Recent estimates of global salt marsh area sit at 5.5 million hectares (Mcowen et al. 2017). Conservatively, this translates to $1 trillion of ecosystem services per annum, potentially as much as $5 trillion (De Groot et al. 2012, Mehvar et al. 2018), equivalent to the entire US federal budget for 2019. There can be little debate as to the value of salt marshes, both in terms of the ecosystem services they provide and the key part they play in helping us understand past climate and sea level trends. This chapter summarizes the preceding work and draws together some key observations and notable knowledge gaps highlighted in the previous chapters. We provide a focus on the expected response of salt marshes to the stresses created by a changing climate.

Salt marshes have been lost or degraded as the intensity of human impacts to coastal landscapes has increased due to agriculture, transportation, urban and industrial development, and climate change. Because salt marshes have limited distribution and embody a variety of ecological functions that are important to humans (see ecosystem services, Chapter 15), many societies have recognized the need to preserve remaining marshes, restore those that have been degraded, and create new marshes in areas where they have been lost. An emerging and critical threat to tidal marshes across the globe is increasing rates of sea level rise and other aspects of climate change, which complicates but also heightens the urgency for restoration. By restoration we mean re-establishing natural conditions and the processes needed to support their functions, especially self-maintenance (see Box 17.1). Typically, salt marshes are self-maintaining, with salt tolerant plants, mineral sediments, and tidal flooding interacting to maintain elevation and ecological functions under dynamic conditions (Chapters 4, 7, 8).

Salt marshes have been useful study systems for community ecologists. They are amenable to experimental manipulation, and the simplicity and strong abiotic gradients of salt marshes lead to clear patterns and experimental outcomes. Many early ecologists believed that salt marsh ecosystems were primarily controlled by bottom-up factors (i.e., that nutrients, salinity, and other abiotic factors were the primary factors regulating productivity, and that productivity in turn regulated ecosystem trophic structure). More recently, many ecologists have argued that consumers have an important role in structuring salt marsh ecosystems through “top-down” processes. A simple conceptual approach, which we take here, is to think of salt marsh communities as being structured by bottom-up, top-down, and non-trophic processes.

Marshes have long been considered useful for their ecosystem service of coastal protection. Their roles in protection from storms and floods are seen as necessary and important to many coastal communities (Barbier et al. 2011; Costanza et al. 1997; Millennium Ecosystem Assessment. 2005; Morgan et al. 2009). Understanding the impacts that storms have on coastal ecosystems and adjacent coastal communities is imperative to increasing coastal resilience in the face of future increases in coastal flooding and associated damage (Mendelsohn et al. 2012; Pielke et al. 2008). Salt marshes have been lauded as buffers to storm surges, wind-generated waves, and elevated water levels (French 2006; Möller 2012). The ecological restoration economy, which includes salt marsh restoration, in the USA alone generates $9.5 billion in annual economic output and employs an estimated 126,000 workers (BenDor et al. 2015). After Hurricane Sandy, the US Fish and Wildlife Service spent more than $40 million on salt marsh restoration projects in response to this single event, including $11 million toward restoring a series of salt marshes along Long Island.

Salt marshes have received considerable scientific attention in recent years due to a combination of factors. Salt marshes host important ecosystems and store large quantities of carbon in their soils (Fagherazzi et al. 2004; Mudd et al. 2009). Currently salt marshes are endangered by accelerated sea-level rise triggered by global warming (Kirwan et al. 2010). A sharp reduction in sediment supply caused by the damming of rivers is also jeopardizing marsh survival along many coasts (Weston 2014). As a result, there is a need to determine the fate of marshlands in different settings in order to inform government and local communities and implement protection strategies. To this end, numerical models are playing an increasingly important role, because they can easily provide future scenarios of marsh conditions under different forcings. However, the evolution of salt marshes as a function of sea-level rise and sediment supply is relatively complex, because of feedbacks among hydrodynamics, sediment transport, and vegetation (Fagherazzi et al. 2012). As a result, marshes are continuously adjusting to a changing environment, in ways often difficult to predict. This intrinsic complexity has generated a flurry of numerical models, each emphasizing a different aspect of salt marsh evolution. It is thus becoming more and more accepted by the scientific community that a comprehensive model of salt marsh evolution is not feasible, given the number and variety of physical and biological processes at play. A detailed approach, based on the description of all possible processes acting at different spatial and temporal scales, has been slowly replaced by a more practical approach, in which separate models are built to address key important processes or to capture specific dynamics.

Salt marshes are expected to undergo substantial change or, potentially, disappear in the next couple of centuries as a result of rising sea level. Increasingly, scientists are asking the question: how long can they survive? This book draws on global expertise to look at how salt marshes evolved, how they function, and how they are responding to the stresses caused by social and environmental change. These environments occur throughout the world: behind barrier islands, bordering estuaries, and dominating lower delta plains (Fig. 1.1) in warm to cool latitudes (≥ 30° latitude). Up until now, previous loss and degradation of coastal marshes has been related to a variety of human actions including dredging and filling, reduction in sediment supplies, and hydrocarbon withdrawal, as well as other causes. However, in the future the greatest impact to marshes will be a consequence of climate change, especially sea-level rise (SLR). Most of the present marshes formed under very different sedimentation and SLR regimes compared to those that occur today. During their formation and throughout their evolution, the rate of SLR was relatively slow and steady, between 0.2 and 1.6 mm/year (Table 1.1). The sustainability of marshes is now threatened by an acceleration in SLR to rates many times greater than those under which they initiated and have evolved. For example, the Romney marsh, which is located north of Boston, Massachusetts, contains a 2-m-thick peat that began forming 3.1 ka BP when sea level was rising at about 0.8 mm/year, a rate that slowed to 0.52 mm/year around 1 ka BP (Donnelly 2006). The rate of SLR in Boston Harbor is now 2.85 mm/year (NOAA 2019), which far exceeds the rate occurring when the Romney marsh built to a supratidal elevation. Eventually, SLR, along with marsh-edge erosion, will outpace the ability of most marshes to accrete vertically (Crosby et al. 2016) and/or compensate for marsh loss by expanding into uplands (Kirwan et al. 2016, Farron 2018).

