Living shorelines that comprise oyster reefs within a mosaic of multiple coastal habitats can be a resilient and adaptive coastal protection alternative to conventional engineered structures. The success of an oyster reef living shoreline depends on the evolution of a stable base substrate that provides initial short-term coastal protection, to the growth of a living oyster reef and associated habitats in the longer-term, which can enhance protection and provide other ecosystem services. Interdisciplinary global teams that include ecologists, biologists, engineers, technoeconomists, and industry are leading the development and implementation of innovative reef-based coastal protection ("Reefense") solutions that can be scaled to face the challenges of climate change.

The implementation of nature-based solutions (NbS), including living shorelines, to mitigate estuarine habitat loss is increasing at a pace exceeding the evaluation of their long-term success. Constructed oyster reefs (CORs) made of shell, concrete, stone, and other materials are one living shoreline tactic that is widely utilized, yet few studies have been conducted to understand the development of CORs within the context of both physical and ecological parameters over longer time scales (4 + years). A COR-based living shoreline project at the Gandy’s Beach Preserve (GBP) in Delaware Bay, NJ, USA, had dual goals of coastal protection and habitat provisioning, which prompted the development of a goal-driven monitoring framework to track project objectives. Methods were developed to quantify the following multi-disciplinary metrics over 7 years: elevations of CORs, waves (height, period, and direction), shoreline elevations, change in extent of vegetation patches, oyster density and size, nekton richness and community composition, and horseshoe crab impingement. The CORs met most of their habitat provisioning objectives as they were colonized by a multi-generational population of shellfish and created habitat for nekton, while posing negligible hazards to horseshoe crabs. However, none of the coastal protection objectives was fully achieved including material stability, wave attenuation, and sediment elevation increase. Results highlight the value of longer-term monitoring to understand performance and the need to match the scale and type of NbS tactic(s) with both the scale of the landscape and the site-specific hydrodynamic conditions to meet project goals.

Coastal nature-based solutions (NbS) have emerged as powerful tools to enhance sustainable development and ecological restoration goals. As a rapidly growing field spanning across social, political, ecological, economic, and engineering disciplines, it is critical that researchers working in coastal NbS regularly attempt to identify emerging focal areas for scientific inquiry. Following the 27th Biennial meeting of the Coastal and Estuarine Research Federation, we provide a transdisciplinary perspective (including biologists, engineers, oceanographers, geoscientists, economists, and facilitators of workforce training programs) of pertinent research questions that, if answered, will advance the effectiveness, sustainability, and widespread adoption of coastal NbS. These suggestions for future research highlight the necessity for diverse expertise and perspectives at every stage in planning, design, implementation, and monitoring coastal NbS.

Eastern oyster (Crassostrea virginica) populations are declining due to habitat degradation, overharvesting, and disease. One factor of potentially equal importance that has received little focus is the impact of food quantity and size composition of the seston supporting larval development, survival, and success at metamorphosis. Phytoplankton serve as a primary source of nutrition for oyster larvae; its size composition and content tend to fluctuate with environmental conditions. These fluctuations, in turn, may significantly impact larval performance. To investigate the potential of variations in the size composition of the seston determining the dynamics of larval performance, simulations are developed to project larval success in relation to variations in three size classes of planktonic food, <5, 5-20, and >20 mu m. A biochemically-based larval performance model is implemented to track key outcomes, defined as the success rate at metamorphosis, mortality during development due to suboptimal food supply, and mortality due to egg quality characteristics supporting early development. The results support the potential of variations in the size composition of planktonic food as a key driver of outcomes, potentially explaining some of the well-described year-to-year volatility in recruitment. The results point to a number of key influences on larval performance including the critical need for sufficient availability of small food particles at the end of the trochophore stage, suggesting a potentially key match-mismatch point in larval development. The results point to the influence of larger food particles later in larval life in supporting increased size at metamorphosis and in reducing planktonic lifetimes, thereby reducing exposure to predation. Simulations also show the influence of egg size and larval physiology supporting increased survival when food size composition is suboptimal. Given the outcomes of this study, increased interest in the size-composition of seston in the field relative to larval performance metrics would seem warranted.

