Exploring the Use of Living Shorelines for Stabilization and Nutrient Mitigation in New England.

As salt marsh habitats face challenges due to sea level rise, storm events, and coastal development, there is an effort to use nature-based approaches such as living shorelines to enhance salt marshes and provide coastal protection. A living shoreline restoration and seasonal monitoring was conducted between July 2016 and October 2018 at an eroding salt marsh on Martha's Vineyard, Massachusetts, Northeastern USA to assess changes in two essential ecosystem services: shoreline stabilization and nitrogen removal. Neither the living shoreline nor unaltered sites demonstrated significant sediment deposition at the marsh edge or on the marsh platform between 2017 and 2018. While we expected nitrogen removal via denitrification to improve at the living shoreline sites over time as abiotic and biotic conditions became more favorable, we found limited support for this hypothesis. We found higher rates of denitrification enzyme activity (DEA) at the living shoreline sites when compared to unaltered sites, but these rates did not increase over time. This study also provides a qualitative assessment of our living shoreline structural integrity through the years, particularly following storm events that greatly challenged our restoration efforts. We demonstrate that living shorelines fortified solely with natural materials may not be the most effective approach to maintain these ecosystem services for Northeastern USA salt marshes exposed to intense northeasterly storms. We suggest the restoration of salt marshes to improve major functions be a priority among managers and restoration practitioners. Initiatives promoting the use of nature-based restoration solution where environmental conditions permit should be encouraged.

Complementary evidence for small-scale spatial assemblages of the exploited grass emperor (Lethrinus laticaudis) in the Shark Bay World Heritage Area, Western Australia.

Understanding the connectivity of exploited fish populations is critical to their management under both rapid and long-term environmental change. Patterns of connectivity are unknown for most fishes in the Shark Bay World Heritage Area (Western Australia), a large, shallow embayment in the eastern Indian Ocean, vulnerable to marine heatwaves. The composition of oxygen (δ18O) and carbon (δ13C) stable isotopes in whole otoliths of the recreationally-important reef fish Lethrinus laticaudis did not differ between Shark Bay's two large inner gulfs, separated by the Peron Peninsula. However, significant differences were found between pairs of locations with different salinities over a spatial scale of ∼60 km within each gulf. Misclassification of samples was greatest between locations mostly in different gulfs, but with similar salinities (15-41%), and rare between adjacent locations in the same gulf with different salinities (0-5%). This is influenced by the strong correlation (ρ = 0.93) between δ18O in otoliths and the salinity gradient of the two gulfs, and further supported by a lack of correlation in the similarities of isotope compositions and distances between locations (ρ = 0.16). Fish samples from each of the different locations were composed of multiple year-classes, yet the otolith chemistry distinguished them at a minimum distance of 16 km apart, indicating that small-scale connectivity of L. laticaudis is likely during the majority of their life cycle. Physical barriers to movement of post-settlement individuals (land masses, expansive seagrass and sand) between the small, isolated reefs of Shark Bay may reduce large scale connectivity, which instead would occur mostly by egg and larval dispersal. The probable scale of connectivity of post-settlement L. laticaudis indicates that this major recreational fishing target species may be vulnerable to localised over-exploitation and negative environmental effects on population sources and sinks within this shallow embayment. Maintaining sustainable spawning biomass at scales relevant to the extent of connectivity for such a species in a World Heritage Area is an important management consideration.

Open SESAME: A Social-Ecological Systems framework for collaborative Adaptive Management and Engagement in coastal restoration and climate adaptation.

The successful implementation and sustainability of many marsh restoration efforts, including coastal adaptation to buffer inundation and mitigate sea level rise, often hinges upon support from surrounding human communities. Yet, stakeholder engagement in these projects remains relatively undervalued and underutilized. We present the Social-Ecological Systems, Adaptive Management, and Engagement (SESAME) framework that provides reciprocal connections between the human and ecological components of restoration efforts and the resulting management and engagement needs. We built and describe this framework through discussion of two case studies of coastal restoration efforts in southern New England salt marshes. The first case study focuses on the use of sediment placement to increase the elevation of the surface of a drowning marsh in Rhode Island as an interim measure to protect against sea level rise. The second case study describes the use of living shorelines for erosion mitigation on a salt marsh in Massachusetts. These cases included significant partner and stakeholder engagement and provided important lessons learned for practical implementation of the SESAME framework. Valuable lessons included the need for engagement throughout the entirety of the process, specific clarification of roles within the restoration efforts, and flexibility in implementation and goal setting.

