A global perspective on wetland salinization: ecological consequences of a growing threat to freshwater wetlands

Abstract

Abstract. Salinization, a widespread threat to the structure and ecological functioning of inland and coastal wetlands, is currently occurring at an unprecedented rate and geographic scale. The causes of salinization are diverse and include alterations to freshwater flows, land-clearance, irrigation, disposal of wastewater effluent, sea level rise, storm surges, and applications of de-icing salts. Climate change and anthropogenic modifications to the hydrologic cycle are expected to further increase the extent and severity of wetland salinization. Salinization alters the fundamental physicochemical nature of the soil-water environment, increasing ionic concentrations and altering chemical equilibria and mineral solubility. Increased concentrations of solutes, especially sulfate, alter the biogeochemical cycling of major elements including carbon, nitrogen, phosphorus, sulfur, iron, and silica. The effects of salinization on wetland biogeochemistry typically include decreased inorganic nitrogen removal (with implications for water quality and climate regulation), decreased carbon storage (with implications for climate regulation and wetland accretion), and increased generation of toxic sulfides (with implications for nutrient cycling and the health/functioning of wetland biota). Indeed, increased salt and sulfide concentrations induce physiological stress in wetland biota and ultimately can result in large shifts in wetland communities and their associated ecosystem functions. The productivity and composition of freshwater species assemblages will be highly altered, and there is a high potential for the disruption of existing interspecific interactions. Although there is a wealth of information on how salinization impacts individual ecosystem components, relatively few studies have addressed the complex and often non-linear feedbacks that determine ecosystem-scale responses or considered how wetland salinization will affect landscape-level processes. Although the salinization of wetlands may be unavoidable in many cases, these systems may also prove to be a fertile testing ground for broader ecological theories including (but not limited to): investigations into alternative stable states and tipping points, trophic cascades, disturbance-recovery processes, and the role of historical events and landscape context in driving community response to disturbance.

Relationships Between Salinity and Short-Term Soil Carbon Accumulation Rates from Marsh Types Across a Landscape in the Mississippi River Delta

“…If surface waters become more saline due to sealevel rise or freshwater limitations, coastal wetlands may transition to a more salt-tolerant plant communities and accumulate less total carbon in their soils (Williams and Rosenheim 2015;Herbert et al 2015). Conversely, if more freshwater reaches the coast via riverine discharge, restoration management actions such as large river diversions, or increased precipitation events, coastal wetlands may become fresher and accumulate more soil carbon in the short term (Morris et al 2013).…”

“…For example, it is predicted that as air and sea temperatures increase, some herbaceous wetland vegetation will be replaced with tropical woody mangrove species (Comeaux et al 2012;Osland et al 2012;Doughty et al 2015) and that these community shifts may influence carbon production and accumulation rates in wetland soils. Sea-level rise is also predicted to produce more saline environmental conditions that could drive transitions of fresh marshes to more saline marsh types (Herbert et al 2015) or drive salt marsh transgression inland (Kirwan et al 2016). In addition, large-scale restoration projects could introduce more fresh water to some estuaries, resulting in fresher marsh types (Morris et al 2013).…”

“…Salinity is an important factor in structuring coastal wetland communities and influences biogeochemical pathways (Craft 2007;Craft et al 2009;Neubauer and Craft 2009). For example, increasing salinity can increase the concentration of terminal electron acceptors (such as iron, manganese and sulfate) that can increase microbial mineralization of organic matter and thus could result in a reduction in carbon accumulation (Herbert et al 2015). Therefore, better understanding of how these environmental conditions influence carbon accumulation rates in various coastal wetland types is needed to predict how climate change through sea-level rise and future coastal restoration actions will influence the carbon accumulation rates (Markewich et al 2007;Couvillion et al 2013).…”

Regulators of coastal wetland methane production and responses to simulated global change

“…Because salinity and sulfate availability are often correlated, it can be difficult to disentangle these two factors; Chambers et al (2011) isolated their effects in a laboratory manipulation and found that seawater (sulfate) had a more dramatic and longer lasting effect on CH 4 production than saltwater (NaCl). Nevertheless, salinity often places additional stress on organisms, such that saltwater intrusion alters microbial and plant communities (Herbert et al, 2015). Additionally, saltwater intrusion can influence cation exchange in the sediments, such that calcium is mobilized, which can coprecipitate with phosphate, and ammonium is released, all of which can shift a wetland towards P rather than N limitation (Herbert et al, 2015;van Dijk et al, 2015).…”

“…Nevertheless, salinity often places additional stress on organisms, such that saltwater intrusion alters microbial and plant communities (Herbert et al, 2015). Additionally, saltwater intrusion can influence cation exchange in the sediments, such that calcium is mobilized, which can coprecipitate with phosphate, and ammonium is released, all of which can shift a wetland towards P rather than N limitation (Herbert et al, 2015;van Dijk et al, 2015). Although we did not directly measure these effects of salinity and therefore cannot rule them out, we hypothesize that sulfate availability and differences in functional microbial guilds are primarily responsible for differences in CH 4 production rather than salinity and salinity-induced cation exchange.…”