Influences of wave fetch, tidal flow and ocean colour on subtidal rocky communities

Community assemblages on subtidal rock change markedly along gradients of wave energy, tidal flow, and turbidity. The importance of these assemblages for rare and delicate species, for shellfish, as nursery areas for fish, and for their contribution to ecosystem functioning in coastal areas has prompted much conservation effort in many countries. I applied a rapid method of calculating a large high-resolution (200 m scale) map of wave exposure < 5 km from the UK coastline to compare with UK subtidal biodiversity records from diver surveys from the 1970s to the 2000s. Satellite-derived estimates of ocean colour, and tidal flows from hydrodynamic models were also extracted for each site. Ordinal logistic regression of categorical abundance data gave species-distribution patterns with wave fetch and depth and dependence on chlorophyll and tidal flows: macroalgae declined with increasing chlorophyll and increased with tidal flow. Multivariate community analysis showed shifts from algae to suspension-feeding animals with increasing depth and in areas of high chlorophyll and tidal flow and a change from delicate forms in wave-shelter to robust species at wave-exposed sites. The strongest positive influence on species diversity was found to be the presence of the kelp Laminaria hyperborea: sites with 0% cover had a median of 6 species, while those with > 40% cover had a median of 22 species. Laminaria hyperborea, and the most diverse communities, is found in areas of estimated low chlorophyll concentrations and in the most wave-exposed environments, which are often but not always in areas of high tidal flow.