The structure and diversity of macroinvertebrate assemblages associated with the understudied pseudo-kelp Saccorhiza polyschides in the Western English Channel (UK)

We examined spatiotemporal variability in the structure of faunal assemblages associated with the warm-temperate pseudo-kelp Saccorhiza polyschides towards its range centre (Western English Channel, southwest UK), to better understand its role as a habitat-former in the northeast Atlantic. A total of 180 sporophytes and their associated fauna were sampled across three months, three sites, and two depths. Assemblage abundance and biomass varied markedly between three morphological sporophyte components (i.e., holdfast, stipe, blade). We recorded rich and abundant macroinvertebrate assemblages, comprising nine phyla, 28 coarse taxonomic groups, and 57 species of molluscs, which consistently dominated assemblages. We observed pronounced seasonality in faunal assemblage structure, marked variability between sites and depths, and strong positive relationships between biogenic habitat availability and faunal abundance/biomass. S. polyschides sporophytes are short-lived and offer temporary, less-stable habitat compared with dominant perennial Laminaria species, so shifts in the relative abundances of habitat-formers will likely alter local biodiversity patterns.