Maximizing oyster-reef growth supports green infrastructure with accelerating sea-level rise

Estimating mussel mound distribution and geometric properties in coastal salt marshes by using UAV-Lidar point clouds

The Atlantic ribbed mussel (Geukensia demissa) is common in southeastern US salt marshes, where they form dense aggregations (mounds), that occur in the highest densities and sizes on the marsh platform close to the tidal creeks' heads. Within these marshes, mussels help build marsh elevation via their biodeposition of organic and inorganic material, stimulate the growth of the dominant foundation species cordgrass (Spartina alterniflora), and create hotspots of invertebrate biodiversity, nutrient cycling, and drought resilience. Given their powerful role, there is rising interest in assessing natural variation in the distribution of mussel mounds and using such information to guide marsh conservation and restoration strategies. However, gathering such information is challenging, because the small dimension (∼1 m) of the mounds and the presence of overlying vegetation make it difficult to quantify mound distribution on the marsh. Therefore, this study presents a new procedure to compute the distribution, height, radius, volume, and distance of mounds in marsh environments using remote sensing. A high-resolution UAV-Lidar point cloud has been collected over a highly vegetated salt marsh in Georgia, USA, using a custom-built laser scanner system. An original detection algorithm, based on a Random Forest classifier, has been implemented to identify the mounds from the point cloud. The algorithm has been trained and tested on surveyed mounds and provides their location and geometric properties. Results indicate that the classifier can distinguish mussel mounds from non-mussel mound locations with an accuracy of 95 %. The classifier identified ∼8000 mounds, which occupy 10 % of the study domain, and a volume (shells+feces/pseudofeces) of 680 m³. The method is highly useful in efforts to monitor mussel mounds over time and scale up to assess mounds across sites, providing invaluable data for future studies related to the geomorphic evolution of marshes to sea level rise and siting marsh conservation and enhancement projects.

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.

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.

Estimation of Intertidal Oyster Reef Density Using Spectral and Structural Characteristics Derived from Unoccupied Aircraft Systems and Structure from Motion Photogrammetry

Eastern oysters (Crassostrea virginica) are an important component of the ecology and economy in coastal zones. Through the long-term consolidation of densely clustered shells, oyster reefs generate three-dimensional and complex structures that yield a suite of ecosystem services, such as nursery habitat, stabilizing shorelines, regulating nutrients, and increasing biological diversity. The decline of global oyster habitat has been well documented and can be attributed to factors, such as overharvesting, pollution, and disease. Monitoring oyster reefs is necessary to evaluate persistence and track changes in habitat conditions but can be time and labor intensive. In this present study, spectral and structural metrics of intertidal oyster reefs derived from Unoccupied Aircraft Systems (UAS) and Structure from Motion (SfM) outputs are used to estimate intertidal oyster density. This workflow provides a remote, rapid, nondestructive, and potentially standardizable method to assess large-scale intertidal oyster reef density that will significantly improve management strategies to protect this important coastal resource from habitat degradation.

Reef design and site hydrodynamics mediate oyster restoration and marsh stabilization outcomes

The detrimental ecological impacts of engineered shoreline protection methods (e.g., seawalls) and the need to protect the coastal zone have prompted calls for greater use of natural and nature-based infrastructure (NNBI). To balance competing needs of structural stability and ecological functioning, managers require assessments of NNBI designs and materials for differing environmental settings (e.g., among wave-energy regimes). To examine the effects of setting and oyster-based NNBI design on the provision of shoreline protection, we constructed reefs from two substrates: a novel, biodegradable material (Oyster Catcher, OC) and traditional oyster shell bags (SB) on low- and high-energy eroding salt marsh shorelines, designated based on fetch and boat wake exposure. Both reef types buffered marsh elevation change on the high-energy shoreline relative to unaltered controls, but only SB reefs were able to do so on the low-energy shoreline. Additionally, both shorelines experienced high ambient rates of retreat and declines in marsh vegetation shoot density. Although constructed reefs did not mitigate marsh retreat on the low-energy shoreline, novel OC reefs significantly reduced retreat relative to SB reefs and control sites (no reefs) on the high-energy shoreline. Those SB reefs were severely damaged by storm events, increasing their areal footprints at the expense of vertical relief. Conversely, OC reefs on both shorelines exhibited steady oyster recruitment and growth and hosted higher densities of larger oysters. To successfully provide shoreline stabilization benefits, oyster-based NNBI must be structurally stable and able to promote sustained oyster recruitment and growth. Our results indicate that deliberate decisions regarding NNBI substrate, siting, and configuration can produce resilient reefs, which reduce rates of erosion and, in some cases, enhance vertical accretion along salt marsh edges. The growth trajectory, structural stability, and co-benefit provisioning of OC reefs demonstrate the potential of alternative restoration substrates to provide valuable oyster habitat along threatened marsh shorelines.