Beyond Bioextraction: The Role of Oyster-Mediated Denitrification in Nutrient Management.

Recently, interest has grown in using oyster-mediated denitrification resulting from aquaculture and restoration as mechanisms for reactive nitrogen (N) removal. To date, short-term N removal through bioextraction has received the most management interest, but there is a growing body of research that has shown oysters can also mediate the long-term removal of N through denitrification (the microbial conversion of reactive N to relatively inert dinitrogen (N2) gas). Oyster suspension feeding and ammonium release via waste and deposition of organic matter to the sediments can stimulate nitrification-denitrification near oyster reefs and aquaculture sites. Oysters also harbor a diverse microbial community in their tissue and shell promoting denitrification and thus enhanced N removal. Additionally, surface areas on oyster reefs provide a habitat for other filter-feeding macrofaunal communities that can further enhance denitrification. Denitrification is a complex biogeochemical process that can be difficult to convey to stakeholders. These complexities have limited consideration and inclusion of oyster-mediated denitrification within nutrient management. Although oyster-mediated denitrification will not be a standalone solution to excess N loading, it may provide an additional management tool that can leverage oyster aquaculture and habitat restoration as a N mitigation strategy. Here, we provide an overview of the biogeochemical processes involved in oyster-mediated denitrification and summarize how it could be incorporated into nutrient management efforts by various stakeholders.

Catastrophic loss of tropical seagrass habitats at the Cocos (Keeling) Islands due to multiple stressors.

Seagrass habitats at the Cocos (Keeling) Islands (CKI), a remote atoll in the Indian Ocean, have suffered a catastrophic decline over the last decade. Seagrass monitoring (1996-2020) in relation to dredging and coastal development works (2009 to 2011) provide a historical baseline, and document the decline of mixed tropical seagrass Thalassia hemprichii and macroalgal (predominantly Caulerpa spp.) beds over a decadal scale time series. Attribution of loss to coastal development is confounded by lagoon-wide die-off events in 2007, 2009 and 2012 and high air and water temperatures from 2009 to 2016, with evidence of broad scale changes, visible in satellite imagery between 2006 and 2018. We conclude that up to 80% of seagrass habitats in the CKI lagoon (~1200 ha) have been lost due to multiple stressors including episodic die-off events related to high temperatures and calm conditions, and loss due to sediment disturbance and increased turbidity. Grazing pressure from the resident green sea turtles (Chelonia mydas) may have also exacerbated the loss of seagrass, which in turn poses a dire threat to their ongoing health and survival. This study highlights the fragility of tropical seagrass habitats and the cascading effect of system imbalance as a result of anthropogenic pressures and climate drivers. Although small in comparison to global estimates, the loss of seagrass habitats at CKI could change the entire ecosystem of a remote atoll. Due to the significance of the Thalassia beds for coastal stability, as food for an isolated population of green sea turtles and as a fish nursery, rehabilitation efforts are warranted.

Bioextractive Removal of Nitrogen by Oysters in Great Bay Piscataqua River Estuary, New Hampshire, USA.

Eutrophication is a challenge to coastal waters around the globe. In many places, nutrient reductions from land-based sources have not been sufficient to achieve desired water quality improvements. Bivalve shellfish have shown promise as an in-water strategy to complement land-based nutrient management. A local-scale production model was used to estimate oyster (Crassostrea virginica) harvest and bioextraction of nitrogen (N) in Great Bay Piscataqua River Estuary (GBP), New Hampshire, USA, because a system-scale ecological model was not available. Farm-scale N removal results (0.072 metric tons acre-1 year-1) were up-scaled to provide a system-wide removal estimate for current (0.61 metric tons year-1), and potential removal (2.35 metric tons year-1) at maximum possible expansion of licensed aquaculture areas. Restored reef N removal was included to provide a more complete picture. Nitrogen removal through reef sequestration was ~ 3 times that of aquaculture. Estimated reef-associated denitrification, based on previously reported rates, removed 0.19 metric tons N year-1. When all oyster processes (aquaculture and reefs) were included, N removal was 0.33% and 0.54% of incoming N for current and expanded acres, respectively. An avoided cost approach, with wastewater treatment as the alternative management measure, was used to estimate the value of the N removed. The maximum economic value for aquaculture-based removal was $105,000 and $405,000 for current and expanded oyster areas, respectively. Combined aquaculture and reef restoration is suggested to maximize N reduction capacity while limiting use conflicts. Comparison of removal based on per oyster N content suggests much lower removal rates than model results, but model harvest estimates are similar to reported harvest. Though results are specific to GBP, the approach is transferable to estuaries that support bivalve aquaculture but do not have complex system-scale hydrodynamic or ecological models.

