Habitat heterogeneity and faunal assemblages at Kolbeinsey and Grimsey hydrothermal vent fields of northern Iceland

Animal communities at shallow and upper-slope hydrothermal vents are shaped by vent-fluid conditions, geological setting, substrate composition, and the surrounding shelf ecosystems. Here, we compare two contrasting vent fields north of Iceland: the Kolbeinsey vent field, a shallow shelf system at approximately 100 m water depth dominated by pillow lava and cobble with localized high-temperature venting on top of the ridge, and the Grimsey vent field, an upper-slope, sediment-hosted system at approximately 400 m water depth characterized by active anhydrite chimneys and hot sediments. Using image-based megafaunal surveys and physical macrofaunal sampling, we show that fine-scale habitat variation is associated with distinct community patterns within and between vent fields. At Kolbeinsey, hydrothermal activity was spatially limited and was visible mainly as microbial mats covering hard substrates and small sediment patches, embedded within extensive hard-substrate habitats. These surrounding hard substrates supported abundant megafaunal assemblages dominated by widespread shelf-associated suspension feeders. At Grimsey, microbial mats on hard substrate, typically chimneys, hosted few visible megafauna (0.4 individuals per image), whereas physical samples revealed macrofaunal communities in vent-influenced sediments and hard substrates that were not apparent from imagery alone. Some anemones in the periphery of the Grimsey field were covered by white filamentous material of putatively microbial origin. Across both vent fields, image-based and physical sampling revealed complementary aspects of biodiversity, demonstrating that visually sparse habitats can host small-bodied and sediment-associated fauna. Community composition differed among habitats, while regional differences between Kolbeinsey and Grimsey likely reflect variations in depth, geology, substrate availability, and vent-fluid influence. These findings provide an important biodiversity baseline and highlight the fine-scale habitat heterogeneity within vent fields, supporting vent-field-specific conservation measures for these rare and spatially restricted ecosystems.