7.9.4: Salt Marsh and Mangrove Response to a Changing Climate and Associated Sea Level Rise

Salt Marsh and Mangrove Response to a Changing Climate and Associated Sea Level Rise

Saltwater coastal wetlands are generally found in sheltered waters and include mangrove forests, seagrass meadows, and tidal salt marshes. These wetlands host incredibly productive plant communities, which take up substantial amounts of C via photosynthesis, and store a significant fraction of that C in their wet, anaerobic soils (Chmura et al. 2003; Donato et al. 2011; Fourqurean et al. 2012). This C has been termed “coastal wetland blue carbon.” These vegetated saltwater coastal ecosystems represent an estimated 0.2% of the area of the ocean, but have C stocks equivalent to 50% of the C buried in ocean sediments (Duarte et al. 2013). As such, saltwater coastal blue C wetlands are some of the most C rich ecosystems on the planet (See Fig. 3 in McLeod et al. 2011). Thus, there are growing efforts to include saltwater wetlands in international climate protection activities and policy frameworks (Wylie et al. 2016; Howard et al. 2017).

Salt marshes and mangrove swamps have accumulated C-rich soil for centuries to millennia as sea levels have slowly risen increasing levels of plant production (See Fig. 3 in McLeod et al. 2011). These wetland soils accumulate vertically through three synergistic processes (See Fig. 7 in Fitzgerald et al. 2008). The belowground growth adds volume to the soil and the aboveground portion helps trap inorganic sediment carried in tidal waters that regularly flood the soil. Extended saturation of the soil reduces the rate of decomposition of soil organic matter, thereby enabling the persistence of the effective blue C sink. Increasing soil volume results in raised surface elevation of the wetland, so that on decadal scales its elevation roughly tracks sea level rise (e.g. Chmura et al. 2001; Ellison 2008). This increase in elevation is accompanied by lateral expansion of the marsh or mangrove swamp over tidal flats in the lower intertidal zone and inland over adjacent terrestrial ecosystems. The vegetation that occupies intertidal niches has evolved a suite of mechanisms to tolerate flooding by saline water, but at a greater expenditure of energy (e.g. Mendelssohn et al. 1982). There is a limit to this tolerance.

Saltwater wetlands provide significant ecosystem services. Mangroves and salt marshes help to slow and attenuate waves and storm surge, reducing the flooding and erosion of ocean coastal communities (Shepard et al. 2011; Arkema et al. 2013). One study suggests that U.S. marine saltwater wetlands provide $23.2 billion dollars of storm protection every year (Costanza et al. 2008) while another study estimates that every hectare of salt marsh provides US $8,234 dollars, or US $3,334 per acre, in storm protection, on average, per year (Barbier et al. 2011). Since a warmer climate contributes to increased storm intensity (Trenberth et al. 2015), enhancing these protective measures is seen as a cost-effective way to protect coastal communities and infrastructure. The storm protection qualities of wetlands are leading many policy and decision makers to consider more investments in protecting or restoring coastal wetlands and other ecosystems to provide the climate adaptation benefits of natural storm and erosion reduction (Barbier 2014; Sutton-Grier et al. 2015).

The impact of climate warming, its associated sea level rise and changes in precipitation patterns will vary considerably within and among tidal marshes. Few studies have looked at combined effects of sea level rise and other aspects of climate change. Feher et al. (2017) reviewed the literature on the influence of changing temperature and precipitation regimes on tidal saline wetlands. They found that for several ecosystem properties and many regions there was still insufficient evidence to make generalized predictions.

Research, however, has demonstrated differences due to climate zones and vegetation. For instance, where growing seasons are limited by cold temperatures, such as the coast of the northern Northwest Atlantic, studies have shown that a warmer climate would marginally increase decomposition, but will increase plant production and soil carbon storage (Charles and Dukes 2009; Gedan and Bertness 2010; Kirwan et al. 2014), although the effect of a rise of sea level was not addressed. On the Mediterranean coast, experimentally increased temperature, decreased precipitation and increased inundation period caused vegetation to shift from a perennial grass to an annual succulent (Strain et al. 2017).

