Spatiotemporal Variability in Subarctic Lithothamnion glaciale Rhodolith Bed Structural Complexity and Macrofaunal Diversity

Rhodoliths are non-geniculate, free-living coralline red algae that can accumulate on the seafloor and form structurally complex benthic habitats supporting diverse communities known as rhodolith beds. We combined in situ rhodolith collections and imagery to quantify variability, over 9 months and at two sites, in the structural complexity and biodiversity of a subarctic Lithothamnion glaciale rhodolith bed. We show that the unconsolidated rhodolith framework is spatially heterogeneous, yet provides a temporally stable habitat to an abundant and highly diverse macrofauna encompassing 108 taxa dominated by brittle stars, chitons, bivalves, gastropods, polychaetes, sea urchins, and sea stars. Specific habitat components, including large bivalve shells, affect rhodolith morphology and resident macrofauna, with increasingly large, non-nucleated rhodoliths hosting higher macrofaunal density, biomass, and diversity than increasingly large, shell-nucleated rhodoliths. The present study's fine taxonomic resolution results strongly support the notion that rhodolith beds are biodiversity hotspots. Their spatial and temporal domains provide clear quantitative evidence that rhodolith beds provide a stable framework under the main influence of biological forcing until sporadic and unusually intense physical forcing reworks it. Our findings suggest that shallow (<20 m depth) rhodolith beds are vulnerable to ongoing and predicted increases in the frequency and severity of wave storms.