As humans have spread across the globe, travel and trade have deliberately or inadvertently carried and released animals and plants as well as microbes into new geographies. With human populations concentrated along rivers and coasts, it is not surprising that many exotic species have been released in coastal areas and a few can survive and thrive, especially in habitats similar to those where they evolved. In tidal marshes, organisms experience some of the most extreme physical conditions on earth: temperatures from −20 to 40°C, flooding twice a day but only a few times a month at higher elevations, sediments ranging from oxidized to severely reduced (Eh of +700 to −300 mV), soil salinity from hypersaline (40–90 ppt) to fresh depending on floodwater source and precipitation, and erosive forces from waves, currents, and ice at higher latitudes. Despite these harsh and variable conditions, there are many organisms adapted to tidal marshes, and new introductions and hybrids that can thrive given the opportunity.

Salt marshes occupy the intertidal zone and support rich ecosystems of salt-tolerant plants and other biota (Costanza et al. 1997; Mitsch and Gosselink, 2000). These ecosystems contain channel networks that dissect marsh platforms, just as terrestrial river networks dissect hillslopes. In contrast to upland landscapes, marsh platforms are very low relief, are inundated by tides, and the channels that dissect them experience bidirectional flows (D’Alpaos et al. 2005; Hughes, 2012; Coco et al. 2013). These conditions are also present in intertidal mudflats, yet marsh platforms sit at different elevations (Fagherazzi et al. 2006), have different characteristics of their channel networks (Rinaldo et al. 1999a, 1999b; Kleinhans et al. 2009), and different hydrodynamics (Fagherazzi et al. 2012). The fundamental difference is the presence of marsh vegetation which has a profound effect on flow within marsh canopies (Nepf, 2012).

Salt marshes are considered some of the most biologically diverse and ecologically important regions on Earth, containing thousands of species of robust salt-tolerant plants, crabs, fish, mollusks, zooplankton, algae, and bacteria. Isolated between topographic headlands, laterally continuous behind protective barriers, or associated with extensive delta landscapes, salt marshes are regulated by a variety of physical forces such as waves, tides, rivers, and storm surges, but they are also impacted by climatic variations in temperature and precipitation, riverine flooding, local tectonics, and subsidence (i.e., a deltaic process that describes the lowering of the land surface). Biological forces also play important roles in controlling salt marsh landscapes as many species shape geomorphic development. As these landscapes form and evolve, there exist significant interactions between biology, hydrology, and geology; thus it is impossible to consider salt marsh geomorphology – i.e., how the landscape changes over time – without taking into account these principal interactions.

The salt marsh response to a changing climate may be more complex than that of either terrestrial or marine ecosystems because salt marshes exist at the interface of land and sea and both bring changes to the marsh. Climate change may exacerbate anthropogenic-related stresses that salt marsh plants are already experiencing, limiting their resilience (Keddy 2011). In this chapter we discuss major climate change impacts likely to affect salt marshes including temperature, sea level rise (SLR), salinity, CO2, freshwater flow, sediment, and nutrients, and consider how salt marsh plants respond to these impacts and potential interactions of these impacts. Specifically, we explore changes in plant productivity and decomposition rates, aboveground and belowground biomass, and stem density as they are central to understanding marsh responses on a larger scale, with implications for species composition, elevation change, nutrient cycling, carbon sequestration, food webs, and ultimately marsh survival. Although this chapter is focused on salt marshes, examples from tidal fresh and brackish marshes are also included to a limited extent where relevant.

Salt marshes are valuable but complex biophysical systems with associated ecosystems. This presents numerous challenges when trying to understand and predict their behaviour and evolution, which is essential to facilitate their continued and sustainable use, conservation and management1. Detailed understanding of the hydrodynamics, sediment dynamics, and ecology that control the system is required, as well as their numerous interactions2,3, but is complicated by spatial and temporal heterogeneity at a range of scales4,5. These complex interactions and feedbacks between the physical, biological, and chemical processes can be investigated in situ following natural, unintentional, or intentional manipulation6, but the mechanistic basis of any observations are confounded by the presence of collinear variables. Hence, laboratory investigations can be beneficial, as they provide the opportunity for systematic testing of subsets of coastal processes, mechanisms, or conditions typical of salt marsh systems, in the absence of confounding variables. With appropriate scaling, this allows a better understanding of the overall function of the salt marsh, and better predictions of their evolution.

Salt marshes are coastal ecosystems located at the boundary between sea and land, generally in tidal environments, often covered by halophytic vegetation and periodically flooded by tide (Allen 2000).

Sixty-five years ago, Teal’s (1962) study showed that salt marsh primary production was greater than community respiration. To explain this result, he suggested that marshes exported excess organic matter either directly as organic matter, or as organisms, to coastal waters. This concept, that marshes were “outwelling” material to the adjacent estuary and coastal oceans, was soon expanded to nutrients as well. However, the actual importance of the marsh in supplying organic matter and nutrients to adjacent coastal systems has been controversial and reviews debating the importance of outwelling from marshes have regularly appeared over the decades (Nixon 1980, Childers et al. 2000, Odum 2000, Valiela et al. 2000, Boynton and Nixon 2013). It has also been argued that in some cases the coastal ocean can act as a source of nutrients to the marsh and estuary (“inwelling”).