Shoreline Hardening Research Articles

Reducing Barriers to Living Shorelines Through Sea Grant Extension Programs

Natural shorelines provide ecosystem services that are integral to maintaining healthy and resilient coastal ecosystems and communities. However, anthropogenic and environmental stressors are reducing the extent of natural shorelines, and thus, their capacity to provide critical ecosystem services. Unfortunately, the most common shoreline management strategies involve hardened structures (e.g., bulkheads and seawalls) that are known to impair coastal ecosystems. Living shorelines offer a more environmentally friendly alternative to this shoreline hardening. To increase the adoption of living shorelines, the Mississippi-Alabama Sea Grant Consortium and Florida Sea Grant have produced guidance documents, offered technical assistance, constructed demonstration sites, and hosted trainings for property owners, contractors, and other practitioners. This article summarizes those efforts, details the results that have been achieved, and outlines existing barriers to further living shoreline implementation in the region.

Increasing marsh bird abundance in coastal wetlands of the Great Lakes, 2011–2021, likely caused by increasing water levels

AbstractWetlands of the Laurentian Great Lakes of North America (i.e., lakes Superior, Michigan, Huron, Erie, and Ontario) provide critical habitat for marsh birds. We used 11 years (2011–2021) of data collected by the Great Lakes Coastal Wetland Monitoring Program at 1,962 point-count locations in 792 wetlands to quantify the first-ever annual abundance indices and trends of 18 marsh-breeding bird species in coastal wetlands throughout the entire Great Lakes. Nine species (50%) increased by 8–37% per year across all of the Great Lakes combined, whereas none decreased. Twelve species (67%) increased by 5–50% per year in at least 1 of the 5 Great Lakes, whereas only 3 species (17%) decreased by 2–10% per year in at least 1 of the lakes. There were more positive trends among lakes and species (n = 34, 48%) than negative trends (n = 5, 7%). These large increases are welcomed because most of the species are of conservation concern in the Great Lakes. Trends were likely caused by long-term, cyclical fluctuations in Great Lakes water levels. Lake levels increased over most of the study, which inundated vegetation and increased open water-vegetation interspersion and open water extent, all of which are known to positively influence abundance of most of the increasing species and negatively influence abundance of all of the decreasing species. Coastal wetlands may be more important for marsh birds than once thought if they provide high-lake-level-induced population pulses for species of conservation concern. Coastal wetland protection and restoration are of utmost importance to safeguard this process. Future climate projections show increases in lake levels over the coming decades, which will cause “coastal squeeze” of many wetlands if they are unable to migrate landward fast enough to keep pace. If this happens, less habitat will be available to support periodic pulses in marsh bird abundance, which appear to be important for regional population dynamics. Actions that allow landward migration of coastal wetlands during increasing lake levels by removing or preventing barriers to movement, such as shoreline hardening, will be useful for maintaining marsh bird breeding habitat in the Great Lakes.

(Un)managed retreat? Perspectives on biophysical and social dynamics of coastal retreat on O‘ahu, Hawai‘i

As an island community with world famous beaches, Hawai‘i’s coastal managers face significant challenges in the face of sea level rise (SLR). Shoreline laws have recently been updated to limit further shoreline hardening, making Hawaiʻi a salient case study to understand the challenges and opportunities around managed retreat. We interviewed 42 private sector, civil society, and government actors involved with coastal governance and anchored our conversations to three distinct communities on O‘ahu. Factors that affected participant perspectives on the viability of retreat per site hinged on: 1) physical/natural barriers and opportunities; 2) perceptions of the public value of a beach; 3) the importance of existing infrastructure; and 4) the political influence of coastal homeowners. Participants’ views on who should inform the coastal planning process moving forward varied by site. In addition to residents, there was largely agreement that communities should include those with affinity and long-standing relationships with place.