There are two major ways that climate change is expected to impact all saltwater wetlands. Climate warming is expected to increase rates of sea level rise, resulting in loss of wetland area through “coastal squeeze,” particularly in areas surrounded by urbanized uplands (e.g. Torio and Chmura 2013). This has been identified as the largest climate change threat for mangroves (Gilman et al. 2008). Secondly, warmer temperatures will allow poleward shifts in flora and fauna that can result in significant changes in the saltwater tidal habitat, thereby altering its ecosystem services, including ability to store blue C, and in some cases causing the release of CO₂ from the blue C sink as described below.

Modification of estuarine hydrology or increased rates of sea level rise can increase the hydroperiod (duration of flooding) beyond the thresholds tolerated by intertidal vegetation. Climate warming will increase rates of sea level rise primarily from continued melting of the world’s ice sheets and glaciers and the thermal expansion of a warming ocean (Church et al. 2013). As the magnitude and rate of ice sheet melting is difficult to model, predicted rates of sea level rise vary, but it is accepted that increasing rates of sea level rise and its impact will be felt on all coastlines, most severely on those already subject to subsidence (sinking). One modest projection, a 0.6 m (2 ft) rise in global (eustatic) sea level by 2100, would translate to an increase of 0.61 m (2.3 ft) at New York City and 1.07 m (3.5 ft) in Galveston Texas. The greatest uncertainty is the rate of melting of ice sheets covering Antarctica and Greenland. There is nothing magical about the year 2100, and it is certain that sea levels will continue to rise for centuries under all current scenarios. A recent report considers six possible outcomes for global mean sea level rise by 2100 ranging from 0.3 meters with a 100% probability to an intermediate projection of 1.0 m with a 17% probability. If recent estimates for Antarctica ice melt are included there is a 0.1% probability that the rise could reach 2.5 m (NOAA 2017). See Fig. \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Past and projected changes in global mean sea level rise. (NOAA 2017a). Mean sea level rise and projections to 2100 under alternative IPCC scenarios. The lowest rise is 0.30 meters (100% probability), the intermediate is 1.0 meters (17% probability) and the highest is to 2.5 meters (0.1% probability)

Within tidal wetlands the effects of increased rates of sea level rise will be most strongly felt at the lower elevations where vegetation will most rapidly succumb and soil accretion will cease (e.g. Kirwan et al. 2010). Without living vegetation, the submerged wetland soil and its C stock can be exposed to erosion and possibly to oxidation of the organic matter, returning centuries of stored CO₂ back to the atmosphere. The fate of soil organic matter eroded from wetlands is an increasingly important science question that is not yet resolved (e.g. DeLaune and White 2012). If the upland adjacent to the tidal wetland is not developed and slopes are gentle, then the wetland can migrate inland, limiting the loss of area (but not necessarily blue carbon stocks). However, if this land is developed or if natural topography is steep, the structures or grade will prevent migration, putting the marsh or mangrove in a coastal squeeze (Torio and Chmura 2013). The potential for coastal squeeze is high on many of the world’s coastlines, particularly on the highly urbanized bays and estuaries of the U.S., such as San Francisco Bay in California and the shore of New York City on Jamaica Bay (Hopper and Meixler 2016). The loss of wetland area due to coastal squeeze means loss of all its ecosystem services including essential habitat for fish and wildlife, loss of the ability of the system to store additional C and loss of its capacity to buffer inland development from the impacts of storms. One opportunity to decrease the amount of salt marsh loss that is likely to occur with sea level rise is to actively plan for future inland marsh migration now. There have been a few innovative studies considering how to plan for marsh migration including one that examined which wetlands along the Gulf coast of the U.S. are most threatened by projected future urban development. This information can be used to identify migration corridors for these wetlands and set priorities for current protection to prevent future coastal squeeze (Enwright et al. 2016). Another study examined two conditions to determine which marshes along the U.S. Northeast and Mid-Atlantic coast are likely to be resilient to sea level rise by examining the current health of the marsh as well as its potential to migrate inland (Anderson and Barnett 2017).