Temperature-buffering by oyster habitat provides temporal stability for rocky shore communities

Intertidal rocky shores are considered among the most thermally stressful marine ecosystems, where many species live close to their upper thermal limit and depend on access to cool microclimates to persist through heat events. In such environments, the provision of cool microclimates by habitat-forming species enables persistence of associated species during high temperature events. We assessed whether, by maintaining cool microclimates through heat events, habitat formed by rock oysters (Saccostrea cucullata) provides temporal stability to associated invertebrate communities over periods of extreme temperatures. On three tropical rocky shores of Hong Kong, which experiences a monsoonal climate, we compared changes in microclimates and invertebrate communities associated with oyster and bare rock habitats between the cool and hot seasons. Oyster habitats were, across both seasons, consistently characterised by lower maximum temperatures and greater thermal stability than bare rock habitats. Invertebrate communities in the bare rock habitat were less diverse and abundant in the hot than the cool season, but communities in the cooler habitats provided by oysters did not display temporal change. These results demonstrate that microclimates formed by oysters provide temporal stability to associated communities across periods of temperature change and are key determinants of species distributions in thermally stressful environments. The conservation and restoration of oyster habitats may, therefore, build resilience in associated ecological communities subject to ongoing environmental change.

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

One of the paramount goals of oyster reef living shorelines is to achieve sustained and adaptive coastal protection, which requires meeting ecological (i.e., develop a self-sustaining oyster population) and engineering (i.e., provide coastal defense) targets. In a large-scale comparison along the Atlantic and Gulf coasts of the United States, the efficacy of various designs of oyster reef living shorelines at providing wave attenuation was evaluated accounting for the ecological limitations of oysters with regard to inundation duration. A critical threshold for intertidal oyster reef establishment is 50% inundation duration. Living shorelines that spent less than one-half of the time (<50%) inundated were not considered suitable habitat for oysters, however, were effective at wave attenuation (68% reduction in wave height). Reefs that experienced >50% inundation were considered suitable habitat for oysters, but wave attenuation was similar to controls (no reef; ~5% reduction in wave height). Many of the oyster reef living shoreline approaches therefore failed to optimize the ecological and engineering goals. In both inundation regimes, wave transmission decreased with an increasing freeboard (difference between reef crest elevation and water level), supporting its importance in the wave attenuation capacity of oyster reef living shorelines. However, given that the reef crest elevation (and thus freeboard) should be determined by the inundation duration requirements of oysters, research needs to be refocused on understanding the implications of other reef parameters (e.g., width) for optimizing wave attenuation. A broader understanding of the reef characteristics and seascape contexts that result in effective coastal defense by oyster reefs is needed to inform appropriate design and implementation of oyster-based living shorelines globally.

How natural processes contribute to flood protection - A sustainable adaptation scheme for a wide green dike

Effective adaptation to sea-level rise is critical for future flood protection. Nature-based solutions including salt marshes have been proposed to naturally enhance coastal infrastructure. A gently sloping grass-covered dike (i.e. Wide Green Dike) can be strengthened with clay accumulating locally in the salt marsh. This study explores the feasibility of extracting salt-marsh sediment for dike reinforcement as a climate adaptation strategy in several sea-level rise scenarios, using the Wide Green Dike in the Dutch part of the Ems-Dollard estuary as a case study. A 0-D sedimentation model was combined with a wave propagation model, and probabilistic models for wave impact and wave overtopping. This model system was used to determine the area of borrow pits required to supply clay for adequate dikes under different sea-level rise scenarios. For medium to high sea-level rise scenarios (>102 cm by 2100) thickening of the clay layer on the dike is required to compensate for the larger waves resulting from insufficient marsh accretion. The model results indicate that for our case study roughly 9.4 ha of borrow pit is sufficient to supply clay for 1 km of dike reinforcement until 2100. The simulated borrow pits are refilled within 22 simulation years on average, and infilling is projected to accelerate with sea-level rise and pit depth. This study highlights the potential of salt marshes as an asset for adapting flood defences in the future.

