Impacts and implications of climate change on Waituna Lagoon, Southland.
Abstract
Waituna Lagoon, near Invercargill, South Island, New Zealand, is a land-locked freshwater lagoon. It supports a variety of threatened species and is part of the ecologically significant Awarua Wetland Ramsar site. Periodically, the gravel lagoon barrier is manually opened to facilitate drainage of surrounding farmland and flushing of nutrient-rich water, turning the lagoon into an estuarine state until the barrier naturally closes. This report provides an assessment of potential climate change-related impacts on the lagoon, based on existing information. Projected increases in rainfall, freshwater inflows, flood events and inundation of surrounding land over the next several decades are likely to contribute to lower lagoon-bed light levels and higher levels of nutrients and sediment entering the lagoon. Such changes may increase algae growth and inhibit the growth of Ruppia spp., desirable native aquatic grasses. Nutrient and sediment inputs are known drivers of lagoon regime shifts (from a desirable macrophyte (freshwater plants)- dominated state to an undesirable algal-dominated state) and are closely linked to declines in water quality. If freshwater inflows increase as predicted, the lagoon will either need to be opened more frequently or the threshold for opening will need to be raised. This raises issues about land use around the lagoon and the long-term sustainability of the current manual opening regime. With ongoing sea level rise, the boundary of the lagoon is likely to shift landward and the intertidal zone is likely to shrink, which may affect wading birds that forage in the intertidal zone. Due to the complexity of the lagoon system, uncertainties about the trajectories of change in climate and sea level and the responses of the lagoon ecosystem, further research and ongoing monitoring is recommended as well as an adaptative management approach. This could include a variety of strategies for managing the lagoon and its biodiversity under both increasing freshwater inflow and saltwater inundation conditions.