Grantee Research Project Results

1999 Progress Report: Effects of Nutrient Enrichment and Large Predator Removal on Seagrass Nursery Habitats: An Experimental Assessment

EPA Grant Number: R826098
Title: Effects of Nutrient Enrichment and Large Predator Removal on Seagrass Nursery Habitats: An Experimental Assessment
Investigators: Heck, Kenneth L. , Valentine, John F. , Pennock, Jonathan R.
Institution: Dauphin Island Sea Lab
EPA Project Officer: Hahn, Intaek
Project Period: January 1, 1998 through December 31, 1999
Project Period Covered by this Report: January 1, 1998 through December 31, 1999
Project Amount: $183,975
RFA: Exploratory Research - Environmental Biology (1997) RFA Text |  Recipients Lists
Research Category: Aquatic Ecosystems , Biology/Life Sciences

Objective:

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. Among them is a decline in submerged aquatic vegetation (SAV) through increased light attenuation and algal overgrowth of SAV leaves, which may outstrip the ability of their grazers to control them. Alternatively, reductions in the abundance and composition of predator populations also can produce profound effects in aquatic systems. A review of predator/prey interactions in SAV systems leads us to hypothesize that losses of top predators also could lead to the disappearance of SAV. Mechanistically, we expect that removing top predators would result in the following sequence of events: (1) increased small fish densities, with a subsequent decrease in their prey (i.e., epibenthic 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. Therefore, the predicted effects of eliminating top consumers are identical to those of eutrophication: namely, a shift from a system dominated by rooted macrophytes to a plankton-dominated system. This "top down" alternative to the "bottom up" nutrient enrichment hypothesis could account for reductions in SAV biomass in heavily fished areas.

Therefore, the objective of this project is to determine which factor (i.e., "top down" or "bottom up") or combination of these factors may lead to SAV decline in the Northern Gulf of Mexico. This is being tested through a series of treatment combinations in experimental cages of Halodule wrightii. The treatment combinations will allow us to elucidate the degree to which each factor (nutrient and/or overfishing) is contributing to SAV decline.

Progress Summary:

The experiment began in May with deployment of the enclosures and our initial sampling of shoot density, shoot biomass, epiphyte biomass, and invertebrate grazer density and biomass. Fertilizer was added to half the enclosures to simulate nutrient loading into the SAV bed (i.e., "bottom up" effect). Fish also were removed from one-third of the cages, left at ambient densities in one-third, and elevated to ten times ambient density in the final third to examine the effect of omnivorous fishes on the SAV community (i.e., "top down" effect). Samples were then collected in July (midpoint) and October (endpoint). Plant clips were taken to look at shoot density, shoot biomass, epiphyte biomass, and C/N ratios for nutrient enrichment. Suction samples were taken for invertebrate grazer density and biomass. All enclosures were seined to look at fish composition within each enclosure. Lastly, water samples were taken from within each cage to verify nutrient loading into particular treatments. Currently, we are processing the samples, with this being about 75 percent complete.

The relevance of this work addresses several national research priorities, including the need to understand more fully the effects of nutrients on the productivity of coastal systems, the factors responsible for changing the abundance of SAV, and the effects of cumulative impacts on the structure and functioning of aquatic ecosystems. The latter is of particular importance, as it is increasingly recognized that our approach to understanding and managing the health of coastal ecosystems must include the simultaneous assessment of multiple stressors.

Future Activities:

First, we will finish the work-up of Year 1 samples and results (April). These results will allow us to address any unforeseen variables in Year 2 that were not addressed in the Year 1 experiment. In Year 2, we will replicate the experiment to address potential temporal variability within the SAV habitat. The replication experiment will run from June through October 2000, the growing season of the experimental SAV. Year 2 data then will be compared with Year 1 to determine which above factor ("bottom-up" or "top-down") or combination of these factors can be attributed to SAV decline in the Northern Gulf of Mexico.

Supplemental Keywords:

marine, estuary, ecological effects, ecosystem, restoration, ecology, Gulf Coast., RFA, Ecosystem Protection/Environmental Exposure & Risk, Scientific Discipline, Geographic Area, Water, Ecological Indicators, State, Ecosystem Protection, Ecosystem/Assessment/Indicators, Ecological Effects - Environmental Exposure & Risk, Hydrology, Nutrients, Gulf of Mexico, Ecology and Ecosystems, Environmental Monitoring, ecological effects, phytoplankton dynamics, nutrient stress, submerged aquatic vegetation, submerged aquatic ecosystems, ecological assessment, coastal ecosystem, aquatic ecosystem, nutrient sensitive ecosystems, ecosystem management, plankton, eutrophication, Alabama (AL), hydrological stability, ecological exposure, algal growth, anthropogenic stresses, estuarine ecosystems, food web, fisheries, seagrass nursery habitats, fish , nutrient management, nutrient supply, anthropogenic stress, nutrient cycling, large predator removal

Progress and Final Reports:

Original Abstract

1998

Final