Resilience of the red alga Palmaria palmata to anthropogenic environmental changes: Small-scale flask assessment of combined effects of marine heatwaves, hyposalinity, and marine pollution

Abstract Seaweeds are keystone species supporting ecosystems and human activities. Despite their vulnerability to increasing anthropogenic pressures, multi-exposure experiments with macroalgae are rare, especially in non-kelp species. This study examined the combined effects of marine heatwaves (+ 3℃; MHW), hyposalinity (-8 PSU; Sal), and wastewater treatment plant effluent (WWTP) in Palmaria palmata (Rhodophyta), representative of present and projected environmental changes. Sporulating and vegetative blades were exposed to single and combined stressors for two weeks, followed by one week (sporulating) and six weeks (vegetative) of recovery. Growth (fresh weight, surface area) and photosynthetic ability (maximum quantum yield of PSII, F v /F m ; maximum electron transport rate, rETR max ) were measured weekly to assess effects on the health and development of the blades. Final blade size did not differ significantly between exposures in vegetative blades despite temporarily increased growth rates. In sporulating blades, MHW and WWTP led to 1.8-fold larger blades than controls. F v /F m decreased during MHW, Sal+WWTP, Sal+MHW, and WWTP+MHW exposures (1.4-fold), indicating transient stress. No lasting effects were observed after recovery. The rETR max was unaffected by exposures and increased steadily over time. Photosynthetic measurements suggest some level of stress during exposures, accompanied by compensation mechanisms in response to PSII damage. The temporary increases in growth rates may indicate hormetic mechanisms with potential for adaptive plasticity, although selected MHW and Sal exposures are within the natural range of P. palmata , and sudden changes did not cause long-term stress in vegetative thalli. These outcomes highlight the resilience of wild and cultivated P. palmata .