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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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2023 Issue 3

Table of Contents

How Will Diked Systems Respond to Sea Level Rise? 

Forecasting inundation in diked and tidally restricted coastal lowlands

Diked and drained coastal lowlands rely on hydraulic, protective infrastructure that may stop functioning at full capacity with projected flooding from climate change. However, it remains difficult to identify if and when the flow structures no longer discharge enough water. To better understand how diked systems respond to sea level rise, researchers developed a simplified water balance approach to model flow through hydraulic control structures and water level diked systems. The team tested their model with observations from the Herring River Estuary in Massachusetts, a diked system that has a current tidal restoration project under construction, for present-day and future sea level rise scenarios from 2020 to 2100. Using a drainability metric that quantifies the net water volume drained over every tidal cycle, they forecasted when the system would have converted from drained to impounded without the restoration.

All sea level rise scenarios modeled with existing infrastructure (a sluice and two flap gates) indicated a conversion from somewhat drained conditions in 2020 to nearly completely impounded conditions by 2100, with substantial impoundment within 20 years. Simulations with up to three additional gates did not dampen this trend toward impoundment, suggesting that rising impounded water levels are likely even with major construction upgrades. Based on these drainage performance tests, substantial and recurring infrastructure improvement projects are necessary to prevent similar diked systems from becoming impounded.

Using models like these could help managers prioritize coastal areas that need restoration, flood control, and alteration (or even removal) of existing dikes. The authors have shared their dike hydraulic flow model code in an open-source Python library for other teams to implement in diked and tidally restricted settings elsewhere. This could be a useful screening tool that may help identify sites that require more complex hydrodynamic modeling or observation network development.

Source: Befus, K.M. et al. 2023. Forecasting Sea Level Rise‑driven Inundation in Diked and Tidally Restricted Coastal Lowlands. Estuaries and Coasts. DOI: 10.1007/s12237-023-01174-1

Identifying Management Priorities in Coastal Seascapes

Bright spots and management sites in Queensland estuaries

In order to protect and restore coastal seascapes, managers often prioritize two types of areas: bright spots that are performing better than expected ecologically, making them potential sites for conservation, as well as management sites that are performing below expectations and should be the focus of interventions. Existing methods for separating these areas are limited, and managers often turn to detailed field assessments and ground-truthing, which can be expensive and time-consuming.

To identify bright spots and management sites, researchers in Australia tested for correlations between metrics indexing ecological condition (focusing on habitat-forming species) and a suite of spatial and environmental variables across four ecosystems—mangroves, seagrass, saltmarsh, and rocky outcrops—in 13 estuaries in southeast Queensland. They used generalized additive mixed models to predict the ecological condition of individual sites based on region-wide patterns; these predictions were then compared to data collected from simple field surveys.

The team successfully identified sites that are performing significantly better or worse than expected, and they found significant statistical patterns in 10 of the 15 ecological condition metrics—making them useful in distinguishing between better- and worse-performing sites. These metrics include mangrove abundance, seagrass shoot height, and saltmarsh vegetation species richness. Urbanization affected only two metrics: Rocky outcrop oyster cover is 75% lower at sites near extensive urbanization, and algal cover is highest at sites with intermediate urbanization. Overall, management sites were more common than bright spots, with 50% more management than bright spots in mangroves, 42.8% more in seagrasses, 38.5% more in saltmarshes, and no bright spots in rocky outcrops.

These models demonstrate that patterns in habitat condition across coastal seascapes can be predicted using spatial modeling approaches. This can help managers avoid spending resources restoring sites that are unlikely to improve. Alternatively, identifying bright spots can highlight areas where management has been successful.