The capacity for oyster (Crassostrea virginica) larvae to successfully develop, grow, and survive is dependent on the quantity and quality of available food. Therefore, examining food supply based upon its biochemical composition of lipid, protein, and labile carbohydrate offers critical insight into oyster larval performance and settlement potential. Biochemical analyses, however, are time-intensive, requiring that such studies be retrospective, generating a need to investigate alternative, real-time techniques to characterize food supply, such as evaluating inherent optical properties. To better understand the food assemblage available to oyster larvae, water samples from seven oyster reefs in Mississippi Sound over two years (May through October) were analyzed for temperature, salinity, particulate organic matter, biochemical properties (lipid, protein, carbohydrate), and in situ optical properties used to infer plankton abundance by size class (pico-, nano-, microplankton), total chlorophyll content, and the magnitude of absorption for colored detrital material. A subset of observations, considered to reflect conditions facilitative for oyster larval survival, clustered into four statistically distinct groups characterized by: high-microplankton, low-microplankton, high-protein, and high-salinity. Total chlorophyll content changed with abiotic conditions, with relatively high concentrations during oligohaline and mesohaline regimes, but declined during periods of changing salinity. Further, transitioning salinities co-occurred with reductions in microplankton concentration and increases in picoplankton concentration. Results of a Spearman’s rank analysis, principal components analysis, and stepwise linear regressions revealed that optical properties were not strongly associated with biochemical properties, preventing these optical data from providing an effective index of oyster larval food supply. Instead, optically-derived microplankton abundance recapitulated total chlorophyll, both of which poorly corresponded to biochemical properties. Picoplankton, colored detrital material, and particulate organic material all exhibited a similarly weak correspondence to biochemical food properties, corroborating that measurements of chlorophyll and particulates do not accurately reflect the food available to oyster larvae, and that biochemical metrics remain as superior food supply indicators.

Oyster Crassostrea virginica population recovery is critical in degraded estuarine systems, such as Mississippi Sound, USA, where repeated mass mortality events have depleted local oyster stocks. Owing to multiple recent die-offs, the western Mississippi Sound oyster population is recruitment-limited; population growth is constrained by the entry of new individuals into the extant population. Therefore, oyster recovery requires an adequate supply of larvae capable of timely development, growth, and successful metamorphosis. Larval performance and settlement potential are influenced by ambient temperature, salinity, and food supply. Food quantity is important to larvae, but so is food quality, as larvae require a balanced diet of lipids, proteins, and carbohydrates to develop and survive through metamorphosis. In this study, in situ environmental and food conditions during the 2021 and 2022 spawning seasons from 7 oyster reefs in western Mississippi Sound were integrated into an established biochemically based larval performance model to estimate periods facilitative of successful metamorphosis. In 2021, model-estimated larval survivorship was suppressed through much of the spawning season by prolonged, extremely low salinity (<5 ppt) and inadequately balanced food supply. Higher seasonal salinity and more balanced food composition increased model-estimated larval survivorship in 2022, despite lower total food content, suggesting larval performance was primarily governed by the quality of available food. Model-estimated settlement windows were compared to settlement windows derived from concomitant field observations of recruitment. Strong agreement between model-estimated and observed settlement windows validates the effectiveness of the model and informs on the underlying causes of recruitment limitation in western Mississippi Sound.

