For more than a century, coastal wetlands have been recognized for their ability to stabilize shorelines and protect coastal communities. However, this paradigm has recently been called into question by small-scale experimental evidence. Here, we conduct a literature review and a small meta-analysis of wave attenuation data, and we find overwhelming evidence in support of established theory. Our review suggests that mangrove and salt marsh vegetation afford context-dependent protection from erosion, storm surge, and potentially small tsunami waves. In biophysical models, field tests, and natural experiments, the presence of wetlands reduces wave heights, property damage, and human deaths. Meta-analysis of wave attenuation by vegetated and unvegetated wetland sites highlights the critical role of vegetation in attenuating waves. Although we find coastal wetland vegetation to be an effective shoreline buffer, wetlands cannot protect shorelines in all locations or scenarios; indeed large-scale regional erosion, river meandering, and large Climatic Change (2011) 106:7-29 tsunami waves and storm surges can overwhelm the attenuation effect of vegetation. However, due to a nonlinear relationship between wave attenuation and wetland size, even small wetlands afford substantial protection from waves. Combining manmade structures with wetlands in ways that mimic nature is likely to increase coastal protection. Oyster domes, for example, can be used in combination with natural wetlands to protect shorelines and restore critical fishery habitat. Finally, coastal wetland vegetation modifies shorelines in ways (e.g. peat accretion) that increase shoreline integrity over long timescales and thus provides a lasting coastal adaptation measure that can protect shorelines against accelerated sea level rise and more frequent storm inundation. We conclude that the shoreline protection paradigm still stands, but that gaps remain in our knowledge about the mechanistic and contextdependent aspects of shoreline protection.

Salt marshes are among the most abundant, fertile, and accessible coastal habitats on earth, and they provide more ecosystem services to coastal populations than any other environment. Since the Middle Ages, humans have manipulated salt marshes at a grand scale, altering species composition, distribution, and ecosystem function. Here, we review historic and contemporary human activities in marsh ecosystems-exploitation of plant products; conversion to farmland, salt works, and urban land; introduction of non-native species; alteration of coastal hydrology; and metal and nutrient pollution. Unexpectedly, diverse types of impacts can have a similar consequence, turning salt marsh food webs upside down, dramatically increasing top down control. Of the various impacts, invasive species, runaway consumer effects, and sea level rise represent the greatest threats to salt marsh ecosystems. We conclude that the best way to protect salt marshes and the services they provide is through the integrated approach of ecosystem-based management.

Estuaries and coastal seas provide valuable ecosystem services but are particularly vulnerable to the co-occurring threats of climate change and oxygen-depleted dead zones. We analyzed the severity of climate change predicted for existing dead zones, and found that 94% of dead zones are in regions that will experience at least a 2 °C temperature increase by the end of the century. We then reviewed how climate change will exacerbate hypoxic conditions through oceanographic, ecological, and physiological processes. We found evidence that suggests numerous climate variables including temperature, ocean acidification, sea-level rise, precipitation, wind, and storm patterns will affect dead zones, and that each of those factors has the potential to act through multiple pathways on both oxygen availability and ecological responses to hypoxia. Given the variety and strength of the mechanisms by which climate change exacerbates hypoxia, and the rates at which climate is changing, we posit that climate change variables are contributing to the dead zone epidemic by acting synergistically with one another and with recognized anthropogenic triggers of hypoxia including eutrophication. This suggests that a multidisciplinary, integrated approach that considers the full range of climate variables is needed to track and potentially reverse the spread of dead zones.

