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Coastal & Estuarine Science News (CESN)

The mission of Coastal & Estuarine Science News (CESN) is to highlight the latest research in the journal Estuaries and Coasts that is relevant to environmental managers. It is a free electronic newsletter delivered to subscribers on a bi-monthly basis. Sign up today!

2024 Issue 5

Table of Contents

Hybrid Living Shorelines: Combining Rocks and Mangroves

Gray–green solutions for reducing shoreline erosion 

Traditional engineered defenses like seawalls can protect shorelines, but they act as barriers that can lead to biodiversity and connectivity loss. Hybrid living shorelines—which combine both gray and green approaches—could help achieve coastal resilience while also retaining ecological functions. In New South Wales, Australia, rock piles (called fillets) built from locally quarried rock have been used along eroding estuary banks to rehabilitate mangroves for at least a decade; and more recently, timber fillets were introduced to experiment with more natural materials. But there’s little data to support their role in facilitating mangrove recruitment and mitigating erosion.

To explore factors leading to the success of hybrid living shorelines, researchers used historical aerial imagery from 2011 to 2022 to study the rates of shoreline change before and after installation and compared erosion in living shoreline and control areas to assess the effectiveness of using rock or timber fillets across three different estuaries in New South Wales.

Although there was considerable site variation, most of the rock fillets reduced erosion rates and resulted in mangrove colonization and canopy formation over the 12-year time series. The timber fillets were less effective though the number of case studies were limited. The establishment of mangroves behind rock fillets contributed to bank stabilization, likely by enhancing habitat complexity and reducing downstream sedimentation. Fillet length and channel width also influenced erosion mitigation: Wider channels experienced greater erosion reduction with increasing length of protected shoreline.

Nature-based coastal defenses have the potential to adapt to future climate changes as coastal habitats can self-repair and grow at a rate that matches sea–level rise. Nonetheless, some fillets contributed to downstream shoreline retreat, highlighting the need to consider site-specific factors, such as sediment movement, when implementing hybrid living shorelines.

Source: Chan, S.C.Y. et al. 2024. Mangrove Cover and Extent of Protection Influence Lateral Erosion Control at Hybrid Mangrove Living Shorelines. Estuaries and Coasts. DOI: 10.1007/s12237-024-01391-2

Cordgrass in Natural vs. Restored Marshes on Long Island Sound

Genetic diversity was four times higher in planted restored marshes

The restoration of salt marshes relies on the establishment of foundational plants to serve as ecosystem engineers. However, the factors contributing to the successful establishment of marsh-building plants remain poorly understood, and some overlooked factors include the genetic structure of the plants themselves.

During the growing seasons of 2021 and 2022, researchers compared six restored salt marshes with six natural ones along the northern coast of Long Island Sound in Connecticut. They focused on smooth cordgrass (Spartina alterniflora) genetics, biomass, and stem morphology. Restoration activities at each site differed, but all of them were planted with plugs of S. alterniflora sourced from offsite commercial vendors (as opposed to allowing natural colonization or transplanting plugs directly from a nearby marsh).

Restored marshes had lower stem ground cover and stem density but greater stem height, stem diameter, and density of flowering stems than natural marshes. They also had lower densities of ribbed mussels and crab burrows in edge habitats than natural marshes. Although there was no difference in live belowground biomass, natural marshes had triple the dead belowground biomass. With lower belowground biomass, lower abundance of burrowing species, and lower current elevations than those observed at the natural marshes, the restored marshes were at greater risk of drowning under sea–level rise.  

On average, the genotypic diversity of restored plants was four times higher than their counterparts in natural marshes—although that difference decreased over time. This may provide a mechanism for these marshes to adapt, as previous studies have found that higher genetic diversity within species typically confers greater resilience to stress and improved productivity compared with similar species with lower genotypic diversity. Because important differences have been observed between different populations of S. alterniflora, the source of plants used may therefore impact restoration success.

Source: Crosby, S.C. et al. 2024. Structure and Function of Restored and Natural Salt Marshes: Implications for Ecosystem Resilience and Adaptive Potential. Estuaries and Coasts. DOI: 10.1007/s12237-024-01395-y

Image: Collecting data in a Connecticut salt marsh / Sarah Crosby

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Hitting a Snag! The Effects of Submerged Logs on Fish in Queensland Estuaries

Strategic snag placement can maximize benefits to fish and fisheries

Submerged and semi-submerged trees and branches make up structurally complex habitats known as log snags, which support the growth of food sources for fish and provide structure for spawning. Despite their ecosystem services, snags are often removed from waterways to improve boat safety and navigability, and restoring log snag habitat (or resnagging) is rare in coastal and estuarine settings.

To better understand how and where log snags benefit fish and fisheries, researchers surveyed fish assemblages on log snags across 13 estuaries in southeast Queensland, Australia, over three years using underwater video cameras. The team counted 5,803 fish from 59 species on 363 log snag sites and measured over a dozen spatial, habitat condition, and water quality variables.

The most diverse fish assemblages were found on snags close to the mouth of the estuary and further from urban structures. The highest total fish abundance, as well as total abundance of harvested fish species, were also found on snags close to the estuary mouth and where algae cover was less than 25%. The abundance of multiple functional groups (omnivores, zooplanktivores, zoobenthivores) followed a similar trend, though these were mediated by environmental variables such as depth.

According to these findings, the strategic placement and design of resnagging efforts should be prioritized throughout estuaries. Although there are benefits to placing snags near the estuary mouth, resnagging would also be highly beneficial in unvegetated or degraded areas that are often upstream where, for example, seagrass distribution may be restricted. As long as they are in areas where algal growth can be limited, navigational passageways aren’t blocked, and snags will not get dislodged, resnagging represents an alternative strategy that improves conditions for fish—one that’s more resilient than vegetated habitats to long-term declines in water quality and other abiotic conditions.

Source: Goodridge Gaines, L.A. et al. 2024. Quantifying Environmental and Spatial Patterns of Fish on Log Snags to Optimise Resnagging in Coastal Seascapes. Estuaries and Coasts. DOI: 10.1007/s12237-024-01388-x

Image: Surveying fish at log snag sites using remote underwater video stations / Lucy Goodridge Gaines

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Send in the Drones!

A low-cost, minimally invasive tool for monitoring coastal wetland restoration