The Eastern Oyster ( Crassostrea virginica ) is a commercially important aquaculture species and food resource along the Atlantic and Gulf coasts of the USA. In addition to its economic value, oyster aquaculture provides ecological value such as water quality improvement. Oyster filtration is highly variable as filtration behavior is influenced by environmental conditions, oyster size, and oyster energetic demands. However, average rates generated in laboratory experiments are often used to estimate the ecological impact of oyster filtration, and there is a need for field-based, farm-specific estimates of filtration that account for this variation. In this study, field experiments were conducted between September 2020 and September 2021 to estimate seasonal oyster filtration physiology at oyster farms in three different bays in the Mid-Atlantic (Barnegat Bay and Delaware Bay in New Jersey and Rehoboth Bay in Delaware). The physiological activity of oysters at each farm varied such that oysters at Barnegat Bay were the most active and oysters at Rehoboth Bay were the least active. Seasonal physiological trends were observed such that filtration behavior generally increased in warmer months. An increase in physiological activity across all farms was associated with an increase in salinity and temperature, but physiological activity at each farm was associated with a different suite of environmental variables including total particulate matter and the organic content of seston. This study provides a robust dataset which can be incorporated into models estimating ecological filtration rates in the Mid-Atlantic and adds to the growing body of evidence supporting bivalve aquaculture as a nutrient reduction strategy.

Ecosystems provide essential services to people including food, water, climate regulation, and aesthetic experiences. Biodiversity can enhance and stabilize ecosystem function and the resulting services natural systems provide. Freshwater mollusks are a diverse group that provide a variety of ecosystem services through their feeding habits (e.g., filter feeding, grazing), top-down and bottom-up effects on food webs, provisioning of habitat, use as a food resource by people, and cultural importance. Research focused on quantifying the direct and indirect ways mollusks influence ecosystem services may help inform policy makers and the public about the value of mollusk communities to society. The Freshwater Mollusk Conservation Society highlighted the need to evaluate mollusk ecosystem services in their 2016 National Strategy for the Conservation of Native Freshwater Mollusks, and, while significant progress has been made, considerable work remains across the research, management, and outreach communities. We briefly review the global status of native freshwater mollusks, assess the current state of knowledge regarding their ecosystem services, and highlight recent advances and knowledge gaps to guide further research and conservation actions. Our intention is to provide ecologists, conservationists, economists, and social scientists with information to improve science-based consideration of the social, ecological, and economic value of mollusk communities to healthy aquatic systems.

Oyster population maintenance and growth require a sufficient larval supply competent for metamorphosis and settlement. Larval performance, in terms of growth, development, survival, and metamorphic success, determines the capacity for a larval cohort to effectively settle and establish into an existing population. Exogenous factors influencing larval development include temperature, salinity, food quantity, and food quality. A sufficient diet, composed of balanced protein, lipids, and carbohydrates to meet larval nutritional demands, is required to promote successful metamorphosis. To evaluate the influence of these exogenous factors on oyster settlement potential in Delaware Bay, a well-established biochemically based Crassostrea gigas (Thunberg, 1793) larval model was adapted to simulate Crassostrea virginica (Gmelin, 1791) larval performance under in situ environmental conditions measured during the 2009 to 2011 reproductive seasons at 10 sites across the salinity gradient of Delaware Bay. Variation in the initial egg size and lipid content, and larval food assimilation efficiency was incorporated into the model to represent potential within-cohort phenotypic variability. The middle portion of Delaware Bay along the New Jersey shoreline, bridging the 15-salinity line, generated the most successful larvae each year, whereas the low-salinity reach, on the Delaware side, and Nantuxent Point Reef had more variable success. Survivorship was a function of adequate temperatures and salinities, sufficient food quantity, and favorable food quality defined in part by the protein-to-(lipid-plus-carbohydrate) ratio. Most settlement was predicted by the model to occur between July and September of each year. To validate the model, estimated settlement windows were compared with calculated settlement windows derived from recruitment observations on yearly shell plants. Modeled and recruitment-derived settlement windows agreed well with each other and verified the capacity of the model to accurately forecast in situ larval performance. The oyster larval model, based on measures of lipid, protein, and carbohydrate, successfully passed an important field test, demonstrating the potential of such biochemically based models to reliably evaluate larval performance under real-world conditions.