The Cost of Shoreline Protection: A Comparison of Approaches in Coastal New England and the Mid-­Atlantic

Shoreline hardening is a method of coastal hazard protection that is often implemented by government agencies and individual property owners. As awareness of the potential negative effects of shoreline hardening has increased, natural and nature-based approaches have gained in popularity. Most research related to shoreline protection has focused on understanding the environmental and ecological effects. However, for hybrid, nature-based approaches, in particular, there is limited information available to compare their monetary costs. To fill this gap, this study used information collected from public shoreline protection projects within the New England and Mid-Atlantic areas to estimate the costs of these measures based on the materials used, such as vegetation, sand, and/or stone. This approach allows for a detailed measurement of potential project inputs and provides needed cost information on the types of materials local governments and other stakeholders may use in their shoreline protection approaches. Results suggest that approaches that use natural materials tend to cost less than those that use more traditional, engineered materials, and nature-based approaches tend to cost somewhere in-between. Specifically, projects can be divided into four subgroups based on their average per-unit costs: (A) walls (mean: $5,628, se: $680) or stone at exposed sites (mean: $4,943, se: $725); (B) sand for beach nourishment (mean: $3,094, se: $397) or stone at low exposure sites ($3,014, se: $379); (C) stone and vegetation at low exposure sites (mean: $1,626, se: $217), stone and sand for other purposes at low exposure sites (mean: $1,411, se: $173), or sand for other purposes (mean: $1,384, se: $151); and (D) stone and sand for other purposes at low exposure sites (mean: $1,411, se: $173), sand for other purposes (mean: $1,384, se: $151), vegetation (mean: $1,300, se: $159), or vegetation and sand for other purposes (mean: $1,285, se: $172). Finally, monitoring and maintenance costs are often not accounted for, which may negatively affect the long-term success of shoreline protection efforts. Coupled with information on environmental and ecological effects of these different approaches, this information will allow for more informed decisions on how coastal and inland communities can best adapt to coastal risks.

Shoreline hardening affects species composition and condition of larval fishes in the Yangtze River: Implications for conservation and inshore habitat restoration

AbstractInshore habitats play a crucial role in the downstream drift of larval fish in large rivers. Natural inshore habitats of most of the world's large rivers have been heavily degraded by shoreline stabilization measures, but the knowledge about consequences for the condition and distribution of larval fishes is extremely limited. Investigations during the peak season of larval drift into species composition as well as into feeding intensity and body injuries of representative taxa were conducted at different inshore mesohabitats along semi‐natural and stabilized shores in a free‐flowing section in the middle Yangtze River. Species were unambiguously identified by a combination of DNA barcoding and morphological analyses. Overall, 37 taxa from seven families were detected from a total of 9,915 fish larvae caught. Earlier developmental stages greatly outnumbered the later stages in diversity and abundance in each investigated mesohabitat. Abundance and assemblage structure as well as the body injury rate differed significantly between mesohabitats. Generally, overall species richness and abundance tended to be lower, and divergence of the assemblage and body injury rates tended to be higher at heavily hardened shores, which could be contributed to the poor riparian vegetation and restricted suitable hydraulic conditions there. Our results may reflect the underestimated impact of inshore habitat degradation on larval fishes and highlight the demand for creating adequate areas of shallow and slow‐flow habitats with good riparian vegetation conditions along the shores. These measures will foster an appropriate connectivity between spawning areas and high‐quality nurseries for larval fishes.

Behaviour broadens thermal safety margins on artificial coastal defences in the tropics

Tropical species are predicted to be among the most vulnerable to climate change as they often live close to their upper limits to thermal tolerance and in many cases, behavioural thermoregulation is required to persist in the thermal extremes of tropical latitudes. In concert with warming temperatures, near-shore species are faced with the additional threat of shoreline hardening, leading to a reduction in microhabitats that can provide thermal refuges. This situation is exemplified in Singapore, which lies almost on the equator and so experiences year-round hot temperatures, and much of its coastline is now seawall. To investigate the thermal ecology of a common intertidal gastropod, Nerita undata, on these artificial structures, we measured thermal conditions on two seawalls, the temperatures of habitats occupied by the snail, and compared these with the snail's thermal tolerance by measuring heart rate and behavioural thermoregulation (as preferred temperature, Tpref). At one of the two seawalls (Tanjong Rimau), temperatures experienced by N. undata exceeded all measures of thermal tolerance in the sun, while at the other (Palawan Beach), they did not. Temperatures in habitats occupied by the snails on the seawalls were similar to their measured Tpref in the laboratory and were lower than all measures of thermal tolerance. Behavioural thermoregulation by the snails, therefore, significantly increased the thermal safety margins of N. undata on the relatively homogenous seawalls in Singapore, and at one of the two seawalls were necessary to allow snails to survive. Accordingly, to facilitate motile species to maintain broad thermal safety margins through behavioural regulation, the provision of additional refuges from thermal stress is recommended on artificial coastal defences such as seawalls.

