It's a patchy world: spatial variation in thermal amelioration by the habitat-forming seaweed Hormosira banksii

Climate warming is exposing intertidal species to increasing temperatures, threatening the survival of those already experiencing conditions close to their upper thermal limit. On intertidal rocky shores, habitat-forming seaweeds can protect associated species from low tide atmospheric temperature extremes, but the efficacy of this temperature amelioration may depend on patch size and environmental setting. We assessed using a combination of loggers and drone surveys how geographic location (latitude) and patch size influences low tide atmospheric temperature amelioration by the habitat-forming seaweed Hormosira banksii. Loggers placed in bare and Hormosira patches at nine sites, across three regions (North, Central, and South Coast, New South Wales) spanning 750 km of the east Australian coast found that, on average, summertime maximum temperatures were 11 °C cooler under the Hormosira canopy than on bare rock. Effects of the alga, however, did not increase with decreasing latitude and were instead greatest at the most southerly (higher latitude) sites. A drone survey conducted at one of the nine sites during a summertime low tide showed that the 90th percentile surface temperature decreased by on average 2.3 °C as patch size increased from 0.25 to 1.25 m². However, a manipulative experiment found no difference in maximum sub-canopy temperature between smaller patches of 0.0625 and 0.25 m². These results highlight the potential for Hormosira patches to mitigate thermal stress to organisms living within or beneath the alga, across a range of environmental conditions. The efficacy of this refuge, however, may be diminished by habitat fragmentation and patch size reduction.