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Coastal & Estuarine Science News (CESN)

Coastal and Estuarine Science News (CESN) is an electronic publication providing brief summaries of select articles from the journal Estuaries and Coasts that emphasize management applications of scientific findings. It is a free electronic newsletter delivered to subscribers on a bi-monthly basis.

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April 2022

Table of Contents

Runnels for Restoration

A climate adaptation tool to buy marshes some time

With persistent inundation, sea level rise has been drowning vegetation and converting salt marsh interiors into open water. Small channels called runnels could help emulate natural drainage. Originally developed in Australia to control mosquitoes, runnels are typically less than 30 centimeters wide and deep, often dug by hand, and shallow enough to avoid peat oxidation and subsidence. They’ve been used in northeastern U.S. marshes over the last decade to drain interior marsh pools, facilitating revegetation and the return of fish and birds.

To better understand how runnels might be used to slow or reverse open water conversion, a workshop was organized with 70 scientists and stakeholders in coastal resource management. In a case study within Rhode Island’s degraded Winnapaug Marsh, for example, flood-tolerant and early-colonizing species increased rapidly within two years; Spartina alterniflora increased to 65% cover after three years and remained 68% after seven years.

According to these discussions, runnels show promise as an effective short-term climate adaptation technique, especially in higher elevation areas where peat is less degraded and root mats are still intact. However, runnels are not useful everywhere as their effectiveness will vary with tidal range, sediment supply, elevation capital, and the rate of accretion relative to sea level rise.

Runnels alone will not save salt marshes. Without addressing the underlying causes of open water conversion, runnel construction is unlikely to improve long-term marsh resilience. But by complementing other interventions, runnels can buy time for salt marshes to respond to additional management actions—or to naturally adapt to sea level rise through vertical accretion and upland migration.

Source: Besterman, A.F. et al. 2022. Buying Time with Runnels: a Climate Adaptation Tool for Salt Marshes. Estuaries and Coasts. DOI: 10.1007/s12237-021-01028-8

Blue Crabs in a Sea of Grass

Habitat associations shift with reproductive stage

Blue crabs (Callinectes sapidus) are highly mobile, ecologically important predators that support multimillion-dollar fisheries along the Atlantic and Gulf coasts. Although adult crabs are often associated with seagrass meadows, less is known about what other habitat characteristics they prefer, or whether their location shifts with crab sex and reproductive status.

Using commercial crab traps, a team of researchers examined the relative abundance and distribution of adult blue crabs during summer months across 33 square kilometers of seagrass meadow along the Virginia coast. They characterized blue crab location in relation to seagrass density as well as trap proximity to salt marshes, oyster reefs, and particular bathymetric features (deep channels and oceanic inlets). They also distinguished crab-habitat relationships by sex and reproductive maturity.

They found that adult crab catch per unit effort (CPUE) was higher in sparse seagrass than in dense meadows. Sparse seagrass may improve the foraging efficiency of crabs while still providing sufficient refuge from predators. The probability of trapping gravid females was higher at the edge of meadows (as opposed to meadow interiors) and near deep channels; furthermore, the likelihood of catching females with mature eggs was highest at oceanic inlets. This is likely because deep channels help gravid females migrate seaward, where their eggs develop offshore. More adult crabs were also found farther from salt marshes, whereas proximity to oyster reefs had no effect on CPUE.

Though seagrasses are well-known for providing numerous ecosystem services, this study focuses on the important role that seagrasses and other coastal foundation species play in providing habitat for blue crabs. Understanding how habitat associations change with the sex and reproductive stage of blue crabs may inform fisheries management actions, such as setting sex-specific exploitation thresholds or including seagrass meadows near oceanic inlets in blue crab spawning sanctuaries.