Contribution of longshore sand exchanges to mesoscale barrier-island behavior: Insights from the Virginia Barrier Islands, U.S. East Coast

The development of coastal adaptation pathways along sandy barrier-island coasts requires an understanding of multi-decadal barrier morphodynamics in response to forcings such as sea-level rise, sediment fluxes, and storminess. Long- and cross-shore sand exchanges among barrier-system compartments (shorefaces, beaches, dunes, tidal inlets, backbarrier lagoons) and between adjacent barrier islands can play a fundamental role in barrier-system morphology and resilience to sea-level rise. We explore these dynamics through investigation of historical areal changes within the 13 largely undeveloped Virginia Barrier Islands of the United States Mid-Atlantic Coast. We pair subaerial island areas mapped from historical surveys and modern aerial and satellite imagery with representative island thicknesses determined from stratigraphic and lidar-derived elevation data to estimate volumetric changes through time. Overall, the Virginia Barrier Islands lost ~3.4% (16 × 106 m3) of their 1887 C.E. volume between 1887 and 2017 C.E. The loss from three central islands (Parramore, Hog, and Cobb)—historically characterized by rotational behavior—was 43% (79 × 106 m3). In particular, Parramore Island continuously decreased in volume over the 20th and 21st centuries. In contrast, growth of Assateague and northern Wallops islands (located at the northern, updrift end of the island chain) from 1887 to 2017 C.E. resulted in a volume increase of ~59 × 106 m3. Whereas the Virginia Barrier Islands as a whole modestly decreased in size (and thus volume) directly in response to storm impacts and accelerating relative sea-level rise since the mid-1800s, the differential behavior of islands south of Assateague suggests more regional controls on coastal geomorphic behavior. Specifically, we identify the roles of antecedent substrate, sand trapping and changing wave-refraction patterns associated with the growth of Assateague Island, and inter-island sand exchanges in response to changing updrift barrier-scale behaviors. These findings help to better understand the underlying mechanisms behind state changes of barrier islands; in particular, the mechanisms by which geomorphic change along one barrier island influences those located several to tens of kilometers downdrift. This highlights the need to consider coastal beach and sand management over long reaches, even in systems which are largely unaffected by development, nourishment, and shoreline hardening.

Historical Maps provide insight into a century and a half of habitat change in Fijian coasts.

Meaningful conservation goals require setting baselines derived from long‐term ecological records and information that is rare in many regions of the world. Historical data allow us to shift baselines back in time in order to strengthen conservation outcomes in the future.To explore how different histories of land use and development influenced coastal ecosystems in two Fijian cities (Suva and Savusavu), we compared a series of historical navigational charts. These charts recorded change in coral reef area and coastal mangrove forests, as well as expansions of hardened shorelines. We used geographic information systems (GIS) to georeference and make quantitative comparisons starting in 1,840 in Suva and 1876 in Savusavu.Our findings show that, despite increasing urbanization in the capital Suva, available coral reef habitat has not significantly changed in over 150 years, but development has hastened a nearly 50% loss of mangroves. Meanwhile, in the smaller city of Savusavu, coral habitats suffered significant loss in area and an increase in patchiness. As in Suva, shoreline hardening increased in Savusavu, but this change was not accompanied by a loss of mangroves.Nautical charts provided hitherto unavailable information on the long‐term loss and alteration of coastal habitats in Fiji. Historical ecology allows scientists to combat shifting baseline syndrome and set measured standards for conservation objectives.

Estuaries as Filters for Riverine Microplastics: Simulations in a Large, Coastal-Plain Estuary