4 5 6 Ghost forests created by the submergence of low-lying land are one of the most striking indicators of 7 climate change along the Atlantic coast of North America. Although dead trees at the margin of 8 estuaries were described as early as 1910, recent research has led to new recognition that the 9 submergence of terrestrial land is geographically widespread, ecologically and economically 10 important, and globally relevant to the survival of coastal wetlands in the face of rapid sea level rise.11 This emerging understanding has in turn generated widespread interest in the physical and ecological 12 mechanisms influencing the extent and pace of upland to wetland conversion. Choices between 13 defending the coast from sea level rise and facilitating ecosystem transgression will play a 14 fundamental role in determining the fate and function of low-lying coastal land. 15 16 Sea level rise rates have been accelerating since the end of the 19 th century, impacting low elevation 17 land along coasts and estuaries around the world 1 . Sea level rise enhances flooding and saltwater 18 intrusion, and threatens coastal communities, infrastructure, and ecosystems 2-4 . Ghost forests and 19 abandoned farmland are striking indicators of sea-level driven land conversion. Dead trees and stumps 20 surrounded by marshland, for example, represent relic forestland that has been replaced by intertidal 21 vegetation. Similarly, bare soil and wetland plants at the edges of agricultural fields indicate the 22 65 coastal land submergence. The review ends with implications for land management, and highlights 66 uncertainty in local flood defense strategy as the key knowledge gap limiting our ability to predict future 67 sea-level driven land conversion and its impact on coastal ecosystems. 68 69 Extent and physical controls of historical land submergence 70 Ghost forests, abandoned agricultural fields, and other indicators of historical land submergence occur 71 throughout low-lying and gently sloping portions of the Atlantic and Gulf coasts of North America 15-20 72 (Fig. 1). Land submergence is most extensive within the mid-Atlantic sea-level rise hotspot that stretches 73 from North Carolina to Massachusetts, where relative sea level is rising three times faster than eustatic 74 rates 27 . For example, 400 km 2 of uplands in the Chesapeake Bay region have converted to tidal marsh 75 since the mid-1800s 19 , and large tracts of hardwood and cedar forest death have been observed in 76 Delaware Bay 16 . However, ghost forests are not confined to the sea level rise hotspot. Ghost forests 77 have also been documented throughout the Florida Gulf Coast 17,18 , the St. Lawrence estuary of Canada 15 , 78 and tidal freshwater forests in South Carolina, Georgia, and Louisiana 21,28 . There has been 148 km 2 of 79 forest conversion over 120 years along the Florida Gulf Coast 17 , and near complete loss of pine forests in 80 the Lower Florida Keys 29 . Surprisingly, the phenomenon has not been widely documented on coastal 81 plains outside of...

Saltwater intrusion is the leading edge of sea-level rise, preceding tidal inundation, but leaving its salty signature far inland. With climate change, saltwater is shifting landward into regions that previously have not experienced or adapted to salinity, leading to novel transitions in biogeochemistry, ecology, and human land uses. We explore these changes and their implications for climate adaptation in coastal ecosystems. Biogeochemical changes, including increases in ionic strength, sulfidation, and alkalinization, have cascading ecological consequences such as upland forest retreat, conversion of freshwater wetlands, nutrient mobilization, and declines in agricultural productivity. We explore the trade-offs among land management decisions in response to these changes and how public policy should shape socioecological transitions in the coastal zone. Understanding transitions resulting from saltwater intrusion—and how to manage them—is vital for promoting coastal resilience.

Anthropogenic climate change is predicted to cause widespread biodiversity loss due to shifts in speciesÕ distributions, but these predictions rarely incorporate ecological associations such as zonation. Here, we predict the decline of a diverse assemblage of mid-latitude salt marsh plants, based on an ecosystem warming experiment. In New England salt marshes, a guild of halophytic forbs occupies stressful, waterlogged pannes. At three sites, experimental warming of < 4°C led to diversity declines in pannes and rapid takeover by a competitive dominant, Spartina patens. In Rhode Island, near their southern range limit, pannes were more sensitive to warming than farther north, and panne area also declined in control plots over the three-season experiment. These results suggest that warming will rapidly reduce plant diversity in New England salt marshes by eliminating a high diversity zone. Biodiversity in zoned ecosystems may be more affected by climate-driven shifts in zonation than by individual speciesÕ distribution shifts.

The cordgrass Spartina alterniflora Loisel is a foundation species critical to the establishment and maintenance of western Atlantic salt marshes. Although the factors regulating cordgrass growth along sheltered, fine-sediment shorelines have been exhaustively studied, less is known about the mechanisms that maintain cordgrass production in high-energy marshes characterized by sandy substrates. We investigated whether deposit-feeding fiddler crabs Uca pugilator Bosc and U. pugnax Smith can mediate local physical conditions and nutrient availability and stimulate cordgrass primary production on sandy marsh sediments. We experimentally removed fiddler crabs from 3 × 3 m plots in an exposed, sandy marsh in Wellfleet, Massachusetts, USA, and found that above-and belowground cordgrass biomass decreased by > 53% and 50%, respectively, and above-and belowground nitrogen in cordgrass (g N m

Salt marsh plant communities have long been envisioned as dynamic, resilient systems that quickly recover from human impacts and natural disturbances. But are salt marshes sufficiently resilient to withstand the escalating intensity and scale of human impacts in coastal environments? In this study we examined the independent and interactive effects of emerging threats to New England salt marshes (temperature increase, accelerating eutrophication, consumer-driven salt marsh die-off, and sea level rise) to understand the future trajectory of these ecologically valuable ecosystems. While marsh plant communities remain resilient to many disturbances, loss of critical foundation species and changing tidal inundation regimes may short circuit marsh resilience in the future. Accelerating sea level rise and salt marsh die-off in particular may interact to overwhelm the compensatory mechanisms of marshes and increase their vulnerability to drowning. Management of marshes will require difficult decisions to balance ecosystem service tradeoffs and conservation goals, which, in light of the immediate threat of salt marsh loss, should focus on maintaining ecosystem resilience.