Habitat benefits of restored oyster reefs and aquaculture to fish and invertebrates in a coastal pond in Rhode Island, US.

Oyster habitat restoration seeks to recover lost ecosystem services including increased provisioning of refuge and foraging habitat for fish and invertebrate communities. The goal of this study was to quantify the ecosystem service benefit of habitat provisioning in Ninigret Pond, RI following oyster restoration. We measured four metrics, abundance, biomass, species richness and diversity, as well as isotopic composition in fish and invertebrates collected seasonally from restored oyster, aquaculture, and bare sediment sites, to examine whether the oyster habitat outperformed the bare sediment habitat. Sampling locations were chosen in Foster's Cove north and south, Grassy Point, South Sanctuary, and an Aquaculture lease; each had two restored oyster sites and one bare sediment site. Each site was sampled using a box trap, seine net, shrimp trap, and minnow trap. Oyster habitats had significantly greater metrics than did bare sediment habitats in some comparisons from the box trap and seine net samples. Restored oyster sites at South Sanctuary had lower metric values than the other oyster sites. Metrics from the Aquaculture sites were comparable to the Foster's Cove and Grassy Point restored oyster sites and often outperformed South Sanctuary restored oyster sites. Seasonally, spring and autumn samples tended to have higher abundance and biomass values than summer. Isotopic composition of five species occurring at both restored oyster and bare sediment sites demonstrated some differences in the trophic levels between species but not between habitat types. In Ninigret Pond, fish and invertebrate abundance, biomass, species richness, and diversity benefit from the use of oyster and bare sediment habitats. Coastal zone managers interested in restoring the ecological function of oyster reefs to support fish and invertebrate communities should consider strategically locating restoration projects within the mosaic of structured habitats and monitoring them for selected ecosystem services.

Role of Shellfish Aquaculture in the Reduction of Eutrophication in an Urban Estuary.

Land-based management has reduced nutrient discharges; however, many coastal waterbodies remain impaired. Oyster "bioextraction" of nutrients and how oyster aquaculture might complement existing management measures in urban estuaries was examined in Long Island Sound, Connecticut. Eutrophication status, nutrient removal, and ecosystem service values were estimated using eutrophication, circulation, local- and ecosystem-scale models, and an avoided-costs valuation. System-scale modeling estimated that 1.31% and 2.68% of incoming nutrients could be removed by current and expanded production, respectively. Up-scaled local-scale results were similar to system-scale results, suggesting that this up-scaling method could be useful in bodies of water without circulation models. The value of removed nitrogen was estimated using alternative management costs (e.g., wastewater treatment) as representative, showing ecosystem service values of $8.5 and $470 million per year for current and maximum expanded production, respectively. These estimates are conservative; removal by clams in Connecticut, oysters and clams in New York, and denitrification are not included. Optimistically, the calculation of oyster-associated removal from all leases in both states (5% of bottom area) plus denitrification losses showed increases to 10%-30% of annual inputs, which would be higher if clams were included. Results are specific to Long Island Sound, but the approach is transferable to other urban estuaries.

Preliminary Evidence for the Amplification of Global Warming in Shallow, Intertidal Estuarine Waters.

Over the past 50 years, mean annual water temperature in northeastern U.S. estuaries has increased by approximately 1.2°C, with most of the warming recorded in the winter and early spring. A recent survey and synthesis of data from four locations in Southern Rhode Island has led us to hypothesize that this warming may be amplified in the shallow (<1 m), nearshore portions of these estuaries. While intertidal areas are not typically selected as locations for long-term monitoring, we compiled data from published literature, theses, and reports that suggest that enhanced warming may be occurring, perhaps at rates three times higher than deeper estuarine waters. Warmer spring waters may be one of the factors influencing biota residing in intertidal regions both in general as well as at our specific sites. We observed greater abundance of fish, and size of Menidia sp., in recent (2010-2012) seine surveys compared to similar collections in 1962. While any linkages are speculative and data are preliminary, taken together they suggest that shallow intertidal portions of estuaries may be important places to look for the effects of climate change.