Drought may amplify the impacts of salt pollution in pond ecosystems: an experimental exploration

Pond ecosystems are biodiversity-rich habitats, which support great biological diversity and provide important ecological services, but increasingly face risk of pollution and drought events. With increase in use of road-salts, ponds become vulnerable to high levels of salt pollution and may impair their biological communities and ecosystem functions. However, understanding the impacts of these two threats combined are limited. In this study, we experimentally investigated the impacts of mad-salt pollution and the expected future increase in drought events on ponds' physical conditions, communities and ecosystem functions. In a two-way factorial design, 20 experimental mesocosms were used to test the individual and combined effects of climate change-driven drought events and salt pollution on natural pond ecosystems. Treatments were presence or absence of water salinization to mimic pollution by mad-salts, and drying to mimic drought events. Our drought treatment doubled water salinity during the experimental period. While salt additions significantly affected ponds' physical conditions and leaf litter decomposition, both salt additions and drying showed no independent impacts on pond biota and ecosystem functions. However, our path analysis revealed that drying indirectly reduced leaf litter decomposition and ecosystem productivity through changes in ponds' physical conditions, although it did not affect biomass of insects and periphyton. Overall, our findings suggest that anticipated drought events will amplify road-salt pollution, and subsequently affect ponds' biodiversity, food webs, and ecosystem functions. Implications for restoration, conservation and climate change adaptation may include actively managing snow-melting salts and long-term monitoring of changes in ponds' biophysical conditions and ecological functions.