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.

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March 2011

Do Marsh Consumers Eat Off a Marsh Detritus or Phytoplankton Menu? Restoration Implications of Food Web Origins

While a restored tidal marsh might look very much like a nearby historical marsh, there are likely to be functional differences between the two, including the sources of organic matter fueling marsh food webs. In a mature marsh, marsh-derived organic matter is typically the basis for the food web; in contrast, young restored marshes may not exhibit sufficient primary production to feed consumers. Instead, organic matter “subsidies” are likely to come from elsewhere. In the case of San Francisco Bay (SFB), “elsewhere” is usually believed to be the open waters of the bay and its pelagic phytoplankton.

A recent study used stable isotope analysis to determine the food source for secondary consumers in marshes ranging in age from “ancient” to those restored 11 years ago. Isotopic signatures of primary producers and invertebrates and fishes collected from these marshes revealed that marsh sources were extremely important in all of these food webs, contributing an average of 76% of the organic matter to the diets of marsh consumers, including both resident and transient species. Surprisingly, there were no differences in the relative importance of marsh sources among wetlands of different ages. This result suggests that food webs in restored marshes function quite similarly to natural sites by the time they are ten years of age, good news for ecosystems and those interested in restoring them.

The results also indicate that marsh geomorphology may play an important role in these dynamics. More complex, dendritic channel morphology may trap marsh-derived materials, while linear marsh channels allow for greater flushing, and therefore loss of marsh-derived organic carbon (and greater introduction of bay carbon sources). This could be of particular interest to restoration practitioners as specific site restorations are planned.

Source: Howe, E. R. and C. A. Simenstad. 2011. Isotopic determination of food web origins in restoring and ancient estuarine wetlands of the San Francisco Bay and Delta. Estuaries and Coasts 34(March 2011). DOI: 10.1007/s12237-011-9376-8.

Eelgrass is on the Decline in Massachusetts, But Some Good News Gives Hope

Eelgrass meadows are as important as they are threatened: they serve as nursery habitat for fish and shellfish and stabilize sediments and shorelines, but they have been in decline globally for decades because of disease, nutrient pollution, and other factors. Monitoring and mapping of seagrass beds has become an important component of managing this critical resource. In Massachusetts, aerial surveys were used to estimate eelgrass (Zostera marina) acreage in about half of the state’s coastal embayments in 1994-1996, 2000-2002, and 2006-2007. Of the 46 embayments mapped in the first survey, 29 were remapped in the second survey and 33 in the third.

A recent comparison of the survey results revealed that eelgrass declined in 30 of the 33 embayments between the first and third surveys, with a median rate of decline of 2.9% per year. However, the second survey provided some positive news: in the embayments where acreages declined during both time intervals, the decreases were greater in the first interval than the second. Moreover, 11 of the 29 sites mapped in the second survey actually showed an increase by the time of the third survey. The best news was that significant expansions of eelgrass were observed in places where wastewater treatment upgrades had been implemented, including Boston Harbor and Gloucester Harbor. In Boston Harbor, eelgrass increased dramatically by the third sampling, after the major wastewater outfall was moved out of the harbor.

Studies of this kind can provide important information for prioritizing management actions and dollars. The results of this study in particular are being used by the Massachusetts Department of Environmental Protection in developing criteria for nitrogen total maximum daily loads (TMDLs).

Source: Costello, C. T. and W. J. Kenworthy. 2011. Twelve year mapping and change analysis of eelgrass (Zostera marina) areal abundance in Massachusetts (U.S.A.) identifies state wide declines. Estuaries and Coasts 34(March 2011). DOI: 10.1007/s12237-010-9371-5.
 

Potential Differences in Predation Vulnerability between Native and Non-native Oysters Could Influence Restoration Planning

Sometimes a careful risk-benefit analysis is a critical part of ecological restoration planning. One of the driving forces behind the tremendous declines in native oyster (Crassostrea virginica) populations in the Chesapeake Bay is parasitic disease, and so it would seem to make sense to undertake restoration using a more disease-resistant species such as the non-native Asian suminoe oyster (C. ariakensis), which is already being cultured in many places in the U.S. The two oysters are very similar, but are they similar enough to avoid the disasters that have occurred when other non-native species were intentionally introduced?

A recent study compared predation vulnerability of early- and late-stage larvae of the native oyster species and two strains of the non-native species to ctenophores, fish larvae, and barnacles. For all three predators, one or both strains of the non-native oyster experienced higher predation rates or was more highly preferred than the native. These disparities translated into differences in relative mortality rates. Of the three predators, ctenophores appeared to be the most important for oyster larvae as they displayed both significant differences in preference among larval prey types and a higher feeding rate overall than the other two predators.

The results of this study suggest that predation mortality could be much higher for C. ariakensis than for C. virginica larvae, a difference that might have significant implications for population viability of the non-natives in Chesapeake Bay. The authors caution that differences between natives and non-natives at all life stages must be considered when considering a deliberate non-native introduction.

Source: Fulford, R. S., D. L. Breitburg, and M. Luckenbach. 2011. Differences in relative predation vulnerability between native and non-native oyster larvae and the influence on restoration planning in an estuarine ecosystem. Estuaries and Coasts 34(March 2011). DOI: 10.1007/s12237-010-9360-8.
 

Marsh or SAV? Nekton Need Both, Especially as Sea Level Rises

Sea level rise and resultant salt water inundation is likely to wreak havoc with Louisiana’s already compromised coastal marsh system. Fragmentation of marsh habitats and salt water intrusion into fresh water systems are just two of the potential outcomes. More fragmented habitats have frequently been observed to contain less dense or diverse populations of resident fauna. A study in the Terrebonne Basin examined the effects of marsh fragmentation, emergent marsh type (defined by salinity regimes of freshwater, intermediate, and brackish), and presence of SAV on nekton density in marsh ponds at low tide. While a substantial amount of work has been done on these issues individually in the past, this study is unique in examining all of these factors at once. Fragmentation, SAV presence, and salinity interacted in sometimes complex ways to affect the densities of four marsh fish species studied in greater detail. Total nekton density increased strongly with the coverage of SAV in brackish marsh ponds, but only weakly in freshwater marshes. Fragmented freshwater and intermediate marshes actually had higher nekton densities than non-fragmented sites, but the opposite was true for brackish marshes (densities of nekton were lower in fragmented brackish marsh than un-fragmented marsh). This result could have significant implications for nekton populations as sea level rise brings more saline water to freshwater habitats and leads to greater fragmentation.

One crucial outcome of the study is that the presence of both emergent and submerged vegetation was necessary for maintaining high nekton densities, especially in brackish marshes. Restoration practitioners need to be aware of the importance of both types of vegetation in these systems.

Source: Hitch, A. T., K. M. Purcell, S. B. Martin, P. L. Klerks, and P. L. Leberg. 2011. Interactions of salinity, marsh fragmentation and submerged aquatic vegetation on resident nekton assemblages of coastal marsh ponds. Estuaries and Coasts 34(March 2011). DOI: 10.1007/s12237-010-9367-1.