Intertidal rockweed beds are often described as climate refuges for species, buffering harsh environmental conditions. Due to problems such as ocean warming, ocean acidification, eutrophication and trophic imbalance, many areas are experiencing blooms of opportunistic filamentous algae, but little is known about how this shift affects the microclimatic conditions in canopy building seaweeds. Here we report some interesting findings from our ad hoc study on rockweeds (Fucus vesiculosus) being covered by filamentous algae during a hot spell. We found significantly higher temperature in filamentous algae aggregations than within rockweed patches. Also, the temperature in the filamentous algae was higher (up to 3.2 °C) than the surrounding water masses, a difference that was not found for the rockweed patches. Our results indicate that filamentous algae aggregations are trapping heat, which indicates that these algae can hamper the intertidal rockweed bed's role as a climate refuge for species when they occur in dense aggregations.
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