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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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2013 December


Lessons from a Success Story: Mattawoman Creek, MD
Ditch-plugging Affects New England Salt Marsh Plant Communities, Study Finds
Deconstructing Construction: Effects of Bridges and Dredging on Circulation in Tampa Bay
Learning from Experience: Application of Adaptive Management to Tidal Energy Development

Lessons from a Success Story: Mattawoman Creek, MD

Unequivocal success stories in estuarine restoration are as welcome as they are unusual. The story of Mattawoman Creek, a small, tidal freshwater tributary of the Potomac River, starts out like that of many similar systems: between the 1970s and 1990s the creek was highly eutrophic, suffering from poor water quality, high algal biomass, and a total absence of SAV (which had been present in the system decades earlier). In 1991, wastewater treatment plant modifications led to reduced nitrogen and phosphorus discharges, and the trajectory of the creek changed dramatically. Reductions in these point sources resulted in significant declines in algal biomass, large increases in SAV coverage and density, and modest increases in water clarity.

A recent report on observed recovery patterns after nutrient load reductions provides several potential lessons for restoration projects in other areas. The researchers found that SAV recovery was marked by a lag period of slow response followed by a period of rapid expansion before reaching a new stable state (algal biomass declines followed a similar pattern), emphasizing the importance of evaluating recovery on a long-term basis. They also identified a potential threshold in the SAV response: when chl a concentrations were greater than 18 μg L-1 aerial coverage of SAVs was very low to zero, but when chl a dropped below this level SAVs increased rapidly.

As part of the project the investigators developed a rough N budget for the creek that provided two important insights that again may have general application. First, the budget showed that the Potomac River, which is downstream of the area in question, was an important source of N. This suggests that N enrichment from downstream water bodies must be taken into account in developing restoration strategies and evaluating their success. The N budget also suggested that atmospheric deposition may be a significant contributor to diffuse loading to the creek, and the authors recommend that particularly attention be paid to this load as conservation efforts move forward.

Source: Boynton, W. R., C. L. S. Hodgkins, C. A. O’Leary, E. M. Bailey, A. R. Bayard, and L. A. Wainger. 2013. Multi-decade responses of a tidal creek system to nutrient load reductions: Mattawoman Creek, Maryland USA. Estuaries and Coasts 36(August 2013). DOI: 10.1007/s12237-013-9690-4.

 Ditch-plugging Affects New England Salt Marsh Plant Communities, Study Finds

Ecologists and managers have known for decades that the old practice of grid ditching marshes to reduce mosquito populations had a range of negative ecological impacts. Instead of eating away at mosquito habitat, ditching may have destroyed critical marsh pools and altered plant communities. In New England, approximately 90% of marshes were ditched between colonial times and the 1930s. An alternative approach for marsh mosquito control, developed in the 1990s, is ditch-plugging, in which marsh soils are used to plug the seaward end of an existing ditch to create pools where mosquito-eating fish and wading birds can make a living. But what are the ecological effects of this newer approach?

Researchers in New England sought to understand how ditch-plugging affects plant communities by comparing ditched, ditch-plugged, and natural creeks and pools in three Gulf of Maine salt marshes. They found that species richness, diversity, biomass, and plant cover were all significantly lower in ditch-plugged habitats as compared to both ditched and natural sites. Many of these differences are likely related to the differences in hydrology at the ditch-plugged sites: Previous studies have shown that marsh surfaces at ditch-plugged sites experience more surface flooding and less drainage than natural sites, for example.

There are serious implications of these results for marsh maintenance. Vegetation helps marshes keep their heads above water by contributing to sediment accretion, and if plugging is doing away with vegetation, the outcome could be bad for marsh resilience to sea level rise. The authors warn that ditch-plugging, while designed to be a more marsh-friendly approach to mosquito control, may actually expedite the loss of marsh habitat.