Public awareness of microplastics and their widespread presence throughout most bodies of water are increasingly documented. The accumulation of microplastics in the ocean, however, appears to be far less than their riverine inputs, suggesting that there is a “missing sink” of plastics in the ocean. Estuaries have long been recognized as filters for riverine material in marine biogeochemical budgets. Here we use a model of estuarine microplastic transport to test the hypothesis that the Chesapeake Bay, a large coastal-plain estuary in eastern North America, is a potentially large filter, or “sink,” of riverine microplastics. The 1-year composite simulation, which tracks an equal number of buoyant and sinking 5-mm diameter particles, shows that 94% of riverine microplastics are beached, with only 5% exported from the Bay, and 1% remaining in the water column. We evaluate the robustness of this finding by conducting additional simulations in a tributary of the Bay for different years, particle densities, particle sizes, turbulent dissipation rates, and shoreline characteristics. The resulting microplastic transport and fate were sensitive to interannual variability over a decadal (2010–2019) analysis, with greater export out of the Bay during high streamflow years. Particle size was found to be unimportant while particle density – specifically if a particle was buoyant or not – was found to significantly influence overall fate and mean duration in the water column. Positively buoyant microplastics are more mobile due to being in the seaward branch of the residual estuarine circulation while negatively buoyant microplastics are transported a lesser distance due to being in the landward branch, and therefore tend to deposit on coastlines close to their river sources, which may help guide sampling campaigns. Half of all riverine microplastics that beach do so within 7–13 days, while those that leave the bay do so within 26 days. Despite microplastic distributions being sensitive to some modeling choices (e.g., particle density and shoreline hardening), in all scenarios most of riverine plastics do not make it to the ocean, suggesting that estuaries may serve as a filter for riverine microplastics.

The effect of coastal landform development on decadal-to millennial-scale longshore sediment fluxes: Evidence from the Holocene evolution of the central mid-Atlantic coast, USA

The behavior of siliciclastic coastal systems is largely controlled by the interplay between accommodation creation and infilling. Factors responsible for altering sediment fluxes to and along open-ocean coasts include cross-shore mobilization of sediment primarily from tidal currents and storms as well as changes in alongshore transport rates moderated by changing wave conditions, river sediment inputs, artificial shoreline hardening and modification, and natural sediment trapping in updrift coastal landforms. This paper focuses on the latter relationships. To address understudied interactions between updrift coastal landforms and downdrift coastal behavior, we quantify the volume and fluxes of sediment trapped in the Assateague-Chincoteague-Wallops barrier-island complex along the Virginia, USA coast and relate these volumes to downdrift coastal-system behavior. During the last ca. 2250 years, these barriers trapped 216 million m3 of sand through the growth of complex beach- and foredune-ridge systems. A period (ca. 400 to 190 years ago) of reduced/no progradation on Chincoteague and Assateague islands corresponds with sediment sequestration in updrift flood-tidal deltas. This finding emphasizes the important control of tidal inlets on alongshore sediment fluxes on barrier-island coasts. Rapid historical spit elongation during the last 190 years has trapped an average of 681,000 m3 yr−1 of sand; this occurred coincident with downdrift barrier-island erosion/migration at long-term rates of >3 m yr−1. Historical sand fluxes to the elongating spit on southern Assateague Island and progradational beach ridges on northernmost Wallops Islands are equivalent to at least 60% of estimated regional longshore transport rates. We therefore propose that sediment trapping and associated wave refraction are the primary drivers of downdrift barrier erosion, while storminess and sea-level rise are secondary forcings of change affecting equally the entire barrier-island chain. Global context is provided by a compilation of sediment trapping through growth of similar longshore sand sinks, which indicates the volume of sediment incorporated into the elongating spit end of Assateague Island is similar to sandy beach- and foredune-ridge plains (108 m3), but average annual trapping at the spit is at least six times greater than those at most mainland-attached, progradational systems. However, Chincoteague and Wallops, two progradational barrier islands, incorporate sand at rates broadly similar to large strandplains. Our findings emphasize the need to account for natural longshore sediment trapping in multi-decadal coastal management efforts on sandy, siliciclastic coasts.

