Influence of recruitment and temperature on distribution of intertidal barnacles in the English Channel

Many warm-water (Lusitanian) species reach their limits in the central English Channel, failing to penetrate to the North Sea. We re-surveyed the eastern limits of the Lusitanian intertidal barnacles Chthamalus montagui and C. stellatus, from 1994 to 2004, a decade of exceptionally high sea temperatures, and found range extensions on both sides of the Channel compared to the 1950s and 1970s. Annual recruitment of Chthamalus on the English coast was monitored. There was a consistent gradient of low recruitment to the east of Portland Bill, with significant reductions coinciding with prominent headlands. Highest recruitment occurred during the warmest years. Cluster analysis showed a high degree of similarity of annual recruitment within coastal cells suggesting that local processes are also important. In 1999 we compared recruitment in the other common intertidal barnacles, the boreal Semibalanus balanoides and the non-native Elminius modestus, with Chthamalus spp. All species showed low recruitment between Selsey Bill and Portland Bill, suggesting habitat limitations and/or hydrographic mechanisms. Annual recruitment of Chthamalus at existing limits on the Isle of Wight was positively correlated with the number of days of westerly and south-westerly winds during the summer, coinciding with the pelagic larval phase. A ‘pulse’ of high Chthamalus recruitment on the Isle of Wight, measured during the summer of 2000, reversed population decline. Only a higher frequency of such pulses will maintain populations at existing limits and increase the rate of range extension towards the North Sea. Such extension will be limited by lack of hard substrata, but proliferation of coastal defence schemes in recent years is increasing suitable habitat for barnacles.