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 2009
Contents
Phytoplankton in Massachusetts Bay: Impacts of a Sewage Outfall Relocation
Study Says Marsh Ditching Leaves Legacy of Water Quality Effects
Dredged Material Placement May Serve as a Life Preserver for Drowning Marshes
Watershed Restoration Requires Stronger Science-Management Links, Says Essay
Phytoplankton in Massachusetts Bay: Impacts of a Sewage Outfall Relocation
Boston Harbor is the site of one of the largest ongoing environmental experiments ever attempted. In 2000, the main sewage outfall servicing two million people in the greater Boston metropolitan area was relocated from the outer harbor, where sewage inputs were contributing to extremely poor water quality, to a site 15 km offshore. Ecosystem monitoring programs have been in place in both the harbor and ocean outfall locations for many years. One such monitoring program has enabled researchers to examine phytoplankton patterns in Massachusetts Bay near the relocated outfall over a 15-year period (1992-2007, approximately nine years before and over seven years after outfall movement and ongoing), and to determine whether the change in outfall location has led to shifts in the phytoplankton community there.
It is clear that the movement of the outfall decreased nutrient concentrations and improved water quality in Boston Harbor. At the ocean location of the outfall, dissolved inorganic nitrogen levels, especially ammonium, have increased, but these changes have not resulted in increased phytoplankton productivity. While there were no significant changes in the total number of phytoplankton in the bay, there were changes in some functional groups. For example, many diatom and dinoflagellate species appear to have declined in most regions examined, while Phaeocystis pouchetii and microflagellates have increased. However, the authors contend that much of the variation observed can be attributed more to regional changes affecting the entire Gulf of Maine than to the outfall relocation. It thus appears that the outfall relocation is not having a major effect on phytoplankton communities in Massachusetts Bay.
Source: Hunt, C. D., D. G. Borkman, P. S. Libby, R. Lacouture, J. T. Turner, and M. J. Mickelson. 2009. Phytoplankton patterns in Massachusetts Bay – 1992-2007. Estuaries and Coasts 32 (DOI 10.1007/s12237-008-9125-9).
Study Says Marsh Ditching Leaves Legacy of Water Quality Effects
Before the current appreciation of the critical functions of salt marshes as nurseries, habitats, and filters, salt marshes were seen as little more than disease-carrying mosquito breeding grounds. In the early twentieth century, most East Coast marshes were ditched to eliminate standing water where mosquito larvae thrive. Later it was learned that this practice actually removed habitat for mosquito-eating fishes, but the damage was done. Not only did the ditches not work very well for reducing mosquito populations, they also left a legacy of water quality problems, as shown in a recent study of eight marshes on Long Island, NY.
The study found that at all sites sampled, ditch water was higher in nutrient concentrations (except for nitrate) than the waters of the adjacent estuary, as were total coliform bacterial counts. In addition, ditch sediments were higher in organic matter than estuarine sediments. The high concentration of nutrients and bacteria in ditches is likely due, in part, to their high surface area to volume ratio and shallow nature compared to adjacent bays and even marsh creeks. This morphology leads to a greater significance of benthic processes in the ditches, promoting the enrichment of nutrients and organic matter. Further, mass balance calculations indicate that ditches have the potential to be an important (but not primary) source of N loading into an estuary. At one pristine site in the summertime, 18% of the N loading came from ditches, the second largest source after groundwater and a larger source than benthic fluxes, surface waters, or the atmosphere. Laboratory experiments indicated that ditch water can stimulate phytoplankton growth, particularly that of diatoms and dinoflagellates, and can also contribute to estuarine hypoxia.
In addition to these water quality impacts, the physical legacy of ditches remains as well: as they were designed to do, they still facilitate faster runoff of water from the marsh surface, which may short-circuit the filtering and processing of nutrients and organic matter. The authors suggest that mitigation would best be achieved by filling or plugging the ditches.