US polyethylene supplies gradually ease as run rates increase

The behavior of siliciclastic coastal systems is largely controlled by the interplay between accommodation creation and infilling. Factors responsible for altering sediment fluxes to and along open-ocean coasts include cross-shore mobilization of sediment primarily from tidal currents and storms as well as changes in alongshore transport rates moderated by changing wave conditions, river sediment inputs, artificial shoreline hardening and modification, and natural sediment trapping in updrift coastal landforms. This paper focuses on the latter relationships. To address understudied interactions between updrift coastal landforms and downdrift coastal behavior, we quantify the volume and fluxes of sediment trapped in the Assateague-Chincoteague-Wallops barrier-island complex along the Virginia, USA coast and relate these volumes to downdrift coastal-system behavior. During the last ca. 2250 years, these barriers trapped 216 million m3 of sand through the growth of complex beach- and foredune-ridge systems. A period (ca. 400 to 190 years ago) of reduced/no progradation on Chincoteague and Assateague islands corresponds with sediment sequestration in updrift flood-tidal deltas. This finding emphasizes the important control of tidal inlets on alongshore sediment fluxes on barrier-island coasts. Rapid historical spit elongation during the last 190 years has trapped an average of 681,000 m3 yr−1 of sand; this occurred coincident with downdrift barrier-island erosion/migration at long-term rates of >3 m yr−1. Historical sand fluxes to the elongating spit on southern Assateague Island and progradational beach ridges on northernmost Wallops Islands are equivalent to at least 60% of estimated regional longshore transport rates. We therefore propose that sediment trapping and associated wave refraction are the primary drivers of downdrift barrier erosion, while storminess and sea-level rise are secondary forcings of change affecting equally the entire barrier-island chain. Global context is provided by a compilation of sediment trapping through growth of similar longshore sand sinks, which indicates the volume of sediment incorporated into the elongating spit end of Assateague Island is similar to sandy beach- and foredune-ridge plains (108 m3), but average annual trapping at the spit is at least six times greater than those at most mainland-attached, progradational systems. However, Chincoteague and Wallops, two progradational barrier islands, incorporate sand at rates broadly similar to large strandplains. Our findings emphasize the need to account for natural longshore sediment trapping in multi-decadal coastal management efforts on sandy, siliciclastic coasts.

Challenges and opportunities for sustaining coastal wetlands and oyster reefs in the southeastern United States

Formed at the confluence of marine and fresh waters, estuaries experience both the seaside pressures of rising sea levels and increasing storm severity, and watershed and precipitation changes that are shifting the quality and quantity of freshwater and sediments delivered from upstream sources. Boating, shoreline hardening, harvesting pressure, and other signatures of human activity are also increasing as populations swell in coastal regions. Given this shifting landscape of pressures, the factors most threatening to estuary health and stability are often uncertain. To identify the greatest contemporary threats to coastal wetlands and oyster reefs across the southeastern United States (Mississippi to North Carolina), we summarized recent population growth and land-cover change and surveyed estuarine management and science experts. From 1996 to 2019, human population growth in the region varied from a 17% decrease to a 171% increase (mean = +43%) with only 5 of the 72 SE US counties losing population, and nearly half growing by more than 40%. Individual counties experienced between 999 and 19,253 km2 of new development (mean: 5725 km2), with 1–5% (mean: 2.6%) of undeveloped lands undergoing development over this period across the region. Correspondingly, our survey of 169 coastal experts highlighted development, shoreline hardening, and upstream modifications to freshwater flow as the most important local threats facing coastal wetlands. Similarly, experts identified development, upstream modifications to freshwater flow, and overharvesting as the most important local threats to oyster reefs. With regards to global threats, experts categorized sea level rise as the most pressing to wetlands, and acidification and precipitation changes as the most pressing to oyster reefs. Survey respondents further identified that more research, driven by collaboration among scientists, engineers, industry professionals, and managers, is needed to assess how precipitation changes, shoreline hardening, and sea level rise are affecting coastal ecosystem stability and function. Due to the profound role of humans in shaping estuarine health, this work highlights that engaging property owners, recreators, and municipalities to implement strategies to improve estuarine health will be vital for sustaining coastal systems in the face of global change.

Reversing a tyranny of cascading shoreline‐protection decisions driving coastal habitat loss

AbstractShoreline hardening is a major driver of biodiversity and habitat loss in coastal ecosystems yet remains a common approach to coastal management globally. Using surveys of waterfront residents in North Carolina, USA, we sought to identify factors influencing individual shore‐protection decisions and ultimately impacting coastal ecosystems, particularly coastal wetlands. We found that neighboring shore condition was the best predictor of respondent shore condition. Respondents with hardened shorelines were more likely to have neighbors with hardened shorelines, and to report that neighbors influenced their shore‐protection choices than respondents with natural shorelines. Further, respondents who expressed climate‐change skepticism and preference for shoreline hardening were opposed to shoreline‐hardening restrictions. Despite preferring hardening, respondents ranked wetlands as highly valuable for storm protection and other ecosystem services, suggesting a disconnect between the ecological knowledge of individuals and social norms of shore‐protection decisions. However, our results also suggest that efforts to increase the installation of living shorelines have the potential to conserve and restore important coastal habitats and support biodiversity along shorelines that may otherwise be degraded by hardening. Further, encouraging waterfront‐property owners who have adopted living shorelines to recommend them to neighbors may be an effective strategy to initiate and reinforce pro‐conservation social norms.

