R827072C019 A Model for Genetic Diversity Aquatic Insects of the Mobile/Tensaw River Delta
    R827072C020 Evaluating Trophic Processes as Indicators of Anthropogenic Eutrophication in Coastal Ecosystems: An Exploratory Analysis
    R827072C021 Effects of Anthropogenic Eutrophication on the Magnitude and Trophic Fate of Microphytobenthic Production in Estuaries
    R827072C022 Characteristics of Ship Waves and Wind Waves in Mobile Bay
    R827072C023 Methods Comparison Between Stripping Voltammetry and Plasma Emission Spectroscopy for Metals in Mobile Bay
    R827072C024 Changes in Water Conditions and Sedimentation Rates Associated With Construction of the Mobile Bay Causeway
    R827072C025 Cold-Induced Hibernation of Marine Vibrios in the Gulf of Mexico: A Study of Cell-Cell Communication and Dormancy in Vibrio vulnificus
    R827072C026 Holocene Sedimentary History of Weeks Bay, AL: Human and Natural Impacts on Deposition in a Gulf Coast Estuary
    R827072C027 Shelter Bottlenecks and Self-Regulation in Blue Crab Populations: Assessing the Roles of Nursery Habitats and Juvenile Interactions for Shelter Dependent Organisms
    R827072C028 Predicting Seagrass Survival in Nutrient Enriched Waters: Toward a New View of an Existing Paradigm
    R827072C029 DMSP and its Role as an Antioxidant in the Salt Marsh Macrophyte Spartina alterniflora
    R827072C030 A Preliminary Survey of Aerial and Ground-Dwelling Insects of the Mobile/Tensaw Delta
    R827072C031 Natural Biogeochemical Tags of Striped Mullet, Mugil cephalus, Estuarine Nursery Areas in the North Central Gulf of Mexico
    R827072C032 Resolution of Sedimentation Rates in Impacted Coastal Environments Using 137Cs and 210Pb Markers: Dog River and Fowl River Embayments
    R827072C033 Investigation of the Use of Pulse Amplitude Modulated (PAM) Fluorometry as an Indicator of Submerged Aquatic Vegetation Health in Mobile Bay
    R827072C034 Influence of Invasive Plant Species in Determining Diversity of Aquatic Vegetation in the Mobile-Tensaw Delta
    R827072C035 The Influence of Shallow Water Hydrodynamics on the Importance of Seagrass Detritus in Estuarine Food Webs
    R827072C036 Food Web Interactions, Spatial Subsidies and the Flow of Energy Between the Mobile Bay Delta and Offshore Waters: A SGER Proposal to the Alabama Center for Estuarine Studies
    R830651C001 Meteorological Modeling of Hurricanes and Coastal Interactions: A Stability Study For Vertical Pressure Levels
    R830651C002 Characterization of Glycoprotein Cues Used by the Parasitic Rhizocephalan Barnacle Loxothylacus texanus To Identify Its Blue Crab Host, Callinectes sapidus
    R830651C003 Survey of Diamondback Terrapin Populations in Alabama Estuaries
    R830651C004 An Assessment of Environmental Contaminant Levels in Water and Dragonfly Larvae Tissues from the Mobile/Tensaw Delta