Spatial variation in size and density of adult and post-settlement Semibalanus balanoides: effects of oceanographic and local conditions

Regional oceanographic processes are emerging as strong influences on growth and recruitment of intertidal species, with important consequences for populations. Yet local conditions such as wave exposure are also important. To disentangle these effects for the intertidal barnacle Semibalanus balanoides, we surveyed 259 sites around Scotland in July and August over 6 yr (from 2001 to 2006). Scaled digital photographs at 3 shore levels gave sizes and densities of juvenile and adult barnacles for comparison with wave fetch, remotely sensed chlorophyll a (chl a) concentration and seasonal sea surface temperature (SST). Patterns were also compared with site ‘openness’: the area of connected open sea <30 km away. Patterns at the 3 shore levels were similar. Hierarchical partitioning (HP) showed that survey year had the biggest effect and improved the predictive power of other variables: wave fetch for adult and juvenile densities, chl a for juvenile size and openness for adult size. SST had little effect. Regression models selected using information theoretic measures included positive effects of chl a, varying among surveys, on average size of barnacles (R2 from 0.5 to 0.6), and larger high shore barnacles in greater wave exposure. Population densities of adults and juveniles increased with wave fetch, with chl a only influencing density at high shore levels (R2 from 0.1 to 0.4). Despite temporal and spatial variation in responses to chl a and wave fetch among surveys, relationships were consistent with growth and size in S. balanoides being limited by food supply, and increased recruitment and adult densities in increased wave exposure. Large-scale ecological patterns in this rocky intertidal species thus result from large-scale oceanographic effects on food concentration with habitat-scale wave-mediated effects on supply of food and larvae.