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.
August 2006
Contents
“Compromise” Nutrient Model Produces Advice for Chesapeake Bay
Model Results Recommend Easing Up on Horseshoe Crab Harvest
Is Food Web Function Restored When Tidal Flow Is? Carbon Isotopes May Provide Clues
Southeast Estuaries May Be Joining the Hypoxia Club
“Compromise” Nutrient Model Produces Advice for Chesapeake Bay
Thanks to intense research and pioneering management programs, Chesapeake Bay serves as the archetypal system for estuarine nutrient management. The latest intergovernmental agreement guiding water quality management there is Chesapeake 2000, which recommits agencies to nutrient reduction goals established in 1987: overall, a 35% reduction in total nitrogen loads relative to 1980s levels. Some of the most important tools used in setting and evaluating these goals are mathematical models of nutrient and oxygen dynamics that can be used in a predictive mode. These models range from relatively simple – essentially regressions or correlations among nutrient loadings, dissolved oxygen levels, and other factors – to complex mechanistic dynamic simulations.
A recently-developed “middle ground” model predicts the extent and location of summer hypoxia in the Bay based on total N inputs from the Susquehanna River. The model loads nitrogen into the Bay at a single point near the Bay’s mouth, and converts it to organic matter that is decayed while being transported back upstream in bottom waters. The model predicts that the 35% N reduction called for in Chesapeake 2000 will decrease the Bay’s hypoxic volume by 36-68%, depending on upstream transport. Complete elimination of hypoxia would require a 73% load reduction, according to model forecasts. This type of intermediate-complexity model offers a number of advantages to managers: quantification of uncertainty is feasible, and it can be validated against field observations of state variables and process rates that are not included in simple regressions. Because of the uncertainties associated with any model, the authors caution that it is wise to use several independent modeling approaches for guiding important and expensive policy actions.
Source: Scavia, D., E. L. A. Kelly, and J. D. Hagy III. 2006. A simple model for forecasting the effects of nitrogen loads on Chesapeake Bay hypoxia. Estuaries and Coasts 29(4): 674-684. (View Abstract)
Model Results Recommend Easing Up on Horseshoe Crab Harvest
Great spawning aggregations of horseshoe crabs are a common summertime sight along Atlantic beaches. Unfortunately for these living fossils, they turn out to have many uses: they are harvested for bait, for scientific research, and for their blood which contains a substance important to the biomedical field. Concern about increasing harvest and population declines led to implementation of an Atlantic States Marine Fisheries Commission Fisheries Management Plan for horseshoe crabs. However, the lack of information about horseshoe crab life history continues to hamper management efforts.
By gathering existing data on life history traits, a stage-structured matrix model of horseshoe crab life history in three Cape Cod bays was recently constructed. The model was used to test the outcomes of potential fisheries management scenarios. Results suggested that the most effective approach for preventing population declines would be to focus management efforts on protecting larger juveniles and smaller adults, the life stages most vulnerable to additional mortality. The authors suggest that the existing practice of harvesting the largest females for the biomedical and bait industries could continue, but implementing a minimum harvest size of 220-240 mm would be helpful in maintaining a sustainable population. A delay in the start of harvest season could provide an additional buffer by allowing females to spawn before being removed from the population.
In one of the three bays studied, the horseshoe crab population was stable despite fairly heavy harvest, leading the investigators to theorize that the bay serves as a sink for crabs produced in the nearby Monomoy National Wildlife Refuge where harvest is banned. This interesting possibility argues for considering meta-populations in management strategies and suggests a role for marine reserves in maintaining some local populations.
Source: Grady, S. P., and I. Valiela. 2006. Stage-structured matrix modeling and suggestions for management of Atlantic horseshoe crab, Limulus polyphemus, populations on Cape Cod, Massachusetts. Estuaries and Coasts 29(4): 685-698. (View Abstract)
Is Food Web Function Restored When Tidal Flow Is? Carbon Isotopes May Provide Clues
Restoration of tidal flows to marshes overrun by invasive Phragmites australis has been undertaken at many East Coast sites. Once the flow is restored and salinity rises, the density of Phragmites tends to decrease and native Spartina returns. But does the food web return along with the native plants? Luckily for scientists interested in food web dynamics, the maxim “you are what you eat” holds true in marine systems: upper trophic levels generally maintain the carbon isotopic signature – the 13C:12C ratio – of the organic matter fueling their particular food web. Upper trophic levels in Phragmites-dominated marshes often show depleted δ13C signatures, while consumers in Spartina marshes tend to be relatively enriched in 13C.
In a set of New England salt marshes ranging from tidally-restricted (unrestored, Phragmites-dominated) to partially restored to Spartina-dominated (reference marshes), investigators examined C isotope ratios of a range of trophic levels. Marsh sites were paired, such that in each of the three salt marsh systems studied a site with unrestricted tidal flow was adjacent to a restricted or restored site. They were particularly interested in results from mummichogs, a common marsh fish that feeds on a wide variety of plant and animal matter. Mummichog δ13C values were greater for fish from Spartina-dominated marshes than for fish from tide-restricted sites; restored sites were intermediate between the two, reflecting the fact that they are still in the process of transforming from Phragmites- to Spartina-dominated systems. The study concludes that monitoring these carbon isotope ratios in fish and organic matter in marsh restoration projects is a good way to track the trajectory of restoration, especially with respect to food web function.
Source: Wozniak, A. S., C. T. Roman, S. C. Wainwright, R. A. McKinney, and M. James-Pirri. 2006. Monitoring food web changes in tide-restored salt marshes: A carbon stable isotope approach. Estuaries and Coasts 29(4): 568-578. (View Abstract)
Southeast Estuaries May Be Joining the Hypoxia Club
The textbook description of estuarine hypoxia development says that excess inorganic nutrient loading into surface waters of stratified systems fuels explosive algal growth. As this abundant production dies off, it sinks, decays, and thereby depletes oxygen in bottom waters. Conventional wisdom says that well-mixed estuaries, such as those in the southeast U.S., are immune to hypoxia because oxygen easily mixes to the bottom from the surface. However, an examination of long-term monitoring data reveals that well-mixed Georgia estuaries are in fact becoming more hypoxic. The data, the best of which came from the Skidaway estuary, reveal a decrease in both surface and bottom O2 concentration and saturation, and a concomitant rise in nutrient concentration – the ingredients of “traditional” hypoxia development. The authors conclude that nutrients, particularly organic nutrients, entering these estuaries from watershed runoff exceed the coastal systems’ ability to absorb them. Because bacterial concentrations also rose over the same time period, they further postulated that heterotrophs (bacteria, flagellates, and other small organisms) may the ones using the organic nutrients, at a rate faster than new oxygen could be mixed in.
If eutrophication can impact well-mixed systems previously considered “pristine,” some management strategies may need to be adjusted. For one thing, bacterial and other heterotrophic pathways may be more important than previously thought for understanding and managing some estuaries. The authors also recommend more systematic monitoring of organic nitrogen levels.
Source: Verity, P. G., M. Alber, and S. B. Bricker. 2006. Development of hypoxia in well-mixed subtropical estuaries in the southeastern USA. Estuaries and Coasts 29(4): 665-673. (View Abstract)
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