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.
April 2010
Contents
Heavy Metals Weigh on Benthic Community in a Hong Kong Harbor
An Eelgrass Success Story in Boston Harbor: Careful Site Selection and Planting Methods are the Keys
Is Invasion by an Introduced Species Always an Ecological Disaster? Perhaps Not, Says Alabama Study
Restoration Projects in Elkhorn Slough Have Led to Improvements in Water Quality, Study Says
Heavy Metals Weigh on Benthic Community in a Hong Kong Harbor
In a study adding to the ample evidence that heavy metal contamination in coastal and estuarine sediments impacts biota, researchers recently found that the health of macrobenthic communities in Tolo Harbor, Hong Kong, is correlated with metal concentration.
Tolo Harbor is a heavily industrialized, nearly land-locked estuary in northeastern Hong Kong that receives both natural and anthropogenic loadings of a variety of heavy metals. The investigators measured metals in harbor sediments and took benthic samples to determine species abundance, richness, evenness, and diversity. Overall, abundance and diversity of macrobenthos are low in this system (some sites were devoid of any species), but strong negative correlations were recorded between diversity of macrobenthos and heavy metal concentrations. Metal concentration did not seem to have an effect on species evenness. Crustaceans, typically intolerant of metal contamination, were found in very low abundance, while more tolerant polychaete species dominated the benthic community. The pattern of occurrence of benthic species in this system follows a classic successional model along a pollution gradient: normal community structure in the relatively unpolluted sites, dominance by opportunistic, pollution-tolerant species at more polluted sites, and an absence of macrobenthic life in the most polluted sediments.
Source: Chen, K., S. Tian, and J. J. Jiao. 2010. Macrobenthic community in Tolo Harbour, Hong Kong and its relations with heavy metals. Estuaries and Coasts 33(3). DOI: 10.1007/s12237-010-9271-8.
An Eelgrass Success Story in Boston Harbor: Careful Site Selection and Planting Methods are the Keys
Most evidence indicates that the well-known relocation of the Boston Harbor sewage outfall has resulted in a wide range of ecosystem improvements in the inner harbor. Now that the harbor has become healthier, managers and researchers felt that the time was right to attempt eelgrass bed restoration. But they wanted to take a careful approach, choosing sites and planting methods deliberately. Their success – the 2 ha restoration is the most successful eelgrass restoration in Massachusetts to date – contains a number of lessons for those working in other systems.
Project personnel began by using established selection criteria as a broad-brush tool to eliminate some candidate sites, and they found that ground-truthing and test planting were essential. Even sites at which grain sizes were within published ranges for successful restoration did not always work: the researchers found successful sites all had a silt-clay content of <35%, much lower than the <70% recommended by others. This difference may be related to water quality at the restoration sites which, while improved, is still not pristine. It also suggests that sediment grain size may remain an issue for eelgrass re-population, even after water quality has improved. Several planting methods were compared. Clump harvest from a donor bed and hand planting were the most efficient methods of transplanting whole plants, but frame planting uses more volunteers, which could be a positive in some projects. Seeding populated far more ground than transplanting, with a smaller investment of time and resources, but met with uneven success.
This project was a resounding success: transplanted sites met and exceeded reference and donor bed habitat function after only two years. The authors attribute this success to their intense focus on site selection, ground-truthing, and transplant methods, but they note that this process is very time-, labor-, and resource-intensive. They suggest that mitigation money could be better spent addressing the sources of eelgrass decline in a watershed context. They describe two methods for funding larger scale projects aimed at preventing eelgrass loss or improving conditions so it can return on its own: mitigation banking and in lieu fee mitigation funds.
Source: Leschen, A. S., K. H. Ford, and N. Tay Evans. 2010. Successful eelgrass (Zostera marina) restoration in a formerly eutrophic estuary (Boston Harbor) supports the use of a multifaceted watershed approach to mitigating eelgrass loss. Estuaries and Coasts 33(April 2010). DOI: 10.1007/s12237-010-9272-7.
Is Invasion by an Introduced Species Always an Ecological Disaster? Perhaps Not, Says Alabama Study
For decades, introduction and spread of exotic species into aquatic systems has been considered ecological bad news. Scientists and policy makers have operated under the assumption that when exotic species take hold, dramatic changes will ensue, ranging from loss of native species to changes in ecosystem services. But a recent study in the Mobile-Tenslaw Delta, AL, provides evidence that perhaps not all introductions lead to such dire outcomes. Investigators evaluated the impacts of introduced Eurasian milfoil, a structurally complex aquatic plant, on food web structure by comparing faunal communities associated with this invasive species and two native plant species: structurally simple wild celery and more complex water stargrass. Predator-prey interactions and habitat choice of shelter-seeking fish were also studied in the laboratory. Field survey results indicated that fauna seemed to choose habitat more on the basis of structural complexity than by differentiating between native and introduced plants. No significant differences were found in the communities occupying the milfoil and stargrass habitats, but there were significant differences between milfoil and wild celery. In the lab, rainwater killifish preferred milfoil over celery, but not over stargrass. These habitat choices held even when a predator was introduced.
One conclusion that can be drawn from this research is that impacts of invasive exotic species are site-specific. Based on research and monitoring in other systems, community-level impacts of Eurasian milfoil have been commonly considered to be detrimental throughout much of North America. However, impacts in this system might be ameliorated by the presence of a native plant that serves much of the same sheltering function. Shelter-seeking fauna may simply not perceive a difference between the newcomer and the native.
Source: Martin, C. W. and J. F. Valentine. 2010. Impacts of a habitat-forming exotic species on estuarine structure and function: an experimental assessment of Eurasian milfoil. Estuaries and Coasts 33(April 2010). DOI: 10.1007/s12237-010-9274-5.
Restoration Projects in Elkhorn Slough Have Led to Improvements in Water Quality, Study Says
California’s Elkhorn Slough is a classic example of a small estuary impacted significantly by agricultural runoff and resulting eutrophication. Water quality there has been further affected by the installation of tidal gates that are designed to keep salt water from flooding adjacent agricultural lands. Restoration projects have taken place in the watershed aimed at improving water quality; in particular, row crops have been pulled back from the high tide line and native plants and /or low erosion crops have been planted in buffer zones. Fortunately, water quality monitoring data have been collected in the restored sub-basins since before the restoration projects were initiated, allowing assessment of their efficacy. These data can also be used to examine changes in water quality associated with operation of the various tide gates, some of which have malfunctioned and been repaired during the period covered by the monitoring. A recent analysis of the monitoring data in a few sub-basins of the slough indicated that even restoration projects as small as 1% of the area of the sub-basin are associated with improvements in water quality, as reflected in decreased nutrient levels and improved dissolved oxygen concentrations. The most important factor explaining improvements in water quality in restored areas appeared to be proximity of the restoration project to the water body – the closer, the better. Further, water quality tended to improve when tide gates were broken in an open position, which facilitates dilution, and declined when the gates were repaired.
The analyses presented here were conducted using BACI (before-after-control-impact) statistical designs, which are increasingly being used to measure the effects of anthropogenic changes to ecosystems. The technique relies on having comparable data before and after an impact or restoration event. The use of the BACI design may be useful to managers working in other systems.
Source: Gee, A. K., K. Wasson, S. L. Shaw, and J. Haskins. 2010. Signatures of restoration and management changes in the water quality of a central California estuary. Estuaries and Coasts 33 (April 2010). DOI: 10.1007/s12237-010-9276-3
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