CESN Main Page

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.

September 2011


What Contributes to Resilience in Restored Eelgrass Beds? Case Studies Reveal Some Important Lessons
Exotic Species, Exotic Locale: Distribution and Abundance of Invasive Fish Species in Hawaiian Wetlands
Study Dissects Relationships among Hypoxia, Stratification, and Nutrients in the Chesapeake
Oysters and Wastewater Outflows are a Bad Mix, but Season and Distance Make a Difference

What Contributes to Resilience in Restored Eelgrass Beds? Case Studies Reveal Some Important Lessons

“Resilience” is a term that has gained popularity in recent years in ecosystem management contexts, but its use in ecology dates back at least to a 1973 paper in which C. S. Holling described resilience as the ability of a system to absorb changes in different types of variables and continue to persist. Resilience is often an implicit goal of ecological restoration projects: to restore a system to a state which is resistant to change.

Estuarine eelgrass restoration is the ideal system in which to study resilience, as eelgrass habitats are well-studied, they often consist of a near-monoculture of eelgrass, and the estuarine systems in which they are found are highly variable. A recent exploration of resilience of eelgrass restoration projects in the Pacific Northwest examined three case studies that varied in length of monitoring post-restoration (three to 12 years) and in extent of success. While two of three sites at the restored areas did ultimately exhibit healthy eelgrass beds, attesting to the resilience of this ecosystem type, one site was smothered by an unusually heavy macroalgae bloom.

Eight resilience planning elements for eelgrass are recommended, including choosing appropriate metrics for post-restoration monitoring (shoot density was used in this study); planting a minimum viable initial density of shoots to buffer against losses during the initial years; considering the importance of broad ecosystem-scale processes affecting relevant water and sediment quality conditions; and consideration of ecological “tipping points” at which recovery by planted shoots is not possible (like the macroalgae bloom mentioned above). The three projects presented indicate that in general restored eelgrass beds can be highly resilient, but multi-year monitoring is necessary to reveal patterns of persistence.

Source: Thom, R. M., H. L. Diefenderfer, J. Vavrinec, and A. B. Borde. 2011. Restoring resiliency: case studies from Pacific Northwest estuarine eelgrass (Zostera marina L.) ecosystems. Estuaries and Coasts 34(August 2011). DOI: 10.1007/s12237-011-9430-6.

Exotic Species, Exotic Locale: Distribution and Abundance of Invasive Fish Species in Hawaiian Wetlands

The perils of invasive exotic species are most pronounced in island ecosystems, where endemic species can more easily be eradicated and invaders have fewer competitors and predators. Hawaii has a particular problem with invasive species, and wetland-associated nekton are no exception. Exotic fish often thrive in Hawaiian wetlands after having been released into the wild for purposes of mosquito control, food, or as a way to dispose of unwanted aquarium pets. A recent survey examined nekton communities in 38 Hawaiian coastal wetlands on the five main islands, and analyzed correlations between densities of invasive and native species and hydrographic and water quality parameters.

The majority of wetlands sampled were dominated by exotic species (densities of invasives were up to 15 times greater than natives), mainly poeciliids (mollies) and tilapia. Native species were more commonly found in open and semi-open wetlands, whereas exotics dominated the closed or hydrographically isolated wetlands. Isolated wetlands also exhibited generally higher temperatures and nutrient concentrations than open systems, the latter of which were correlated with invasive fish densities. The authors suggest that future wetland restoration projects in Hawaii take these results into account by attempting to maintain hydrological connections between the ocean and wetland ecosystems when possible, and to incorporate plans to control and eradicate exotic fish.

Source: MacKenzie, R. A. and G. L. Bruland. 2011. Nekton communities in Hawaiian coastal wetlands: the distribution and abundance of introduced fish species. Estuaries and Coasts 34(August 2011). DOI: 10.1007/s12237-011-9427-1.

Study Dissects Relationships among Hypoxia, Stratification, and Nutrients in the Chesapeake

According to conventional wisdom resulting from decades of research, nutrient loadings and stratification generally lead to summer bottom hypoxia in the Chesapeake Bay. This familiar tale is taking on novel twists based on a new analysis of long-term data sets. Researchers think that while nutrients are still key, large-scale climatic forces are playing a previously unacknowledged role.

Detailed analyses of 60 years of monitoring data revealed a subtle but important difference between early and late summer hypoxia trends. The volume of hypoxic water has decreased slightly in the late summer over that time period, likely driven by spring nutrient loading, which has decreased. The opposite is true in the early summer: hypoxia in June and early July has actually increased, despite decreases in nutrient loadings brought about by management actions. The early summer hypoxia was tied to an increase in bay stratification strength in June during the study period. Why the increased stratification? The most likely culprit is sea level rise, which has increased the bay’s longitudinal salinity gradient.

The study also demonstrated that the duration of summertime hypoxia is related to nitrogen loading, and so nutrients are not off the hook as drivers of hypoxia. Continued nutrient controls are still critically necessary in the Chesapeake, and have already led to water quality improvements.

Source: Murphy, R. R., W. M. Kemp, and W. P. Ball 2011. Long-term trends in Chesapeake Bay seasonal hypoxia, stratification, and nutrient loading. Estuaries and Coasts 34(August 2011). DOI: 10.1007/s12237-011-9413-7.

Oysters and Wastewater Outflows are a Bad Mix, but Season and Distance Make a Difference

As filter feeders, oysters are what they eat. That’s exactly why areas influenced by wastewater effluent are generally closed to shellfishing. But how far does that closed area need to extend? And are all seasons equally problematic for wastewater contamination? In a study of oysters in Mobile Bay, AL, investigators addressed these questions, while also evaluating the best methods for monitoring oyster beds for contamination that could affect human health.

Investigators sampled transplanted oysters along a transect at increasing distances and with dilution from a wastewater treatment plant in three seasons. They used both ecological and human health-related indicators to evaluate risk related to wastewater effluent. Oyster growth varied seasonally, but there was no difference attributable to distance from the treatment plant. Survival of the oysters was uniformly high at all locations. Both fecal coliforms and male-specific coliphage (MSC, a proxy for human enteric viruses and more indicative of the presence of human-specific pathogens than coliform) were highest closest to the effluent and did not vary with season. MSC decreased significantly with distance from the plant, suggesting that it is a better indicator than fecal coliforms. Stable isotope analyses revealed that the 15N values of the experimental oysters shifted over time to reflect the changing contribution of effluent-derived material in their diet (which was highest in summer when growth was fastest).

So while the oysters themselves didn’t seem to mind being near the plant, as indicated by growth and mortality measurements, it is clear that oysters found closer to the plant could present greater human health risk if they are consumed. The authors suggest that monitoring beds using a combination of MSC and 15N assays along with hydrographic studies of effluent dilution might provide the best option for determining oyster exposure to effluent. In addition, seasonal and spatial effects of effluent need to be considered when defining wastewater influence on a system or species.

Source: Biancani, P. J., R. H. Carmichael, J. H. Daskin, W. Burkhardt III, and K. R. Calci. 2011. Season and spatial effects of wastewater effluent on growth, survival, and accumulation of microbial contaminants by oysters in Mobile Bay, Alabama. Estuaries and Coasts 34(August 2011). DOI: 10.1007/s12237-011-9421-7.