Protecting Coastlines from Flooding in a Changing Climate: A Preliminary Experimental Study to Investigate a Sustainable Approach

Rising sea levels are causing more frequent flooding events in coastal areas and generate many issues for coastal communities such as loss of property or damages to infrastructures. To address this issue, this paper reviews measures currently in place and identifies possible control measures that can be implemented to aid preservation of coastlines in the future. Breakwaters present a unique opportunity to proactively address the impact of coastal flooding. However, there is currently a lack of research into combined hard and soft engineering techniques. To address the global need for developing sustainable solutions, three specific breakwater configurations were designed and experimentally compared in the hydraulic laboratory at Coventry University to assess their performance in reducing overtopping and the impact of waves, quantifying the effectiveness of each. The investigation confirmed that stepped configurations work effectively in high amplitudes waves, especially with the presence of a slope angle to aid wave reflection. These results provide a very valuable preliminary investigation into novel sustainable solutions incorporating both artificial and natural based strategies that could be considered by local and national authorities for the planning of future mitigation strategies to defend coastal areas from flooding and erosion.

Effects of inundation duration on southeastern Louisiana oyster reefs

Understanding the effects of predicted rising sea levels, combined with changes in precipitation and freshwater inflow on key estuarine ecosystem engineers such as the eastern oyster would provide critical information to inform restoration design and predictive models. Using oyster ladders with shell bags placed at three heights to capture a range of inundation levels, oyster growth of naturally recruited spat was monitored over the course of 6 months. Oyster numbers and shell heights were consistently highest in bottom and mid bags experiencing greater than 50% inundation (mid: 63 ± 7%; bottom: 95 ± 3%). Identifying thresholds for optimal oyster growth and survival to enhance restoration engineering would require finer scale evaluation of inundation levels.

Coastal sedimentation across North America doubled in the 20th century despite river dams

The proliferation of dams since 1950 promoted sediment deposition in reservoirs, which is thought to be starving the coast of sediment and decreasing the resilience of communities to storms and sea-level rise. Diminished river loads measured upstream from the coast, however, should not be assumed to propagate seaward. Here, we show that century-long records of sediment mass accumulation rates (g cm⁻² yr⁻¹) and sediment accumulation rates (cm yr⁻¹) more than doubled after 1950 in coastal depocenters around North America. Sediment sources downstream of dams compensate for the river-sediment lost to impoundments. Sediment is accumulating in coastal depocenters at a rate that matches or exceeds relative sea-level rise, apart from rapidly subsiding Texas and Louisiana where water depths are increasing and intertidal areas are disappearing. Assuming no feedbacks, accelerating global sea-level rise will eventually surpass current sediment accumulation rates, underscoring the need for including coastal-sediment management in habitat-restoration projects.

The proliferation of dams since 1950 has promoted sediment deposition in reservoirs, which is thought to be starving the coast of sediment and decreasing resistance to storms and sea-level rise. Here, the authors show that century-long records of sediment mass accumulation rates and sediment accumulation rates more than doubled after 1950 in coastal depocenters around North America.