Source: Vincent, R. E., D. M. Burdick, and M. Dionne. 2013. Ditching and ditch-plugging in New England salt marshes: Effects on plant communities and self-maintenance. Estuaries and Coasts 36(July 2013). DOI: 10.1007/s12237-013-9671-7.

 Deconstructing Construction: Effects of Bridges and Dredging on Circulation in Tampa Bay

Coastal water bodies like estuaries can present obstacles to the efficient movement of people and goods around an urban landscape, and in many areas the environment has been altered to address these challenges by dredging and building bridges. Although these alterations can facilitate the transport of people and boats, they can have unintended consequences for another kind of transport: that of water, salt, and other ecological components. The authors of a recent study examined how construction of large-scale infrastructure (bridges, causeways, dredging) in Tampa Bay affected salinity and residual (non-tidal) flow in the system.

The study compared model simulations of the bay’s circulation given present-day bathymetry in contrast to bathymetry generated from 1879 depth soundings. In the intervening years, dredging increased the main channel depth by 15 feet and four major bridges were constructed over the bay. These large-scale construction projects were found to steepen the salinity gradient by about 40% during times of low freshwater input and to flatten the gradient by about 25% during moderate freshwater flow conditions. Deepening of the shipping channel also increased the exchange circulation in the bay, allowing for greater up-estuary salt transport. Results varied depending on the local bathymetry of basins within the bay and local freshwater influence.

Changes in residual circulation have implications beyond the movement of water and salt. Flushing and residence times are also affected, as are nutrient dynamics. The approach used here could be applied to other systems to help managers assess the potential impacts of large-scale construction, which is likely to continue as coastal populations grow.

Source: Meyers, S. D., A. L. Linville, and M. E. Luther. 2013. Alteration of residual circulation due to large-scale infrastructure in a coastal plain estuary. Estuaries and Coasts 36(September 2013). DOI: 10.1007/s12237-013-9691-3.

Learning from Experience: Application of Adaptive Management to Tidal Energy Development

Marine hydrokinetic energy development – extracting energy from waves, currents, and tides – is moving full steam ahead, but because these technologies are so new, management frameworks often lag behind, making the permitting and licensing process slow and frustrating. One of the nascent industry’s biggest challenges is uncertainty in the regulatory process stemming from confusion about jurisdictions, permitting requirements, and environmental policies. One way to address these issues is by taking an adaptive management approach to regulation of this emerging industry, whereby data is collected as the technology is deployed and regulation responds to that information. A recent social science study of a tidal energy project in Maine resulted in a better understanding of the regulatory process, perspectives of project stakeholders, and institutional challenges and vulnerabilities. This information was used to guide recommendations for implementing adaptive management in this and other hydrokinetic energy projects.

The authors interviewed stakeholders, attended relevant meetings, and reviewed the literature to understand and describe the regulatory process for Ocean Renewable Power Company’s Cobscook Bay Tidal Energy Project at the mouth of the Bay of Fundy. They describe the roles of the range of permitting and reviewing agencies and the interactions among them. They found a number of institutional factors favorable to an adaptive management approach. First, the decision-makers value learning from experience: interviewees acknowledged that feedback from monitoring programs should be used to improve decision-making. Second, managers appreciate that adaptive management offers a method for correcting avoidable error, as they can stop a project if the impacts prove to be unacceptably negative. The authors also found a strong commitment to interagency coordination, and an emphasis on the importance of proactive engagement. A number of barriers to adaptive management implementation are described, including conflicting agency cultures, high costs of monitoring, and long timeframes associated with baseline data collection.

The authors conclude that adaptive management is a viable approach to moving ocean renewable energy development ahead in a way that is socially and environmentally responsible. They note that successful implementation of such a framework will require institutional conditions that favor efficient learning from experience.

Source: Jansujwicz, J. S. and T. R. Johnson. 2013. Understanding and informing permitting decisions for tidal energy development using an adaptive management framework. Estuaries and Coasts 2013 (August 2013). DOI: 10.1007/s12237-013-9678-0.