Large-scale variation in wave attenuation of oyster reef living shorelines and the influence of inundation duration

Hydrodynamics of artificial and oyster-populated breakwaters: A laboratory study with scaled-down oyster castles under unidirectional flow

Oyster populations within the Chesapeake Bay have been drastically reduced over the last century mainly due to unregulated human activities and diseases. Regulations and restoration efforts have focused on restoring oyster populations while also considering their ability to provide ecosystem services, such as coastal protection and water quality improvement, among others. To promote oyster growth and the settlement of new populations, a recent technique adopted along the east coast of the US is the use of oyster castles (OCs). OCs have proven effective in recruiting and retaining oysters and in promoting both vertical growth and horizontal expansion of oyster habitats. OCs are widely used in coastal protection as greener alternative to common engineering solutions. We quantified hydrodynamic differences that occur around these OCs during their early stage (i.e. castles without oysters), and with fully developed oysters covering the surface of the castles through a series of laboratory experiments. The experiments were conducted in a recirculating Odell-Kovasznay type channel at the Ecohydraulics and Ecomorphodynamics Laboratory (EEL) at the University of Illinois. OCs (both with and without oysters) were 3D printed at 1:7 scale to fit the canal, and Particle Image Velocimetry (PIV) was used for 2D flow characterization. Data showed noticeable differences in flow acceleration atop the castles when covered with oysters, as well as an increase in the generation and distribution of turbulent kinetic energy atop and around the oyster-covered castles. Magnitudes and spatial distribution of Reynolds stresses were also affected by the presence of oysters in both submerged and near-emergent conditions. Challenges associated with the estimation of the drag coefficient for both gray and oyster-covered OCs highlighted the need for more data besides the centerline 2D PIV output. Further research involving the whole three-dimensional structure of the flow, in both unidirectional and oscillatory conditions, will allow us to provide relevant guidelines on the design and use of oyster-populated breakwaters as a viable nature-based solution for coastal protection within low-energy environments.

Current extent and future opportunities for living shorelines in Australia

Living shorelines aim to enhance the resilience of coastlines to hazards while simultaneously delivering co-benefits such as carbon sequestration. Despite the potential ecological and socio-economic benefits of living shorelines over conventional engineered coastal protection structures, application is limited globally. Australia has a long and diverse coastline that provides prime opportunities for living shorelines using beaches and dunes, vegetation, and biogenic reefs, which may be either natural ('soft' approach) or with an engineered structural component ('hybrid' approach). Published scientific studies, however, have indicated limited use of living shorelines for coastal protection in Australia. In response, we combined a national survey and interviews of coastal practitioners and a grey and peer-reviewed literature search to (1) identify barriers to living shoreline implementation; and (2) create a database of living shoreline projects in Australia based on sources other than scientific literature. Projects included were those that had either a primary or secondary goal of protection of coastal assets from erosion and/or flooding. We identified 138 living shoreline projects in Australia through the means sampled starting in 1970; with the number of projects increasing through time particularly since 2000. Over half of the total projects (59 %) were considered to be successful according to their initial stated objective (i.e., reducing hazard risk) and 18 % of projects could not be assessed for their success based on the information available. Seventy percent of projects received formal or informal monitoring. Even in the absence of peer-reviewed support for living shoreline construction in Australia, we discovered local and regional increases in their use. This suggests that coastal practitioners are learning on-the-ground, however more generally it was stated that few examples of living shorelines are being made available, suggesting a barrier in information sharing among agencies at a broader scale. A database of living shoreline projects can increase knowledge among practitioners globally to develop best practice that informs technical guidelines for different approaches and helps focus attention on areas for further research.

Achieving conservation and restoration outcomes through ecologically beneficial aquaculture

A range of conservation and restoration tools are needed to safeguard the structure and function of aquatic ecosystems. Aquaculture, the culturing of aquatic organisms, often contributes to the numerous stressors that aquatic ecosystems face, yet some aquaculture activities can also deliver ecological benefits. We reviewed the literature on aquaculture activities that may contribute to conservation and restoration outcomes, either by enhancing the persistence or recovery of one or more target species or by moving aquatic ecosystems toward a target state. We identified 12 ecologically beneficial outcomes achievable via aquaculture: species recovery, habitat restoration, habitat rehabilitation, habitat protection, bioremediation, assisted evolution, climate change mitigation, wild harvest replacement, coastal defense, removal of overabundant species, biological control, and ex situ conservation. This list may be expanded as new applications are discovered. Positive intentions do not guarantee positive ecological outcomes, so it is critical that potentially ecologically beneficial aquaculture activities be evaluated via clear and measurable indicators of success to reduce potential abuse by greenwashing. Unanimity on outcomes, indicators, and related terminology will bring the field of aquaculture-environment interactions into line with consensus standards in conservation and restoration ecology. Broad consensus will also aid the development of future certification schemes for ecologically beneficial aquaculture.