The effects of large-scale breakwaters on shoreline vegetation

In response to shoreline erosion and coastal wetland loss, living shorelines have been implemented as a natural alternative to shoreline hardening. In high wave energy systems, “hybrid” living shoreline designs incorporating large-scale breakwaters have been increasingly chosen to restore and conserve wetlands. However, evaluations of the effectiveness of breakwaters at preserving natural shorelines and promote the growth of shoreline plantings are limited. To evaluate the effectiveness of large-scale breakwaters at protecting or restoring marshes in high wave energy environments, we conducted an experimental planting and shoreline monitoring program landward of eight-year-old breakwaters and adjacent no breakwater sites along Bon Secour Bay, Alabama. Results showed that breakwaters help natural marsh to maintain its cover at a high level (70%), but have little impact on shoreline planting. Furthermore, breakwater presence reduced the pressure for upland migration, allowing natural marsh patches to expand seaward. Without breakwater protection, fringing marsh retreated upland significantly. Cumulatively, this study suggests that large-scale breakwaters could have an impact on preserving fringing marsh vegetation in high wave energy environments though their effectiveness into the future will require adaptive management in response to local sea-level rise.

Conservation practice insights from a comparative case study of two shoreline stabilization projects in Boston Harbor, MA

AbstractAs sea levels rise and the frequency of flooding events increases, so do efforts to stabilize coastal shorelines. Nature‐based solutions to shoreline stabilization (“green” solutions, as opposed to traditional “grey” shoreline hardening) have been increasingly adopted in the Gulf of Mexico and mid‐Atlantic regions of the United States, but they are only beginning to be implemented, documented, and understood in the northeastern United States. This case study contrasts two shoreline stabilization projects directly across from each other on the banks of the Mystic River, Boston Harbor, MA. These two projects, one private and one public, employ a spectrum of green‐grey shoreline stabilization techniques. Originally planned as traditional grey shoreline stabilizations, both projects shifted toward greener solutions due to local environmental group advocacy, highlighting the influence of outside groups as advocates for nature‐based solutions. This study documents the evolution of both projects, highlighting common challenges in permitting and vegetation maintenance, as well as critical differences in project goals, funding sources, and biodiversity impacts.

Communicating Managed Retreat in California

California cities face growing threats from sea-level rise as increased frequency and severity of flooding and storms cause devastating erosion, infrastructure damage, and loss of property. Management plans are often designed to prevent or slow flooding with short-term, defensive strategies such as shoreline hardening, beach nourishment, and living shorelines. By contrast, managed retreat focuses on avoiding hazards and adapting to changing shorelines by relocating out of harm’s way. However, the term “managed retreat” can be controversial and has engendered heated debates, defensive protests, and steady resistance in some communities. Such responses have stymied inclusion of managed retreat in adaptation plans, and in some cases has resulted in complete abandonment of the policy review process. We examined the Local Coastal Program review process in seven California communities at imminent risk of sea-level rise and categorized each case as receptive or resistant to managed retreat. Three prominent themes distinguished the two groups: (1) inclusivity, timing, and consistency of communication, (2) property ownership, and (3) stakeholder reluctance to change. We examined use of terminology and communication strategies and provided recommendations to communicate “managed retreat” more effectively.

Contrasting trajectories in macrophyte community development after shoreline restoration: water level obscures trends

• Macrophytes were evaluated to determine shoreline restoration success. • Epiphytic algae were surveyed to evaluate their potential impact on macrophytes. • Macrophytes were influenced by slope, wind and wave exposure among habitats. • Hydrologic changes impacted macrophyte metrics, but algal biomass had no impact. • Restoration improved habitat quality but water level change conflated assessment. Macrophytes are critical components of biologically productive lake littoral zones. Sensitivity to environmental factors such as sediment content and light availability makes macrophytes potential bioindicators of anthropogenic stress. The industrial past of Muskegon Lake (Michigan, USA) has severely disturbed the system, resulting in shoreline hardening and sediment contamination. Shoreline restoration during 2010 and 2011 presented an opportunity to use macrophytes as indicators of pre-restoration (2009–2010), shorter-term (2011–2012) and longer-term (2018) post-restoration ecosystem status. Macrophytes were sampled along transects perpendicular to the shoreline in two restored and one reference habitat, and predicted to experience post-restoration density and richness increases. Epiphytic algae were surveyed in 2018 only. Restored habitat quality, based on macrophyte species composition, minimally improved during post-restoration and there was no clear pattern in macrophyte richness and density. Water level was the strongest environmental driver of macrophyte community change, especially in 2018, when emergent macrophyte richness declined. Epiphytic algal dry weight was not sufficient to negatively impact macrophytes, as wave exposure lowered algal densities. Post-restoration habitat quality remained greater at the reference transect than at the two restored transects, although the restored habitats exhibited contrasting trajectories due to differences in slope (%), wave exposure, and disturbance.

