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Coastal & Estuarine Science News (CESN)

Coastal & Estuarine Science News (CESN) is an electronic publication providing brief summaries of select articles from the journal Estuaries & Coasts that emphasize management applications of scientific findings. It is a free electronic newsletter delivered to subscribers on a bimonthly basis.

June 2007


Can Chesapeake Oysters Be Enlisted in the Eutrophication Battle?
Designing Marine Reserve Networks: Connectivity is the Key
Perspective: Restoration Ecology is not Equal To Ecological Restoration, But Both are Needed
Study Provides Preview of Effects of Sea Level Rise on Marsh Plants

Can Chesapeake Oysters Be Enlisted in the Eutrophication Battle?

Restoration of Chesapeake Bay oyster beds would likely win some battles against the effects of eutrophication, but not the war, according to a recent study.

While efforts to address eutrophication and related problems in the Bay have focused largely on reducing nutrient loadings, it has been suggested that restoration of the system’s filter-feeding native oysters might also help. A modeling study, using one of the main modeling tools developed to help manage the Bay, compared current water quality conditions with those predicted to occur with a ten-fold increase in oyster biomass (as called for by the US Environmental Protection Agency). Modeling results predicted that the increase in oysters would be associated with a 1 mg/m3 decrease in Bay-wide summer chlorophyll concentrations, with the greatest reductions in shallow tributary areas. Changes in dissolved oxygen varied depending on a site’s depth and stratification. During summer, in bottom waters subject to hypoxia, DO increases were predicted to be 0.25 g/m3. The biggest predicted improvement was a marked increase in submerged aquatic vegetation biomass of 25%, perhaps because oysters are often found near SAV beds and their presence helps to improve water clarity, a boon for the plants.

While the predicted improvements are in the right direction, they are not large enough to substitute for reductions in nutrient loading. The authors conclude that oyster restoration can be used as one tool to address Chesapeake Bay eutrophication, but it should be targeted at localities where it can make the most difference. They also note that wet years with low salinities and high solids loadings are hard on oysters, making the oysters’ benefits hard to discern under those conditions. Therefore, the benefits of oyster restoration will be best measured over multiple years encompassing a range of hydrologic conditions.

Source: Cerco, C. F., and M. R. Noel. 2007. Can oyster restoration reverse cultural eutrophication in Chesapeake Bay? Estuaries and Coasts 30(2): 331-343. (View Abstract)

Designing Marine Reserve Networks: Connectivity is the Key

If one marine reserve is a good approach to protecting an estuarine ecosystem, is a network of multiple sites better? The network approach works best if those sites are appropriately connected via the dispersal of larvae, and if most of the larvae remain within the boundaries of the network. A new study modeled larval dispersal in the eastern basin of the Strait of Juan de Fuca, Washington, to explore the issue of marine reserve network connectivity. The researchers used an existing numerical model originally designed to simulate trajectories of oil spills to predict the fates of passive particles, representing planktonic larvae, released from various locations. The virtual larvae were “released” at a shallow depth (11 m) and a deeper depth (27 m) at each release site and tracked for four days and 30 days.

As might be expected, larvae dispersed much farther after 30 days than after four. In fact, organisms with a short larval duration (i.e., those in the four-day simulation) were likely to settle near the release site. Release site location accounted for the greatest amount of variation in dispersal distance, exceeding variation due to year, month, or tidal phase. Three overall dispersal patterns related to release site location were observed: sites from which dispersal distance is short with low variance (i.e., larvae are clumped), sites from which dispersal distance is large with low variance (i.e., larvae uniformly disperse long distances), and sites with intermediate to long dispersal distances with high variance. Each dispersal pattern will contribute differently to connectivity among marine reserves, according to the authors. Sites in the first group would be best suited to a single large reserve, while those in the third group are the most likely to contribute to overall connectivity in a network of reserves. Sites characterized by long-distance low variance dispersal are less likely to contribute to connectivity within local networks, but might provide larvae to very distant sites.

This study highlights the need to know something about the early life history of organisms targeted for protection by marine reserves, as well as regional and site-specific oceanography.

Source: Engie, K., and T. Klinger. 2007. Modeling passive dispersal through a large estuarine system to evaluate marine reserve network connections. Estuaries and Coasts 30(2): 201-213. (View Abstract)

Perspective: Restoration Ecology is not Equal To Ecological Restoration, But Both are Needed

Restoration ecology, rooted in science, and ecological restoration, which responds to society’s priorities, are not one and the same, states a recent paper published in Estuaries and Coasts, but the most effective approach to restoration is to integrate the two. Especially in locations with high human population density, restoration initiatives should seek to balance human needs with those of the denizens of the habitat to be restored. Population density also forms the basis for the author’s proposal that estuaries should be divided into three categories for the purposes of restoration planning: urban-industrial estuaries which support intense human uses; production estuaries whose dominant use is harvest or culture of estuarine species; and conservation estuaries which are largely undisturbed systems with few human uses and low population density along their shorelines. While acknowledging that some systems contain elements of all three types, the author suggests that for all types, ecological and sociological criteria for successful restoration should be used in proportion to human population density in the landscape. The more people that live near a given estuary, the greater the weight that should be given to human dimensions of a project.

Finally, the author distinguishes between “restoration,” which he says should be used to describe practices that attempt to return estuarine ecosystems to some optimum biological integrity, and “rehabilitation,” the process by which degraded portions of human-dominated systems are renewed.  

Source: P Weinstein, M.. 2007. Linking restoration ecology and ecological restoration in estuarine landscapes. Estuaries and Coasts 30(2): 365-370. (View Abstract)

Study Provides Preview of Effects of Sea Level Rise on Marsh Plants

Coastal managers know they need to incorporate sea level rise into many of their planning and policy decisions, a tall order when the actual effects of sea level rise are not yet entirely known. A recent study demonstrates that the sea level rise picture is likely to be complex for marsh systems, as increased flooding of tidal marshes will have varying impacts on different marsh plant species.  

The investigators simulated sea level rise in experimental enclosures by varying salinity and flooding depths for three species of marsh plants. With this design, they could test the effects of flooding and salinity individually and look at interactions between the two factors. Panicum hemitomon, a freshwater marsh grass, was least affected by flooding at low salinities but was most sensitive to increased salinities, as measured by plant growth above ground and below ground. The most salt-tolerant species, Spartina patens, was the least flood tolerant. The brackish species tested, Sagittaria lancifolia, showed an intermediate response. Results also indicated that higher salinities led to reduced soil organic matter, as the plants held at higher salinities produced less below ground biomass.  

This study shows that the very structure of coastal wetlands will likely be altered by sea level rise, as community shifts will be governed by the responses of individual species to new environmental conditions.  

Source: Spalding, E. A., and M. W. Hester. 2007. Interactive effects of hydrology and salinity on oligohaline plant species productivity: Implications of relative sea-level rise. Estuaries and Coasts 30(2): 214-225. (View Abstract)