7.9.3: Ecological Consequences for Freshwater Wetlands in a Changing Climate

Freshwater wetlands may be altered by climate change in all geographic regions of the world (Junk et al. 2013). A changed climate will alter hydrology, and functionality may be impaired by increased temperatures, drought or flooding events, CO₂ increases, and/or salinity intrusion (Junk et al. 2013). These changes will affect critical functions and ecosystem services such as carbon storage, biodiversity support, wildlife habitat and water quality (Junk et al. 2013). Negative impacts related to climate change will be compounded by synergies with other stressors, such as invasive species and land use change, thereby potentially increasing both the difficulties in managing and restoring wetlands, and the risk of endemic species extinctions (Erwin 2009).

Despite these challenges, some freshwater wetlands may be relatively resilient to climate change (Baron et al. 2002; Middleton and Souter 2016) within certain boundaries of temperature, precipitation, water level, salinity intrusion, and storm activity (Poff et al. 2002; Bernstein et al. 2007). At the same time, salinity intrusion poses specific threats to coastal freshwater wetlands because many species in these ecosystems are intolerant of salinity (Keddy 2010). Also, these species often have lower levels of production if salinity levels become too high (Middleton 1999; Sutter et al. 2014; Middleton and Souter 2016). A recent review synthesizes the state of our knowledge on how salinization associated with climate change will impact these wetlands (Herbert et al. 2015).

Climate change poses threats to non-coastal freshwater wetlands as well; hydrology is shifting as many local water regimes have become wetter or drier in recent decades (Fig. \(\PageIndex{1}\)) (Mallakpour and Villarini 2015). In particular, megadroughts predicted by climate models (Cook et al. 2015) may dry Midwestern and Southwestern wetlands in North America with severe consequences for both wetlands and society. Severe droughts could impair the ability of these wetlands to maintain services including water quality, water supply, flood control, storm protection, and direct harvests of fish, animals, and plants, ultimately with severe negative impacts on ecosystem function and biodiversity (Baron et al. 2002; Middleton and Souter 2016). In addition, reduced winter snowpack and earlier snowmelt are impacting northern freshwater wetlands by altering the timing and magnitude of stream flows (Lawler 2009). In northern areas with permafrost, vegetation structure completely changes after permafrost melts (Malhotra and Roulet 2015). In fact, climate change is already changing community composition, species distribution, phenology, physiology and invasive species presence (Lawler 2009).

Figure \(\PageIndex{1}\): The magnitude and frequency of flood events in the Midwestern United States from 1962−2011. Triangles show trends of flooding at U.S.G.S. gage stations with trends (positive, negative, neutral; blue triangle, red triangle, and gray circle, respectively; from Mallakpour and Villarini 2015)

Unfortunately, many of the world’s freshwater wetlands are already stressed by increased land-use pressure, so that additional hydrological alteration can contribute to an overall decrease in resilience to climate change (Baron 2002; Middleton and Souter 2016). Human alteration is commonplace throughout river corridors, challenging management as the impacts of upstream alterations accumulate along the waterway (See Fig. \(\PageIndex{2}\)). (DuBowy 2013; Tockner and Stanford 2002). As demands for river resources increase, such problems are expected to worsen (Baron et al. 2002). Flowing water is compromised by river re-engineering practices, even though moving water generally improves oxygenation and plant health (Middleton 1999). Also, upriver freshwater extraction in tidal freshwater wetlands coupled with sea level rise can cause the salinification of surface and ground water, with accompanying stress and even the collapse of tidal vegetation in the freshwater reaches of estuaries (Perry and Atkinson 2009; Middleton and Souter 2016).

Figure \(\PageIndex{2}\): The hydrologic changes in the Mississippi River and tributaries for navigation and development include straightening, deepening, levee construction and damming. These engineering practices influence ecosystem processes across the floodplain and channel of this big river system (DuBowy 2013)

Fortunately, emerging research suggests that vegetation collapse sometimes can be avoided by hydrologic remediation (Souter et al. 2010). Freshwater remediation can reduce salinity and revive freshwater forests stressed by salinity intrusion, if the vegetation is not fatally damaged (Middleton et al. 2015; Middleton and Souter 2016). Such techniques could become critical for maintaining future ecosystem health and services (Baron et al. 2002; Middleton and Souter 2016). To date, there is no report of long-term monitoring of the survival of vegetation following remediation, so any long-term benefits are untested (Middleton and Souter 2016). Managers may need to carefully monitor the effects of traditional techniques and adjust the timing and/or intensity of management actions accordingly (Jackson and Hobbs 2009; Middleton et al. 2017).

One harbinger of ecosystem change is that the early life history stages of foundation species (species with a strong role in structuring communities) are increasingly unsuccessful at the hot or dry edges of their ranges, noting that juveniles are more sensitive to environmental extremes than adult plants (Jackson and Hobbs 2009). Without regeneration, vegetation enters a relict state (Williams et al. 1999). Worldwide examples of relict foundation species are growing, and such vegetation may be poised for abrupt decline if disturbance removes adult vegetation (Middleton et al. 2017). There are several indicators that some freshwater wetlands are poised for collapse at the edges of their ranges, and the loss of all but relict species is a key indicator of that problem (Middleton et al 2017). Thus, freshwater wetlands face a myriad of challenges in the face of climate change.

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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