Restoring estuarine ecosystems using nature-based solutions: Towards an integrated eco-engineering design guideline

Traditional solutions to estuarine flood risk management have typically involved the implementation of static 'hard' shoreline protection structures, often at the expense of the natural landscape and the societal and ecosystem benefits they provide. In a changing climate, there is an increasing need to restore these estuarine ecosystems, and alternative measures in the form of Nature-based Solutions (NbS) are being considered. Guidance that balances ecology and engineering is required for NbS to establish as self-sustaining ecosystems. In this study, a review of NbS guidelines was undertaken, revealing an absence of technical content bridging ecological and engineering values. Instead, most guidelines focus on NbS project implementation, identifying engineering aspects, and providing frameworks for investors and project managers. Integration of technical engineering and ecological outcomes within NbS guidelines is needed. A conceptual approach for integrating eco-engineering aspects for estuarine ecosystems is proposed. This conceptual approach focuses on the critical thresholds and parameter relationships associated with establishment, growth, recovery and mortality, and functionality of estuarine NbS, in efforts to quantify changes in ecological development and flood risk mitigation services. The conceptual approach documents how the suggested relationships between parameters can be adopted by practitioners in the short-term, medium-term, and long-term. The application of this conceptual approach to multi-habitat restoration is explored, including lifecycle timing and ecosystem/design functionality. The findings of this study demonstrate the need for an integrated NbS design guideline that balances ecology and engineering research for the long-term success of estuarine ecosystems.

Meta-analysis reveals drivers of restoration success for oysters and reef community

Restoration aims to reverse global declines of foundation species, but it is unclear how project attributes, the physical setting, and antecedent conditions affect restoration success. In coastal seas worldwide, oyster reef restoration is increasing to counter historic habitat destruction and associated declines in fisheries production and biodiversity. Yet, restoration outcomes are highly variable and the factors that enhance oyster production and nekton abundance and diversity on restored reefs are unresolved. To quantify the drivers of oyster restoration success, we used meta-analysis to synthesize data from 158 restored reefs paired with unstructured habitats along the U.S. Gulf and Atlantic coasts. The average recovery of oyster production was 65% greater in subtidal (vs. intertidal) zones, 173% greater in polyhaline (vs. mesohaline) environments and increased with tidal range, demonstrating that physical conditions can strongly influence the restoration success of foundation species. Additionally, restoration increased the relative abundance and richness of nektonic fishes and invertebrates over time as reefs aged (at least 8 years post-construction). Thus, the restoration benefits for provisioning habitat and enhancing biodiversity accrue over time, highlighting that restoration projects need multiple years to maximize ecosystem functions. Furthermore, long-term monitoring of restored and control sites is needed to assess restoration outcomes and associated drivers. Lastly, our work reveals data constraints for several potential drivers of restoration outcomes, including reef construction material, reef dimensions, harvest pressure and disease prevalence. More experimental and observational studies are needed to target these factors and measure them with consistent methods across studies. Our findings indicate that the assisted recovery of foundation species yields several enhancements to ecosystem services, but such benefits are mediated by time and environmental conditions.

The coastal protection and blue carbon benefits of hybrid mangrove living shorelines

Hybrid living shorelines use a combination of engineered structures with natural ecosystems to achieve coastal protection and habitat restoration outcomes, with added co-benefits such as carbon sequestration. Rock fillets constructed along eroding estuarine banks are designed to accumulate sediment, establish mangroves, and stabilise the shoreline. There is, however, a lack of data to support whether rock fillets are achieving these goals. We used a chronosequence of rock fillets to determine their effect on mangrove development, bank stabilisation and carbon sequestration in four estuaries in New South Wales, Australia. Aboveground biomass and adult density increased with age of rock fillets, and mangrove structure was similar to a natural fringing mangrove after 15 years. The rock fillets accumulated sediment, which reduced the eroded estuary bank height, however, little effect of the fillets on bank slope was observed. Sediment carbon stocks were not different between rock fillets, eroding estuary banks and natural fringing mangroves. Rock fillet design had a significant effect on mangrove structure and coastal protection function, with greater wave transmission through lower rock fillets, suggesting design optimisation is needed. As the construction cost of the rock fillets was equal or less than traditional rock revetments, where suitable they present a more economic and environmentally sustainable solution to estuarine erosion management.