Climate warming has a direct impact on salt marshes and mangrove swamps by increasing poleward migration of their flora and fauna. Such changes are most observable where species’ populations occur near the edge of their biogeographic ranges. In fact, globally, mangroves are expanding their range from tropical and subtropical climes, to invade salt marshes on adjacent warm temperate coasts (e.g. Godoy and DeLacerda 2015). Studies are finding that climate-changed-induced movement of mangroves into saltmarsh with warming temperatures is resulting in increases in the carbon stored in biomass and soils in marine and estuarine mangroves. This is because mangrove forests have some of the highest average C storage per land area in unmanaged terrestrial ecosystems (Doughty et al. 2015, Kelleway et al. 2015). As mangroves replace salt marsh vegetation, soil C may increase (Bianchi et al. 2013). However, such invasions significantly change habitat structure and we know little about impacts on biotic interactions, potential lags for co-evolved species to shift, or challenges to mosquito control management (Dale et al. 2013). While SLR is expected to enable mangroves to migrate inland where other obstacles do not occur, the example of mangrove dieback in northern Australia (Duke et al. 2017) shows that the impact of climate is more complex, with changes in the regional climate patterns resulting in lower rainfall and tidal depression during the hot part of the year being suggested as the cause of the dieback.

Several studies have documented that increasing salinity in upstream reaches of an estuary will decrease biomass accumulation of foundation freshwater plant species (Sutter et al. 2014, 2015; Neubauer et al. 2005). In microcosm studies, Sutter et al. (2015) found that even smooth cordgrass (Spartina alterniflora), a salt marsh foundation species on the western Atlantic, had reduced growth when exposed to increased salinity and grown with the invasive strain of tall common reed (Phragmites australis).

An example of range extension of benthic fauna is found in the herbaceous salt marsh fiddler crab (Uca pugnax) that burrows in marsh soil. Historically, the range of the fiddler crab has been limited along the northwestern Atlantic coast to waters south of Cape Cod, Massachusetts. Its range recently has expanded northward where it has been observed on the coast of New Hampshire (Johnson 2014). The effect of fiddler crabs on C storage has been studied in Virginia salt marshes where Thomas and Blum (2010) found that 74% more root material was decomposed in marshes with fiddler crab burrows. Unless potential predators and competitors accompany crab migration, this range extension could lead to significant release of CO₂ to the atmosphere from northern salt marsh C sinks.

Saltwater wetlands are effective natural C sinks until they are disturbed, degraded, or destroyed by draining them for urban development, agriculture, aquaculture or by other means. Rising sea levels will also degrade these ecosystems. Disturbing wetland hydrology can enable oxygen to oxidize stored soil organic matter. Drying wetland soils increases microbial decomposition of stored organic C causing these natural sinks to become sources of CO₂ emissions (Pendleton et al. 2012). Preventing loss of these ecosystems is a priority to avoid additional GHG emissions. Restoring degraded or lost saltwater wetlands can regenerate their ability to remove and sequester CO₂ from the atmosphere.

References

Anderson MG, Barnett A (2017) Resilient coastal sites for conservation in the Northeast and Mid-Atlantic US. The Nature Conservancy, Eastern Conservation Science

Arkema KK, Guannel G, Verutes G, Wood SA, Guerry A, Ruckelshaus M, Kareiva P, Lacayo M, Silver JM (2013) Coastal habitats shield people and property from sea-level rise and storms. Nature Climate Change 3:913–918j. https://doi.org/10.1038/NCLIMATE1944

Barbier EB, Hacker SD, Kennedy C, Koch EW, Stier AC, Silliman BR (2011) The value of estuarine and coastal ecosystem services. Ecological Monographs 81(2):169–193

Bianchi TS, Allison MA, Zhao J, Li X, Comeaux RS, Feagin RA, Kulawardhana RW (2013) Historical reconstruction of mangrove expansion in the Gulf of Mexico: Linking climate change with carbon sequestration in coastal wetlands. Estuarine, Coastal and Shelf Science 119:7–16. https://doi.org/10.1016/j.ecss.2012.12.007

Charles H, Dukes JS (2009) Effects of warming and altered precipitation on plant and nutrient dynamics of a New England salt marsh. Ecological Applications 19:1758–1773

Chmura GL, Anisfeld S, Cahoon D, Lynch J (2003) Global carbon sequestration in tidal, saline wetland soils. Global Biogeochemical Cycles 17(4):22.1–22.12. https://doi.org/10.1029/2002GB001917

Church JA, Monselesan D, Gregory JM (2013) Marzeion B (2013) Evaluating the ability of process based models to project sea-level change. Environmental Research Letters 8:014051 (8pp). https://doi.org/10.1088/1748-9326/8/1/014051