Source: Koch, F., and C. J. Gobler. 2009. The effects of tidal export from salt marsh ditches on estuarine water quality and plankton communities. Estuaries and Coasts 32 (DOI 10.1007/s12237-008-9123-y).
Dredged Material Placement May Serve as a Life Preserver for Drowning Marshes
In a time when sea level rise is becoming a reality for coastal ecosystems and communities, coastal marshes face a special challenge: keeping their heads above water as sea level rises faster than sedimentation can build up new marsh land. As the edges of a marsh become water-logged and vegetation dies off, shallow ponds form in the marsh interior, which then enlarge through continued erosion and subsidence. One strategy undertaken to assist marshes in keeping pace with rising waters is to add sediment to the marsh surface to increase marsh elevation and enhanced growth of marsh vegetation. Does it work?
One study of six sites in the Mississippi Delta where a thin layer of dredged material was piped over the marsh surface suggests that this approach may work particularly well for interior ponds. Marsh response to sediment additions was assessed in the short term (6-12 months after sediment application) and trajectory models were used to predict long-term growth of marsh vegetation for marshes and interior ponds at all six sites. Short and long-term assessment revealed that soil addition increased soil nitrogen content and bulk density in both deteriorating marsh and interior pond sites; in contrast, vegetative cover and productivity were not enhanced long-term at deteriorating marsh sites, but long-term response models predicted that the vegetative biomass of the interior ponds will recover to reference levels over a seven-year period.
The authors conclude that sediment enhancement is a viable approach for restoring marsh sites, particularly interior ponds. They stress that selection of appropriate targets for elevation restoration will be critical to the success of this approach, and that the use of trajectory models shows promise for assessing projects.
Source: La Peyre, M. K., B. Gossman, and B. P. Piazza. 2009. Short- and long-term response of deteriorating brackish marshes and open-water ponds to sediment enhancement by thin-layer dredge disposal. Estuaries and Coasts 32 (DOI 10.1007/s12237-008-9126-8).
Watershed Restoration Requires Stronger Science-Management Links, Says Essay
In an essay based on the keynote address she gave at the 2007 Estuarine Research Federation conference, Margaret Palmer argues that existing science is inadequately used when planning and implementing watershed restoration projects, and large gaps remain in knowledge crucial to carrying out successful restoration. Further, some science that is inadequately tested is probably relied upon too much in restoration projects.
Palmer presents a rigorous evaluation of the current use of science in restoration that outlines ways in which ecological knowledge should influence restoration and places where more research is needed. In the first category, which she terms “science in need of application,” she describes scientific insights that have been under-utilized in restoration, such as the fact that systems are not necessarily at equilibrium, that the arrangement of habitats across the landscape matters, and that external exchange can be an important driver of ecosystems. She stresses that restoration needs to be shifted away from strictly structural interventions, a focus on single species, and the idea of static reference conditions, toward restoration of processes, endpoints that include ranges of conditions, and evaluations of potential sites in a larger spatial context.
On the other hand, she gives examples of “applications in need of science,” major gaps in scientific understanding that hamper successful restoration. In this category she includes determining if physical manipulations of habitat can be used to promote diversity, evaluating whether small restoration projects are effective, and assessing the efficacy of specific alterations such as streambank utilization. She also stresses the importance of incorporating social science by identifying feedbacks between social and ecological systems that act to constrain science-based restoration.
Palmer concludes that science is not playing the role it should in restoration, perhaps because much of the science produced is not being effectively communicated to the management community, or because the science is not truly addressing the needs of resource practitioners. Whatever the reason, she states that the wall between science and management must be breached, as “society cannot afford an ad hoc approach to environmental management – the costs to the economy and ultimately to human health will simply be too high.”
Source: A Palmer, M.. 2009. Reforming watershed restoration: Science in need of application and applications in need of science. Estuaries and Coasts 32 (DOI 10.1007/s12237-008-9129-5).
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