Understanding the structure and richness of natural communities is a fundamental goal of marine ecology, and foundation species such as large macroalgae have a disproportionate role in structuring biodiversity. However, high-resolution information on assemblages associated with macroalgae is lacking for many species and regions. Saccorhiza polyschides is a warm-temperate kelp with a relatively short lifespan (12–18 months), large thallus and bulbous holdfast offering habitat for diverse assemblages. In the UK, S. polyschides populations are thought to have proliferated recently. Here, we quantified the density and habitat structure provided by S. polyschides along a gradient of wave exposure within Plymouth Sound, and examined the composition and diversity of associated faunal assemblages. Density varied significantly between sites but not by wave exposure, while biometric measurements were generally highly variable. Senescing holdfasts from sporophytes offered valuable habitat, with high abundance and richness of associated assemblages, although these varied markedly between sporophytes and sites. Faunal abundance, taxon richness and diversity were significantly higher at fully exposed sites than at moderately exposed sites. Internal volume of holdfasts was positively correlated with faunal abundance and taxon richness. We recorded more than 27 distinct taxa and up to ~600 individuals within a single holdfast. Taxa included three fish species, including a novel observation of the pipefish Nerophis lumbriciformis. Further work is needed to examine seasonality in habitat structure and associated diversity patterns but our study demonstrates that even remnant holdfasts from decaying sporophytes represent a valuable microhabitat that may provide shelter, protection and food during winter.

Canopy-forming fucoid algae have an important role as ecosystem engineers on rocky intertidal shores, where they increase the abundance of species otherwise limited by exposure during low tide. The facilitative relationship between Ascophyllum nodosum and associated organisms was explored using a frond breakage experiment (100%, 50%, 25%, 0% intact-frond treatments) in southern England, to assess the consequences of disturbance. Understorey substratum temperature was on average 3°C higher in 0% and 25% intact-frond treatments than in plots with 50% and 100% intact fronds. Light (as PAR during low tide) doubled in 0% intact-frond treatments in comparison to other treatments (which had similar light levels). Mobile invertebrate species richness declined by on average 1 species per m2 in the treatments with only 25% and 0% intact fronds, and the abundance of Littorina obtusata declined by 2.4–4.2 individuals per m2 in the treatments with 25 and 0% intact fronds. Sessile taxa, including Osmundea pinnatifida and encrusting coralline algae, declined by half on average in the 0% intact-frond treatment. These results suggest that the ability of Ascophyllum to mediate environmental conditions to the understorey is the mechanism responsible for species distributed in the understorey (autogenic ecosystem engineering). The results of this study imply that a pulse disturbance resulting in a 50% breakage of Ascophyllum fronds significantly increases temperature and decreases the abundance of mobile invertebrates usually associated with Ascophyllum. Sessile taxa associated with Ascophyllum can, however, withstand disturbances down to 25% intact Ascophyllum fronds.