Costanza R, Perez-Maqueo O, Martinez ML, Sutton P, Anderson SJ, Mulder K (2008) The value of coastal wetlands for hurricane protection. Ambio 37(4):241–248

Dale P, Eslami-Andargoli L, Knight J (2013) The impact of encroachment of mangroves into saltmarshes on saltwater mosquito habitats. Journal of Vector Ecology 38(2):330–338

Delaune RD, White JR (2012) Will coastal wetlands continue to sequester carbon in response to an increase in global sea level? A case study of the rapidly subsiding Mississippi river deltaic plain. Climatic Change 110:297. https://doi.org/10.1007/s10584-011-0089-6

Donato DC, Kauffman JB, Murdiyarso D, Kurnianto S, Stidham M, Kanninen M (2011) Mangroves among the most carbon-rich forests in the tropics. Nature Geoscience 4:293–297. https://doi.org/10.1038/ngeo1123

Doughty CL, Langley JA, Walker WS, Feller IC, Schaub R, Chapman SK (2015) Mangrove expansion rapidly increases coastal wetland storage. Estuaries and Coasts. https://doi.org/10.1007/s12237-015-9993-8

Duarte CM, Losada IJ, Hendriks IE, Mazarrasa I, Marbà N (2013) The role of coastal plant communities for climate change mitigation and adaptation. Nature Climate Change 3:961–968. https://doi.org/10.1038/nclimate1970

Duke NC, Kovacs JM, Griffiths AD, Preece L, Hill DJE, van Oosterzee P, Mackenzie J, Morning HS, Burrows D (2017) Large-scale dieback of mangroves in Australia’s Gulf of Carpentaria: A severe ecosystem response, coincidental with an unusually extreme weather event. Marine and Freshwater Research. https://doi.org/10.1071/MF16322

Ellison JC (2008) Long-term retrospection on mangrove development using sediment cores and pollen analysis: A review. Aquatic Botany 89:93–104

Enwright NM, Griffith KT, Osland MJ (2016) Barriers to and opportunities for landward migration of coastal wetlands with sea level rise. Frontiers in Ecology and the Environment 14(6):307–316

Feher LC, Osland MJ, Griffith KT, Grace JB, Howard RJ, Stagg CL, Enwright NM, Krauss KW, Gabler CA, Day RH, Rogers K (2017) Linear and nonlinear effects of temperature and precipitation on ecosystem properties in tidal saline wetlands. Ecosphere 8(10):e01956. https://doi.org/10.1002/ecs2.1956

FitzGerald DM, Fenster MS, Argow BA, Buynevich IV (2008) Coastal impacts due to sea-level rise. Annual Review of Earth and Planetary Sciences 36:601–647

Fourqurean JW, Duarte CM, Kennedy H, Marbà N, Holmer M, Mateo MA, Apostolaki ET, Kendrick GA, Krause-Jensen D, McGlathery KJ, Serrano O (2012) Seagrass ecosystems as a globally significant carbon stock. Nature Geoscience 5:505–509. https://doi.org/10.1038/ngeo1477

Gedan KB, Bertness MD (2010) How will warming affect the salt marsh foundation species Spartina patens and its ecological role? Oecologia 164:479–487

Gilman EL, Ellison JC, Duke NC, Field C (2008) Threats to mangroves from climate change and adaptation options: A review. Aquatic Botany 89:237–250

Godoy MDP, DeLacerda LD (2015) Mangroves response to climate change: A review of recent findings on mangrove extension and distribution. Anais da Academia Brasileira de Ciências (Annals of the Brazilian Academy of Sciences) https://doi.org/10.1590/0001-3765201520150055 Accessed 15 Oct 2017

Hopper T, Meixler MS (2016) Modeling coastal vulnerability through space and time. PLoS ONE 11(10):e0163495. https://doi.org/10.1371/journal

Howard J, Sutton-Grier A, Herr D, Kleypas J, Landis E, Mcleod E, Pidgeon E, Simpson S (2017) Clarifying the role of coastal and marine systems in climate mitigation. Frontiers in Ecology and Environment 15:42–50

Johnson DS (2014) Fiddler on the roof: A northern range extension for the marsh fiddler crab Uca pugnax. Journal of Crustacean Biology 34(5):671–673