Geomorphology and Species Interactions Control Facilitation Cascades in a Salt Marsh Ecosystem

Facilitation cascades are chains of positive interactions that occur as frequently as trophic cascades and are equally important drivers of ecosystem function, where they involve the overlap of primary and secondary, or dependent, habitat-forming foundation species [1]. Although it is well recognized that the size and configuration of secondary foundation species' patches are critical features modulating the ecological effects of facilitation cascades [2], the mechanisms governing their spatial distribution are often challenging to discern given that they operate across multiple spatial and temporal scales [1, 3]. We therefore combined regional surveys of southeastern US salt marsh geomorphology and invertebrate communities with a predator exclusion experiment to elucidate the drivers, both geomorphic and biotic, controlling the establishment, persistence, and ecosystem functioning impacts of a regionally abundant facilitation cascade involving habitat-forming marsh cordgrass and aggregations of ribbed mussels. We discovered a hierarchy of physical and biological factors predictably controlling the strength and self-organization of this facilitation cascade across creekshed, landscape, and patch scales. These results significantly enhance our capacity to spatially predict coastal ecosystem function across scales based on easily identifiable metrics of geomorphology that are mechanistically linked to ecological processes. Replication of this approach across vegetated coastal ecosystems has the potential to support management efforts by elucidating the multi-scale linkages between geomorphology and ecology that, in turn, define spatially explicit patterns in community assembly and ecosystem functioning.

Rapid and Accurate Monitoring of Intertidal Oyster Reef Habitat Using Unoccupied Aircraft Systems and Structure from Motion

Oysters support an economically important fishery in many locations in the United States and provide benefits to the surrounding environment by filtering water, providing habitat for fish, and stabilizing shorelines. Changes in oyster reef health reflect variations in factors such as recreational and commercial harvests, predation, disease, storms, and broader anthropogenic influences, such as climate change. Consistent measurements of reef area and morphology can help effectively monitor oyster habitat across locations. However, traditional approaches to acquiring these data are time-consuming and can be costly. Unoccupied aircraft systems (UAS) present a rapid and reliable method for assessing oyster habitat that may overcome these limitations, although little information on the accuracy of platforms and processing techniques is available. In the present study, oyster reefs ranging in size from 30 m2 to 300 m2 were surveyed using both fixed-wing and multirotor UAS and compared with ground-based surveys of each reef conducted with a real-time kinematic global positioning system (RTK-GPS). Survey images from UAS were processed using structure from motion (SfM) stereo photogrammetry techniques, with and without the use of ground control point (GCP) correction, to create reef-scale measures of area and morphology for comparison to ground-based measures. UAS-based estimates of both reef area and morphology were consistently lower than ground-based estimates, and the results of matched pairs analyses revealed that differences in reef area did not vary significantly by aircraft or the use of GCPs. However, the use of GCPs increased the accuracy of UAS-based reef morphology measurements, particularly in areas with the presence of water and/or homogeneous spectral characteristics. Our results indicate that both fixed-wing and multirotor UAS can be used to accurately monitor intertidal oyster reefs over time and that proper ground control techniques will improve measurements of reef morphology. These non-destructive methods help modernize oyster habitat monitoring by providing useful and accurate knowledge about the structure and health of oyster reefs ecosystems.

Oyster breakwater reefs promote adjacent mudflat stability and salt marsh growth in a monsoon dominated subtropical coast

Oyster reefs have the potential as eco-engineers to improve coastal protection. A field experiment was undertaken to assess the benefit of oyster breakwater reefs to mitigate shoreline erosion in a monsoon-dominated subtropical system. Three breakwater reefs with recruited oysters were deployed on an eroding intertidal mudflat at Kutubdia Island, the southeast Bangladesh coast. Data were collected on wave dissipation by the reef structures, changes in shoreline profile, erosion-accretion patterns, and lateral saltmarsh movement and related growth. This was done over four seasons, including the rainy monsoon period. The observed wave heights in the study area ranged 0.1–0.5 m. The reefs were able to dissipate wave energy and act as breakwaters for tidal water levels between 0.5–1.0 m. Waves were totally blocked by the vertical relief of the reefs at water levels <0.5 m. On the lee side of the reefs, there was accretion of 29 cm clayey sediments with erosion reduction of 54% as compared to control sites. The changes caused by the deployed reefs also facilitated seaward expansion of the salt marsh. This study showed that breakwater oyster reefs can reduce erosion, trap suspended sediment, and support seaward saltmarsh expansion demonstrating the potential as a nature-based solution for protecting the subtropical coastlines.

