Synergistic effects of extreme temperature and low salinity on foundational macroalga Fucus vesiculosus in the northern Baltic Sea

•Low salinity increases negative impacts of high temperature on Fucus vesiculosus.•Even short (8-day) exposure to extreme temperatures was highly harmful.•Two local populations studied showed different and unexpected responses.•Some effects were only displayed several days after the treatment.•Interactive stressors may cause abrupt declines in Fucus.

Climate change has been identified as one of the biggest current drivers of environmental change. Climate model projections for the Baltic Sea forecast increased frequency and duration of extreme temperatures, together with declines in salinity, which are expected to have impacts on the biota. In this experimental study, the interacting effects of low salinity and short-term (8 days) extreme seawater temperatures, followed by an 11-day recovery period, on the foundational macroalga, Fucus vesiculosus, were investigated. To account for potential variation in the responses at local scale, individuals originating from two different local populations, a warm and a cold site were included.In experiments manipulating temperature (20 °C to 28 °C) and salinity (4 or 6), it was found that even an 8-day exposure to 26 °C or higher was detrimental to F. vesiculosus, causing extensive tissue necrosis. Tissue necrosis was enhanced by low salinity. Photosynthesis, measured as the steady-state electron transport rate (ETR) and maximum ETR, declined at 26 °C, and this effect was also enhanced by low salinity. Temperatures above 26 °C caused declines in light-limited photosynthetic efficiency (α), indicating direct physiological damage to PS II reaction centers.After 11 days of recovery, some photosynthetic parameters recovered in the 26 °C, but not in the 28 °C treatment. It is concluded that Baltic F. vesiculosus populations may be severely affected even by short-term (8 days) exposure to high seawater temperatures when combined with the synergistic effects of low salinity predicted for the future Baltic Sea.

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