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. , Pennock, Jonathan R. , Valentine, John F.
Institution: Dauphin Island Sea Lab , University of South Alabama
Current Institution: Dauphin Island Sea Lab
EPA Project Officer: Hahn, Intaek
Project Period: January 1, 1998 through December 31, 1999
Project Amount: $183,975
RFA: Exploratory Research - Environmental Biology (1997) RFA Text |  Recipients Lists
Research Category: Biology/Life Sciences , Ecosystems


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 can also produce profound effects in aquatic systems. A review of predator/prey interactions in SAV systems leads us to hypothesize that losses of top predators could also 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, but to date remains untested.


We will use sustained multi-year field experiments to assess the individual and combined effects of removing top predators and enriching water column nutrients (N and P) on coastal ecosystem structure and function. We will focus our attention on shoalgrass (Halodule wrightii) habitats near Point aux Pines, Alabama, a protected research site with fringing seagrass habitat that contains abundant populations of small crustacean and gastropod mesograzers. We will use 7.5 m2 enclosures stocked with elevated 10x ambient densities of juvenile pinfish (Lagodon rhomboides) and other members of the local fish assemblage to simulate the effects of large predator reductions, and we will use an in situ delivery system to supplement N and P in nutrient addition treatments. Monthly determinations of water column nutrients and chlorophyll a, along with measurements of the biomass and abundance of leaf epiphytes and seagrass production, biomass and shoot and leaf densities will be used to evaluate the relative effects of manipulating nutrient supply and altering food web structure.

Expected Results:

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. The proposed studies emphasize the two most common perturbations of coastal systems - eutrophication and overfishing - and investigates their separate and combined impacts. Because these perturbations are occurring at an advancing rate throughout the world, with concomitant losses of highly productive seagrass "nursery" habitat, it is important for managers to understand some of the indirect, but potentially very important, consequences that might result from reducing large predator influence while at the same time increasing nutrient supply. This is particularly true, for example, in areas like the Chesapeake Bay or the Florida Keys, where very expensive nutrient reduction plans are being implemented or contemplated in hopes of permitting the reestablishment of seagrass beds. If food web effects due to fishing activities are significantly involved in seagrass losses in such areas, costly reductions in nutrient supply may not ensure the return of seagrass habitat. Finally, it should be noted that this work focusses on the Gulf coast, a region increasingly threatened by rapid development, and designated by EPA and other federal agencies, as deserving special attention.

Supplemental Keywords:

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

Progress and Final Reports:

  • 1998
  • Final