Hydrodynamic and biogeochemical evolution of a restored intertidal oyster (Crassostrea virginica) reef

Oyster reef restoration is increasingly used as a tool for restoring lost ecosystem services in degraded aquatic systems, but questions remain about the efficacy of the practice and when/if restored reefs may behave similarly to intact natural reefs. In this case study, field observations highlighted short- (<1 month post-restoration) and longer-term (30 months; 3 recruitment cycles) transformations in canopy, hydrodynamic, and biogeochemical characteristics of a restored intertidal oyster reef relative to nearby intact and degraded reefs. Within 12 months of restoration, live oyster density (326 oysters/m²), mean shell length (47 mm), and mean canopy height (76 mm) did not differ significantly from those observed on a reference reef. Lowering of the reef crest during restoration reestablished over-reef flow and periodic tidal inundation, improving hydraulic connectivity between the channel and the reef surface. This immediately restored much of the reef's hydrodynamic function and eliminated the irregular flow patterns observed on the previously degraded reef. Results showed that mean flow (channel-to-reef flow attenuation: 98% / 62%; within/above canopy) and velocity normalized turbulence (w^'2¯/U²: 10^-1/10^-2; ϵ/U³: 10⁰/10^-2 m^-1) characteristics were similar across the restored and reference reefs within 1 year of restoration, with temporal changes in mixing within the canopy attributed to increases in live oyster density. Nutrient pools (mean total carbon, total nitrogen) on reference and restored reefs had similar magnitudes within 1 year (C: 39 & 33 g/kg, N: 1.5 & 1.8 g/kg), while increases in DOC and NH₄⁺ were correlated with the presence of live oysters. Most changes that occurred on the restored reef were linked to oyster recruitment and canopy growth, which modulated hydrodynamics through direct flow interactions and controlled sediment nutrient and organic matter content through waste deposition and burial.

The Use of Non-Plastic Materials for Oyster Reef and Shoreline Restoration: Understanding What Is Needed and Where the Field Is Headed

Oyster and shoreline restoration is occurring around the globe to recover lost ecosystem services. In the state of Florida, USA, dozens of estuarine habitat restoration projects are underway. These projects have traditionally relied on both natural and man-made materials, including plastics. As the impacts of plastics on marine ecosystems are better understood, practitioners are increasingly focused on plastic-free restoration. To better understand this transition, we surveyed Florida restoration practitioners in April 2021 to capture current non-plastic restoration project trends and their status. Our descriptive survey goals were to understand: (1) what non-plastic materials have been tested, (2) trade-offs between plastic and non-plastic materials (e.g., cost, sourcing, volunteer engagement), and (3) the performance of non-plastic materials. Responses indicated that a variety of non-plastic materials are currently being used, including rock, cement-infused jute structures, cement Reef Balls™ (Reef Ball Foundation, USA), BESE-elements®, and metal gabions. Overall, these materials are more expensive and equally or more difficult to install than previously popular plastic-based materials. No “best” non-plastic material emerged from our survey in part because many novel materials have been deployed for under three years. Long-term performance under a variety of abiotic and biotic conditions is thus a future research priority.

Strategies for Successful Mangrove Living Shoreline Stabilizations in Shallow Water Subtropical Estuaries

By combatting erosion and increasing habitat, mangrove living shorelines are an effective alternative to hard-armoring in tropical and subtropical areas. An experimental red mangrove living shoreline was deployed within Mosquito Lagoon, Florida, using a factorial design to test the impact of mangrove age, breakwater presence, and mangrove placement on mangrove survival within the first year of deployment. Mixed mangrove age treatments were included to identify if seedling (11-month-old) survival could be enhanced by the presence of transitional (23-month-old) and adult (35 to 47-month-old) mangroves. Environmental factors were monitored to detect possible causes of mangrove mortalities. Approximately half (50.6%) of mangroves died, and of those, 90.7% occurred within the annual high-water season, and 88.9% showed signs of flooding stress. Planting seedlings haphazardly among older mangroves did not attenuate enough wave energy to significantly increase seedling survival. Breakwaters alleviated stress through a reduction in water velocity and wave height, increasing the odds of survival by 197% and 437% when mangroves were planted in the landward and seaward rows, respectively. Compared to seedlings, deployment of adult mangroves increased survival odds by 1087%. Collectively, our results indicate that sites with a high-water season should utilize a breakwater structure and mangroves with a woody stem.