The mechanisms by which oysters facilitate invertebrates vary across environmental gradients

The effective use of ecosystem engineers to conserve biodiversity requires an understanding of the types of resources an engineer modifies, and how these modifications vary with biotic and abiotic context. In the intertidal zone, oysters engineer ecological communities by reducing temperature and desiccation stress, enhancing the availability of hard substrate for attachment, and by ameliorating biological interactions such as competition and predation. Using a field experiment manipulating shading, predator access and availability of shell substrate at four sites distributed over 900 km of east Australian coastline, we investigated how the relative importance of these mechanisms of facilitation vary spatially. At all sites, and irrespective of environmental conditions, the provision of hard substrate by oysters enhanced the abundance and richness of invertebrates, in particular epibionts (barnacles and oyster spat) and grazing gastropods. Mobile arthropods utilised the habitat provided by disarticulated dead oysters more than live oyster habitat, whereas the abundance of polychaetes and bivalves were much greater in live oysters, suggesting the oyster filter-feeding activity is important for these groups. In warmer estuaries, shading by oysters had a larger effect on biodiversity, whereas in cooler estuaries, the provision of a predation refuge by oysters played a more important role. Such knowledge of how ecosystem engineering effects vary across environmental gradients can help inform management strategies targeting ecosystem resilience via the amelioration of specific environmental stressors, or conservation of specific community assemblages.

Investing in Natural and Nature-Based Infrastructure: Building Better Along Our Coasts

Much of the United States’ critical infrastructure is either aging or requires significant repair, leaving U.S. communities and the economy vulnerable. Outdated and dilapidated infrastructure places coastal communities, in particular, at risk from the increasingly frequent and intense coastal storm events and rising sea levels. Therefore, investments in coastal infrastructure are urgently needed to ensure community safety and prosperity; however, these investments should not jeopardize the ecosystems and natural resources that underlie economic wealth and human well-being. Over the past 50 years, efforts have been made to integrate built infrastructure with natural landscape features, often termed “green” infrastructure, in order to sustain and restore valuable ecosystem functions and services. For example, significant advances have been made in implementing green infrastructure approaches for stormwater management, wastewater treatment, and drinking water conservation and delivery. However, the implementation of natural and nature-based infrastructure (NNBI) aimed at flood prevention and coastal erosion protection is lagging. There is an opportunity now, as the U.S. government reacts to the recent, unprecedented flooding and hurricane damage and considers greater infrastructure investments, to incorporate NNBI into coastal infrastructure projects. Doing so will increase resilience and provide critical services to local communities in a cost-effective manner and thereby help to sustain a growing economy.

Evidence of exceptional oyster-reef resilience to fluctuations in sea level

Ecosystems at the land-sea interface are vulnerable to rising sea level. Intertidal habitats must maintain their surface elevations with respect to sea level to persist via vertical growth or landward retreat, but projected rates of sea-level rise may exceed the accretion rates of many biogenic habitats. While considerable attention is focused on climate change over centennial timescales, relative sea level also fluctuates dramatically (10-30 cm) over month-to-year timescales due to interacting oceanic and atmospheric processes. To assess the response of oyster-reef (Crassostrea virginica) growth to interannual variations in mean sea level (MSL) and improve long-term forecasts of reef response to rising seas, we monitored the morphology of constructed and natural intertidal reefs over 5 years using terrestrial lidar. Timing of reef scans created distinct periods of high and low relative water level for decade-old reefs (n = 3) constructed in 1997 and 2000, young reefs (n = 11) constructed in 2011 and one natural reef (approximately 100 years old). Changes in surface elevation were related to MSL trends. Decade-old reefs achieved 2 cm/year growth, which occurred along higher elevations when MSL increased. Young reefs experienced peak growth (6.7 cm/year) at a lower elevation that coincided with a drop in MSL. The natural reef exhibited considerable loss during the low MSL of the first time step but grew substantially during higher MSL through the second time step, with growth peaking (4.3 cm/year) at MSL, reoccupying the elevations previously lost. Oyster reefs appear to be in dynamic equilibrium with short-term (month-to-year) fluctuations in sea level, evidencing notable resilience to future changes to sea level that surpasses other coastal biogenic habitat types. These growth patterns support the presence of a previously defined optimal growth zone that shifts correspondingly with changes in MSL, which can help guide oyster-reef conservation and restoration.