Could seasonal upwelling act as a refugium for Ulva lactuca (Ulvales, Chlorophyta) in a warming scenario?

Marine heat waves can rapidly alter the distribution and abundance of marine populations. Coastal upwellings could act as refugia during periods of global warming and populations adapted to highly variable climate conditions, such as those found in coastal upwellings, might respond better to rapid changes in climate. This study analyzed the effects of temperature, marine heat waves, and recovery on the growth and photosynthesis of Ulva lactuca from two thermally distinct Brazilian populations: a cold site population influenced by coastal upwelling (Arraial do Cabo, AC), and a warmer site population, not influenced by upwelling (Niterói, NI). Our experiments comprised three consecutive phases: 15 days in a gradient of constant temperature (16, 21, 26 and 31 °C); followed by a 5 day heat wave simulation (+ 4 °C); followed by a 5 day recovery period (at pre-heat wave temperatures). Our results showed minor differences between populations, suggesting no ecological adaptation. Neither population tolerated temperature increase; thus, the upwelling region as a microclimate might not act as a refugium for U. lactuca under a warmer ocean or during marine heat waves. In conclusion, physiological differences between AC and NI do not suggest the existence of ecotypes, and future marine heat waves should affect these populations similarly.