Source: Cheng, S.L. et al. 2022. Coastal Vegetation and Bathymetry Influence Blue Crab Abundance Across Spatial Scales. Estuaries and Coasts. DOI: 10.1007/s12237-021-01039-5

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Tracking Wastewater Contamination at Shellfish Farms

Artificial sweeteners are emerging indicators of human wastewater

Contamination, as indicated by high fecal coliform levels, can result in the closure of shellfish areas to harvesting. Researchers have been working for decades to come up with indicators that can be used to distinguish between human and non-human sources of contamination to aid management.

In a recent study, a team of researchers used a suite of methods to track potential sources of contamination to shellfish farms near Alabama’s Portersville Bay in the northern Gulf of Mexico. Researchers collected water samples from shellfish farms as well as nearby residential, agricultural, and industrial areas and a wastewater treatment plant (WWTP) outfall along a tidal river adjacent to the farms. In addition to fecal coliforms and other microbial indicators, they also measured artificial sweeteners, a relatively new indicator of human wastewater, along with stable isotopes and dissolved nutrients.

The results indicated that shellfish farms had elevated levels of coliform, artificial sweeteners, and ammonium year-round. Although high fecal coliform concentrations are related to waste from any warm-blooded animal, elevated ammonium is consistent with nitrogen loading from fertilizer or directly discharged human waste. The interpretation that the sources of contamination were human-derived was corroborated by artificial sweetener concentrations, which tended to be higher at the WWTP and in residential areas. Indeed, artificial sweeteners may detect WWTP influence better than coliforms because effective treatment reduces microbial indicators. The researchers also found evidence for additional coliform inputs from agricultural and industrial sites during cold, wet periods when runoff was increased.

Using multiple indicators can help managers detect and define wastewater sources, identify targets for monitoring or remediation, and improve metrics for determining shellfish area closures in estuaries with a variety of land-derived wastewater sources. Incidentally, the study revealed that installation of water quality improvements—such as septic system upgrades and new sewer connections—may have inadvertently released buried sewage into the water column.

Source: Frith, A. et al. 2021. Multiple Indicators of Wastewater Contamination to Shellfish Farms Near a Tidal River. Estuaries and Coasts. DOI: 10.1007/s12237-021-01033-x

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Will Genetic-Based Restoration Strategies Work for SAVs?

Dispersal distances can determine transplant success

Reestablishing resilient ecosystems requires counteracting current stressors and bringing populations to levels that are self-sustaining. One consideration for restoration is genetic diversity, as high diversity can help ensure long-term species persistence and resilience by increasing fitness, supporting growth and productivity, and facilitating rapid recovery from disturbances. But for this to be successful, transplants need to be able to reproduce successfully.

To better understand the patterns of genetic diversity and dispersal distances, researchers compared the genetic structure of the submersed aquatic plant species Vallisneria americana from across similar salinity gradients in three rivers—the Potomac, Hudson, and Kennebec rivers in Maryland, New York, and Maine, respectively—using DNA extracted from plant shoots collected in the summertime.

The team found important differences that may limit the broad use of the same propagule selection strategy across rivers. The Potomac has high genotypic diversity with no detectable dispersal barriers between sampled sites; moving restoration stock along the river would fall within natural dispersal distances. The Hudson, on the other hand, supported low genotypic diversity, and gene flow was only upstream. Although inbreeding was not detected in the Kennebec or the Hudson, recent bottlenecks were observed and dispersal barriers created short dispersion distances. So although shoots could be transplanted across Potomac sites without concern, there’s a greater risk of introducing incompatible transplant materials from one population to another within each of the other two rivers.

The strong genetic divergence detected between the three rivers is likely driven by isolation and local adaptations. Ultimately, locally relevant data and stock may be needed to maintain self-sustaining systems and enhance restoration success. Selecting transplant material from ecologically similar sites within dispersal distances within each river should increase genetic diversity while maintaining local adaptation and reducing the risk of mixing incompatible traits.

Source: Marsden, B.W. et al. 2021. Assessing the Potential to Extrapolate Genetic‑Based Restoration Strategies Between Ecologically Similar but Geographically Separate Locations of the Foundation Species Vallisneria americana Michx. Estuaries and Coasts. DOI: 10.1007/s12237-021-01031-z

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