Bivalve shellfish are common in coastal ecosystems where their aggregate structure attenuates wave energy and provides habitat, while delivering water quality benefits through their feeding activity. These factors make them appropriate candidates for inclusion in living shorelines to facilitate positive ecological outcomes. In 2014, a 61 m shellfish-based living shoreline was constructed along a salt marsh at the DuPont Nature Center in Milford, Delaware with the goal to maximize shellfish populations for water quality benefits. Monitoring was conducted to assess oyster and ribbed mussel population development and evaluate their relative contribution to cumulative filtration at three positions- on structures near mean low water (low), along the marsh edge (high), and on the untreated mudflat between (mid). Oyster and ribbed mussel counts and size demographics were converted to population and biomass densities to calculate filtration capacity in each position through 2020. Cumulatively, shellfish on the Mispillion living shoreline filtered 6763 kg of seston, but population and biomass development varied spatiotemporally between species. Between 2018 and 2020, oyster population and biomass densities declined at the low, but increased at the high positons, while ribbed mussel densities increased at both. Despite differences among species and position, the annual summed filtration of the low and high positions continually increased. These results indicate that a multi-species approach, across a variety of appropriate spatial niches, can help maintain or enhance overall filtration capacity through either complementary species contributions at a single positon, or spatial compensation by a single species across positons.

Tidal wetlands in the Mid-Atlantic, USA, are experiencing high rates of relative sea level rise, and it is unclear whether they will be resilient in the face of future flooding increases. In a previous study, we found 80% of our study areas in tidal freshwater and salt marshes in the Delaware Estuary and Barnegat Bay had elevation change rates lower than the 19-year increase in mean sea level. Here, we examine relationships between marsh elevation dynamics and abiotic and biotic parameters in order to assess their utility as indicators of vulnerability to relative sea level rise. We further apply a range of marsh vulnerability indicators including elevation change rates to evaluate their ability to corroborate marsh habitat change over the last 30 years. Of the field measurements, soil bulk density and belowground plant biomass were among the strongest predictors of elevation change and accretion dynamics across all marsh types and settings. Both tidal freshwater and salt marshes tended to have higher rates of elevation increase and surface accretion in areas where soil bulk density and live belowground biomass were higher. Nine of the ten marshes experienced a net loss of area from the 1970s to 2015 ranging from 0.05 to 14%. Although tidal freshwater marshes were low in elevation and experienced variable elevation change rates, marsh area loss was low. Conversely, salt marshes closest to the coast and perched high in the tidal frame with a higher degree of human modification tended to experience the greatest marsh loss, which incorporated anthropogenic impacts and edge erosion. Thus, our regional assessment points to the need for a comprehensive understanding of factors that influence marsh resilience including human modifications and geomorphic settings.

Tidal marshes are highly valued habitats, yet are vulnerable to loss from both anthropogenic and natural disturbances including sea-level rise (SLR). Many tidal marshes have kept pace with SLR over the last century; on average, however, recent escalations in SLR increase the vulnerability of marshes to submergence. Relative sea-level rise near our study sites in the Mid-Atlantic U.S. averaged 4.34 mm year −1 over the last 50 years, yet over the last 19 years, relative sea-level rise averaged 6.25 mm year −1 and the rise in high tide water levels averaged 8.13 mm year −1 . We compared these rates of water rise to rates of marsh surface elevation change using surface elevation tables in ten tidal marshes—three tidal freshwater and four saline marshes in the Delaware Estuary and three salt marshes in Barnegat Bay, NJ, USA. We also examined the effects of marsh type and geomorphic setting on rates of elevation change, surface accretion, and subsurface change as well as the influence of marsh elevation, distance from a channel, and tidal range ( n = 3 sites per marsh). Surface elevation change measured over the last 4 to 9 years averaged less than 6 mm year −1 in nine out of ten of the study stations, less than rates of relative SLR. Tidal freshwater marshes in the Delaware Estuary had greater rates of surface accretion and elevation change than salt marshes in Barnegat Bay. Marshes sitting lower in the tidal frame and experiencing higher tidal ranges tended to have higher surface accretion rates, but shallow subsidence had a stronger influence on these elevation change rates. We estimated time to submergence (i.e., lifespan) for using rates of marsh elevation change, a SLR rate of 10 mm year −1 , and thresholds for conversion to open water using geospatial datasets. The calculated time to submergence for the majority of marshes was 60 to 80 years with some predicted to submerge in as few as 5 years. These data suggest that in order to keep pace with accelerating SLR, surface accretion in many of these marshes will have to increase at a rate that surpasses shallow subsidence (1–7 mm year −1 ).