Shoreline modification affects recruitment of invasive Phragmites australis

Shoreline hardening affects ecological processes in nearshore intertidal ecosystems and upland habitats. Invasive species establishment and spread is one consequence of shoreline alterations. Invasive Phragmites australis has spread throughout the U.S., including in subestuaries with hardened shorelines. It is unclear, though, by what mechanisms shoreline hardening facilitates P. australis establishment. We tested the hypothesis that shoreline structures are associated with seedling recruitment, resulting in higher levels of within stand genetic diversity. We used microsatellite analysis to examine the genetic diversity of P. australis stands associated with two types of shoreline structures (riprap, bulkhead revetments) compared to unaltered shorelines. Because P. australis seed viability depends on cross-pollination, higher levels of genetic diversity in stands associated with hardened shorelines are more likely to contribute to the spread of invasive P. australis. We conclude that the extent of shoreline modifications in the Chesapeake Bay has contributed to the spread of P. australis by seeds. The results have implications for the management of P. australis associated with the establishment of shoreline structures.

Risk of shoreline hardening and associated beach loss peaks before mid-century: O\u02bbahu, Hawai\u02bbi

Shoreline hardening, which causes beach loss globally, will accelerate with sea level rise (SLR), causing more beach loss if management practices are not changed. To improve beach conservation efforts, current and future shoreline hardening patterns on sandy beaches need deeper analysis. A shoreline change model driven by incremental SLR (0.25, 0.46, 0.74 m) is used to simulate future changes in the position of an administrative hazard zone, as a proxy for risk of hardening at all sandy beaches on the island of O‘ahu, Hawai ‘i. In Hawai ‘i, hardening can be triggered when evidence of erosion is within 6.1 m (“20 ft”) of certain structures, allowing an applicant to request emergency protection. Results show an increase in shoreline vulnerability to hardening with SLR governed by backshore land use patterns. The largest increase (+ 7.6%) occurred between modern-day and 0.25 m of SLR (very likely by year 2050) with half of all beachfront shoreline at risk by 0.74 m of SLR. Maximum risk of shoreline hardening and beach loss is projected to occur from modern-day and near-term hardening because of the heavily developed aspect of some shoreline segments. Adaptation to SLR should be considered an immediate need—not solely a future issue.

Effects of Repeated Sand Replenishment Projects on Runs of a Beach-Spawning Fish, the California Grunion

Beach habitats are diminishing globally, particularly in urban areas, as sea-level rise, erosion, and shoreline hardening, along with reduced sediment inputs, combine to squeeze the coast. In California, USA an endemic marine fish, the California grunion, spawns on sandy beaches during late-night spring tides. Its unique recreational fishery is managed by the California Department of Fish and Wildlife. The City of Oceanside, CA contracts for annual harbor dredging and, after testing, places the sandy sediment on its public beach. The effects on local beach wildlife from this annual sand replenishment are not known. We examined the effect of this repeated activity as a case study over three years on the spawning runs of the California grunion. Some spawning runs occurred in all three years, but the fish avoided areas with high scarps in the intertidal zone that developed following sand placement activity. Grunion spawning runs have declined in the habitat range as a whole over the past two decades, and those in Oceanside have declined to an even greater extent. Increasing sandy beach habitat can be beneficial to wildlife, but the method of placement, timing of the project, and fate of the beach afterward can modulate or prevent beneficial effects. Frequent repetition of sand placement may accumulate impacts without allowing sufficient time for the ecosystem to recover. Rather than improving the habitat, these repeated projects in Oceanside may degrade the spawning habitat for the grunion. Alternative discharge methods and locations, slope and elevation designs, sediment volumes, and greater care in beach fill practices should be implemented to reduce future impacts.