Holopelagic Sargassum aggregations provide warmer microhabitats for associated fauna

<p><span>Drifting aggregations of <em>Sargassum</em> algae provide critical habitat for endemic, endangered, and commercially important species. They may also provide favorable microclimates for associated fauna. To quantify thermal characteristics of holopelagic <em>Sargassum</em> aggregations, we evaluated thermal profiles of 50 aggregations in situ in the Sargasso Sea. </span><span>Sea surface temperature (SST) in the center of </span><span>aggregations</span><span> was significantly higher than in nearby open water, and SST differential was independent of </span><span>aggregation</span><span> volume, area, and thickness. SST differential between </span><span>aggregation</span><span> edge and open water was smaller than those between </span><span>aggregation</span><span> center and </span><span>aggregation</span><span> edge and between </span><span>aggregation</span><span> center and open water. Water temperature was significantly higher inside and below </span><span>aggregations</span><span> compared to open water but did not vary inside </span><span>aggregations</span><span> with depth. </span><span>Holopelagic </span><em><span>Sargassum</span></em><span> aggregations provide warmer microhabitats for associated fauna, which may benefit marine ectotherms, though temperature differentials were narrow (up to 0.7 °C) over the range of </span><span>aggregation</span><span> sizes we encountered (area 0.01–15 m²). </span><span>We propose a hypothetical curve describing variation in SST differential with <em>Sargassum</em> aggregation size as a prediction for future studies to evaluate across temporal and geographic ranges. Our study provides a foundation for investigating the importance of thermal microhabitats in holopelagic <em>Sargassum</em> ecosystems</span><span>.</span></p>