Source: Gilby, B.L. et al. 2023. Drivers of Ecological Condition Identify Bright Spots and Sites for Management Across Coastal Seascapes. Estuaries and Coasts. DOI: 10.1007/s12237-023-01187-w

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Estimating Juvenile Salmon Carrying Capacities

A tool for designing and evaluating restoration projects

Estuarine floodplain habitats are the focus of many restoration efforts for salmon recovery, but what is the carrying capacity of these areas for juvenile salmon? To answer this question, researchers in the Pacific Northwest synthesized existing data on the densities of juvenile Chinook salmon (Oncorhynchus tshawytscha) and coho salmon (O. kisutch) across their North American ranges. The team then used a habitat expansion approach to estimate the capacity of 16 coastal Oregon estuaries to support salmon restoration using spatial data on habitat extent.

Juvenile Chinook and coho salmon density values varied by season, but were higher in large river and floodplain habitats (compared to estuarine habitats) and higher in distributary channels (compared with main channel subhabitats). When marsh type was considered, the highest values were in forested river tidal marshes (compared with other channel and marsh habitat combinations). Current and historic juvenile salmon capacity in the Oregon estuaries varied, but was directly related to the area of tidal marsh habitat. The current habitat extent ranges from 10 to 95% of historical habitat, with smaller estuaries generally experiencing less overall loss than larger ones. In systems where sea level rise is projected to result in the loss of vegetated tidal wetland habitat, estimated carrying capacities were predicted to decline by up to 54%. On the flip side, systems projected to see an increase in tidal wetlands with sea level rise are predicted to have up to a 320% increase in capacity.

In areas where current vegetated marsh area is close to that estimated for historical extent, the authors propose focusing on improving habitat quality; whereas in estuaries where large wetlands have been lost due to diking, restoring tidal flow and marsh restoration to increase habitat extent may be a more effective strategy. Capacity estimates could prove useful for considering impacts of new projects and may enable managers to forecast climate change impacts on salmon stock rearing capacity. 

Source: Hall, J. et al. 2023. Estimating Juvenile Salmon Estuarine Carrying Capacities to Support Restoration Planning and Evaluation. Estuaries and Coasts. DOI: 10.1007/s12237-023-01185-y

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Undersized Mediterranean Lagoons are Understudied

Size matters

Despite their importance and prevalence in the Mediterranean, small lagoons are poorly understood and often unprotected in favor of large lagoons, which are typically the focus of conservation norms and scientific studies due to their economic importance. In a special issue on low inflow estuaries, researchers in France provide the first inventory of all the lagoons in Corsica, which contains 95 out of the 400 or so lagoons along the Mediterranean coast; 91 of these are smaller than 0.5 square kilometers. Then, to predict how climate change and anthropic pressure might affect small lagoons, they investigated the hydrology and ecology of three small lagoons in the southeastern part of the island. Arasu, Santa Giulia, and Balistra lagoons have different depths and seasonal biogeochemical cycles, and they’re characterized by their contrasting anthropogenic contexts: highly modified and disturbed, medium disturbance, and quasi-pristine, respectively. The team sampled nutrients in both surface waters and ground water as well as phytoplankton communities in the lagoons.

Arasu and Santa Giulia lagoons showed an increase in watershed urbanization, high phytoplankton biomass, low diversity, and blooms of potentially harmful dinoflagellates. Conversely, Balistra lagoon showed a good status overall, but in its watershed, two major sources of pollution were found, linked to hamlets without public sanitation upstream of the lagoon and exceeding the World Health Organization drinking water threshold for nitrates.

This study demonstrates the importance of small lagoons at the watershed and sea scale, and it highlights the significance of an integrated approach, considering lagoons within their adjacent connected systems and anthropogenic contexts. Large systems are too often the focus of monitoring efforts, even though small systems frequently have closer and more direct connections to land use. Small coastal lagoons not only provide numerous essential ecosystem services, but they also act as sentinels for change and human interventions due to their rapid response to disturbance.

Source: Ligorini, V. et al. 2023. Small Mediterranean coastal Lagoons Under Threat: Hydro‑ecological Disturbances and Local Anthropogenic Pressures (Size Matters). Estuaries and Coasts. DOI: 10.1007/s12237-023-01182-1

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