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

TLDR

Salinization threatens inland and coastal wetlands worldwide, driven by altered freshwater flows, land use, irrigation, wastewater discharge, sea‑level rise, storm surges, and de‑icing salts, and is projected to worsen with climate change. This review seeks to synthesize how wetland salinization drives ecosystem‑scale feedbacks and landscape‑level processes, and to use wetlands as a testbed for ecological theories such as alternative stable states, tipping points, trophic cascades, and disturbance recovery. Salinization alters soil‑water chemistry by raising ionic concentrations, shifting chemical equilibria and mineral solubility, and modifying the biogeochemical cycling of carbon, nitrogen, phosphorus, sulfur, iron, and silica. Salinization reduces inorganic nitrogen removal and carbon storage, generates toxic sulfides, imposes physiological stress on biota, and drives large shifts in community composition and ecosystem function, disrupting interspecific interactions.

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.