Nutrient inversion but not warming drive changes in periphyton biomass and composition in shallow lake mesocosms

Abstract Periphyton plays an important functional role in shallow lakes and is sensitive to variation in nutrient availability and global warming. Understanding the responses of periphyton to changes in nutrient availability in contrasting warming scenarios can contribute to the development of more efficient techniques for the restoration of lakes in a future warmer world. We analyzed the periphyton response to early eutrophication and oligotrophication under three different warming scenarios and explored the associated environmental drivers of periphyton in 24 shallow lake mesocosms. These were initially set up in 2003 with two nutrient treatments (with or without nutrient addition) and three temperatures (ambient, A2 IPCC scenario and A2 increased by 50%) following a full factorial design. In 2023, the nutrient treatment was inverted, starting nutrient addition in low nutrient mesocosms (nutrient increase) and stopping nutrient addition (nutrient decrease) in the remaining. Artificial plants were placed in each mesocosm to allow periphyton to develop for 21 d, both before and after the inversion of the nutrient treatments. We found marked changes in periphyton biomass and composition to the nutrient inversion, but no response to warming. Nutrient decrease induced lower periphyton biomass and compositional changes at species and group levels. Nutrient increase had no effect on biomass accrual or species composition but altered the group composition. Our results suggest that lower nitrate availability was a crucial factor driving biomass and compositional changes with the cessation of nutrient addition, while a combination of environmental variables explained the altered periphyton group composition in the nutrient increase treatment.