Kelleway JJ, Saintilan N, Macreadie PI, Skilbeck CG, Zawadzki A, Ralph PJ (2015) Seventy years of continuous encroachment substantially increases “blue carbon” capacity as mangroves replace intertidal salt marshes. Global Change Biology 22:1097–1109. https://doi.org/10.1111/gcb.13158

Kirwan ML, Guntenspergen GR, Langley JA (2014) Temperature sensitivity of organic-matter decay in tidal marshes. Biogeosciences 11:4801–4808

McLeod E, Chmura GL, Bouillon S, Salm R, Björk M, Duarte CM, Lovelock CE, Schlesinger WH, Silliman BR (2011) A blueprint for blue carbon: Toward an improved understanding of the role of vegetated coastal habitats in sequestering CO₂. Frontiers in Ecology Environment 9(10):552–560 Figure 3. https://doi.org/10.1890/110004

Mendelssohn IA, McKee KL, Postek MT (1982) Sublethal stresses controlling Spartina alterniflora productivity. In: Gopal B, Turner RE, Wetzel RG, Whigham DF (eds) Wetlands: Ecology and management. National Institute of Ecology, Jaipur, pp 223–242

Neubauer SC, Givler K, Valentine SK, Megonigal JP (2005) Seasonal patterns and plant - mediated controls of subsurface wetland biogeochemistry. Ecology 86(12):3334–3344

NOAA 2017 National Oceanic and Atmospheric Administration U.S. Department of Commerce, U.S. Geological Survey, U.S. Environmental Protection Agency and Rutgers University, Global and regional sea level rise scenarios for the United States NOAA Technical Report NOS CO-OPS 083 techrpt83_Global_and_Regional_SLR_Scenarios_for_the_US_final.pdf. Accessed 19 Apr 2017

Pendleton L, Donato DC, Murray BC, Crooks S, Jenkins WA, Sifleet S, Craft C, Fourqurean JW, Kauffman JB, Marba N, Megonigal JP, Pidgeon E, Herr D, Gordon D, Baldera A (2012) Estimating global “blue carbon” emissions from conversion and degradation of vegetated coastal ecosystems. PLoS ONE 7(9):e43542. https://doi.org/10.1371/journal.pone.0043542

Shepard CC, Crain CM, Beck MW (2011) The protective role of coastal marshes: A systematic review and meta-analysis. PLoS ONE 6(11):e27374. https://doi.org/10.1371/journal.pone.0027374

Strain MA, van Belzen J, Comandini P, Wong J, Bouma TJ, Airoldi L (2017) The role of changing climate in driving the shift from perennial grasses to annual succulents in a Mediterranean saltmarsh. Journal of Ecology 105:13741385

Sutter LS, Perry JE, Chambers RM (2014) Tidal freshwater marsh plant responses to low level salinity increases. Wetlands 34:167–175

Sutter LA, Chambers RM, Perry JE (2015) Seawater intrusion mediates species transition in low salinity, tidal marsh vegetation. Aquatic Botany 122:32–39

Sutton-Grier AE, Bamford H, Wowk K (2015) Future of our coasts: the potential for natural and hybrid infrastructure to enhance the resilience of our coastal communities, economies and ecosystem. Environmental Science & Policy 51:137–148

Thomas CR, Blum LK (2010) Importance of the fiddler crab Uca pugnax to salt marsh soil organic matter accumulation. Marine Ecology Progress Series 414:167–177. https://doi.org/10.3354/meps08708

Torio D, Chmura GL (2013) Assessing coastal squeeze of tidal wetlands. Journal of Coastal Research 29(5):1049–1061. https://doi.org/10.2112/JCOASTRES-D-12-00162.1

Trenberth KE, Fasullo JT, Shepherd TG (2015) Attribution of climate extreme events. Nature Climate Change 5:725–730

Wylie L, Sutton-Grier AE, Moore A (2016) Keys to successful blue carbon projects: Lessons learned from global case studies. Marine Policy 65:76–84

Excerpted From:

Moomaw, W.R., Chmura, G.L., Davies, G.T. et al. Wetlands In a Changing Climate: Science, Policy and Management. Wetlands 38, 183–205 (2018). Accessed December 2023 https://doi.org/10.1007/s13157-018-1023-8 CC-BY