Flood frequencies in coastal forests are increasing as sea level rise accelerates from 3–4 mm year−1 to possibly more than 10 mm year−1 by the end of this century. As flooding increases, coastal forests retreat, ghost forests form, and coastal marshes migrate inland. The existence of ghost forests makes the mechanism of forest retreat clear: low-lying trees become more exposed to coastal flooding until they ultimately die. Variability in these retreat rates, however, makes it difficult to predict where and when retreat will continue to occur. Understanding tree growth responses to tidal water levels relative to other environmental factors is a critical step in elucidating the factors that influence retreat variability. Here, dendrochronology was used to study factors that contribute to variations in growth patterns in four coastal forests fringing the Delaware and Barnegat Bays. Species chosen for study included loblolly pine (Pinus taeda), pitch pine (Pinus rigida), and American holly (Ilex opaca). Pearson’s and partial correlation tests showed that growth relationships with monthly environmental conditions varied across sites and were moderate in strength (generally R < 0.5), but each site had at least one significant growth-water level correlation. As coastal flooding exposure is spatially dependent, tree chronologies were also separated into high and low elevation groups. Pearson’s and partial correlation tests of the mean differences between elevation groups showed that at some sites, low elevation trees grew less than high elevation trees when water levels were high, as might be expected. At one site, however, lower elevation trees grew more when water levels were higher, which suggests that other interacting factors—regardless of current flood exposure—potentially have positive, yet likely temporary, influence over tree growth in these low-lying areas.

ABSTRACT Moody, J.A.; Gentry, M.J.; Bouboulis, S.A., and Kreeger, D.A., 2020. Effects of substrate (protection and type) on ribbed mussel (Geukensia demissa) recruitment for living shoreline applications. Journal of Coastal Research, 36(3), 619–627. Coconut Creek (Florida), ISSN 0749-0208. Ribbed mussels are the functional dominant species in eastern U.S. salt marshes, providing particulate nutrient filtration and enhanced vegetative growth services. Wetland loss in the Delaware Estuary has resulted in ribbed mussel population declines and increased interest in ecological restoration practices, such as living shorelines. Incorporation of ribbed mussels into restoration applications represents an opportunity to enhance ecological structure and function in shoreline stabilization projects. Since 2008, the Partnership for the Delaware Estuary has implemented 14 living shorelines, with ribbed mussel recruitment exhibiting temporal and spatial variability. To better understand sources of this variability, a three-tiered study was employed to evaluate differences in ribbed mussel recruitment: (1) on previously deployed substrates (2009–10); (2) on exposed vs. protected surfaces; and (3) across a variety of substrates. Results showed that on aged living shorelines, ribbed mussel density was greater on oyster shell bags than on coir fiber logs and the unaltered marsh edge. On newly deployed experimental materials, recruitment was higher on the surface of shell bags when protected by mesh but was only <8% of total bag recruitment and did not differ between protected and unprotected shell bags. Mussel recruitment was greater on oyster shell and Oyster Castle® than on coir fiber recruitment tiles, and also on those protected by shell bag mesh than by coir fiber or without protection. Therefore, living shorelines that incorporate ribbed mussel refuge in the form of protected surfaces and interstitial space can facilitate recruitment and persistence, enhancing populations and their associated water quality services.