European-scale analysis of seasonal variability in limpet grazing activity and microalgal abundance

The film of microalgae and macroalgal propagules which coats intertidal rocks is the main food resource of limpets, the dominant grazers on exposed shores of north-west Europe. Spatial and temporal variability in feeding activity of limpets and abundance of microalgae were examined at mid-tide level across a European gradient from the Isle of Man in the British Isles to south-west Portugal. Feeding activity was assessed as the frequency of radula scrapes on wax surfaces placed on the shore. This was undertaken monthly at 2 shores at each of 4 locations, the Isle of Man, south-west England, northern Spain and south-west Portugal, over 1 yr. The abundance of the microalgal film was determined simultaneously at 3 of the 4 locations by measuring the concentration of chlorophyll a on the rock surface. The density and species diversity of limpets increased with decreasing latitude. This was mirrored by a trend of increased levels of limpet grazing, although 1 location (northern Spain) did not fit this trend. Seasonal changes in limpet grazing intensity were found at 3 of the 4 locations, characterised by elevated grazing during the summer and autumn. The seasonal variations resulted from changes in the level of foraging of individual limpets and were positively correlated with mean sea temperature in the Isle of Man and south-west England. The pattern of grazing activity was not solely a function of sea temperature. In the Isle of Man, grazing by Patella vulgata declined during the later stages of gonad development and increased following spawning. Seasonal variation in the standing stock of microalgae, measured as the concentration of chlorophyll a, was found at all 3 locations examined, the Isle of Man, south-west England and northern Spain, with greater abundance in the winter compared to summer. This pattern was consistent over both large (among locations: 100s of kilometres) and medium (between shores: 1000s of metres) scales. There were no correlations between microalgal abundance and limpet grazing activity at any location. There were significant negative correlations between chlorophyll a and maximum monthly air temperature and monthly sunshine hours. Comparison of microalgal abundance among locations showed a general decline in standing stock with decreasing latitude, but differences between specific locations varied with season. South-west England showed similar levels of microalgal abundance to the most northerly location (Isle of Man) in winter, and to the most southerly location (northern Spain) in the summer.