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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May 2019

Comparing the ecosystem filtration services of native and non-native oysters
Taking a holistic look at marsh sediment dynamics
Is plant diversity important for protecting coastal dunes?
Evaluating restored wetlands in Northern Gulf of Mexico

Comparing the ecosystem filtration services of native and non-native oysters
Strategic seeding can maximize filtration

One of the great benefits of cultivating oysters comes from their feeding activity, which removes suspended particulate matter from the water. On the west coast of the U.S., the presumed filtration values of native Olympia oysters (Ostrea lurida) has been a strong motivator for restoration efforts aimed at replacing the dominant, but non-native Pacific oyster (Crassostrea gigas) -- yet the actual filtration values of these two species has not been compared.

A model examining existing oyster restoration in Oregon’s Yaquina Bay probed this assumption, comparing the filtration services provided by these two species set at historic native oyster population densities. What this model found may surprise some: although restoring native oyster populations to historical levels would increase bay-wide filtration, non-native oysters actually provide much better filtration services. The story is more complicated than this, however, as factors such as salinity and hydrodynamics greatly affect the filtration services of both species. Strategic seeding, the model found, could more efficiently maximize filtration services by placing each species of oyster in areas in the bay with long residence times and where there are favorable environmental conditions.

The findings of this paper are consistent with those of an unrelated study featured in the March 2019 CESN that showed that environmental conditions affected the success of native and non-native oysters planted in eelgrass beds. Combined, these two studies suggest that species is not the only consideration for oyster restoration efforts.

Source: Gray, M., P. Ermgassen, J. Gair et al. 2019. Spatially Explicit Estimates of In Situ Filtration by Native Oysters to Augment Ecosystem Services during Restoration. DOI: 10.1007/s12237-019-00515-3

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Taking a holistic look at marsh sediment dynamics
Both vertical and lateral movement must be considered in marsh restoration

Some marsh restoration efforts focus on shoreline protection and limiting lateral erosion, such as constructing living shorelines. Other projects are aimed at increasing vertical elevation, such as through thin layer placement of sediment. In a new perspective article, author Neil K. Ganju contends that these marsh restoration projects do not generally consider the full sediment dynamics of a system, and that a focus on preventing lateral erosion near a channel can actually reduce the material available for maintaining the elevation of the marsh edge and interior.

Sediment budgets are already appreciated for beaches, where renourishment and shoreline construction projects often consider the effect of, and on, sediment transport. A similar understanding of the sources, timing, and amount of sediment input for marshes could prevent both unintended consequences and counterproductive efforts in marsh restoration. He offers several examples of this, such as that of Blackwater National Wildlife Refuge in Maryland, where calculations showed that sediment export from the system would offset a proposed thin-layer sediment placement in less than 6 months. If a full sediment budget for a marsh is not available, Ganju suggests the ratio of unvegetated to vegetated marsh (the UVVR) may serve as an indicator of marsh status.

Marsh habitats, the author stresses, are systems that move in multiple directions. Evaluating the marsh as a whole can therefore save time and effort in its restoration.

Source: Ganju, N.K. 2019. Marshes are the New Beaches: Integrating Sediment Transport into Restoration Planning. Estuaries and Coasts. DOI: 10.1007/s12237-019-00531-3

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Is plant diversity important for protecting coastal dunes?
Functional richness does not necessarily reduce wave erosion

Vegetation is known to play an important role in protecting coastal dunes from erosion. Yet, given that dune species can have different effects on sediment transport, increased species richness might increase erosion control on coastal dunes. The authors of a new study evaluated this idea by comparing the erosion control of three common dune species grown alone or in combination.

The experiment tested erosion on full-scale model sand dunes exposed to up to 40 minutes of artificially generated waves. The results showed that erosion protection was indeed species-specific. However, increased species richness did not guarantee increased protection, and some treatments actually saw greater erosion than in the bare control. There was also no significant difference in protection for dunes planted with two species as compared with three species. Rather, the most effective protection came from singular plant cover by the beach morning glory, Ipomoeae pes-caprae. This plant’s deep roots and large, sturdy leaves enable it to cover dunes like a carpet, reducing sand mobility.

Yet solving coastal erosion is not as simple as just planting Ipomoeae, and the authors suggest several avenues for further work such as evaluating longer periods of wave exposure and examining the effect of plant density. They also note that some of the other species might play other important roles in dune-building processes.

Source: Maximiliano-Cordova, C., K. Salgado, M.L. Martínez et al. Feagin. 2019. Does the Functional Richness of Plants Reduce Wave Erosion on Embryo Coastal Dunes? DOI: 10.1007/s12237-019-00537-x

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Evaluating restored wetlands in Northern Gulf of Mexico
Comparisons help benchmark recovery for vegetation and soil

When is a restored wetland considered “recovered”? Even as coastal managers work to rejuvenate wetlands all over the world, the degree to which these habitats return to natural conditions remains largely unknown. Part of this is due to the fact that the results of restoration are based largely on the standards it is measured against, and so choosing the appropriate basis for performance plays an enormous role in the apparent success of a project. A literature review and meta-analysis for restored coastal wetlands in the northern Gulf of Mexico identified reference parameters and standards for these restored sites.

“Recovery,” for the purposes of this meta-analysis, was defined as a convergence with values measured at undisturbed or recovered reference sites. Recovery was measured using metrics for above and belowground biomass, vegetation cover, soil organic matter, and soil nutrients. Among the 62 studies examined, the meta-analysis found several important standards for managers to consider. For example, while surface vegetation cover and belowground biomass might return relatively quickly in restored sites — within 5 years for above-ground vegetation measures, and within 15 years for belowground biomass — levels of important soil nutrients like organic carbon and nitrogen can remain low even after more than 15 years of recovery.

This study also identified a potentially concerning pattern among older restored marshes, in which an initial improvement is followed by declining conditions. However, there were many fewer studies that examined marshes 10 or more years after restoration, suggesting a great need for gathering high-quality data at sites even once restoration is considered complete.

Source: Ebbets, A.L., D.R. Lane, P. Dixon et al. 2019. Using Meta-Analysis to Develop Evidence-Based Recovery Trajectories of Vegetation and Soils in Restored Wetlands in the Northern Gulf of Mexico. DOI: 10.1007/s12237-019-00536-y

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Taking a holistic look at marsh sediment dynamics