Understanding the impacts of multiple stressors and associated regime shifts in shallow wetlands

Understanding the dynamics and drivers of regime change is essential for effective wetland management. Much evidence suggests that nutrient-enriched, shallow, permanent lakes and wetlands typically exist in either two alternative stable states or regimes: a clear-water state dominated by macroscopic plants or a turbid-water state dominated by microscopic phytoplankton. In European lakes, where phosphorus is often limiting, macroscopic plants typically dominate when total phosphorus (TP) is less than 50 µg L-1 and phytoplankton dominate when total phosphorus exceeds 150 µg L-1. Predicting which state will dominate between these two thresholds is more difficult because feedback mechanisms hinder macroscopic plants invading a phytoplankton-dominated system and vice versa. Hysteresis occurs because there is not a simple linear relationship between nutrient concentration and the abundance of phytoplankton or macroscopic plants. Non-linear dynamics prevail and regime change can only occur when nutrient thresholds and associated feedback mechanisms are overcome. Although nutrient-driven state changes are well documented, other state changes can be driven by water regime, salinity and organic matter loadings. Research on wetlands in south-western Australia indicated that a multi-state model was applicable to perennial salinised wetlands where salinity, rather than nutrient concentration, was the main water quality driver. The finding that a dual state model did not apply to Western Australian wetlands with a seasonal water regime indicated that water regime is also influential. Developing conceptual models of regime change provides a powerful tool for integrating data on physical, chemical and biological features of standing waters into concepts that can generate testable predictions and guide restoration activities.