Free-Space Availability and Larval Substratum Selection as Determinants of Barnacle Population-Structure in a Developing Rocky Intertidal Community

Abstract

Variation in settlement and recruitment of the barnacle Semibalanus balanoides (L.) was experimentally investigated in the high, mid and low intertidal zones at 2 adjacent sites (A and B) on a rocky shore in Nova Scotia, Canada, following a rare occurrence of ice-scouring. Manipulations in 100 cm2 quadrats at each intertidal height involved removal of various components of the sessile macrofauna and macroflora, removal of the total community and a control treatment. The availability of free space on the substratum in each quadrat was measured prior to the onset of settlement to determine whether differences in the density of barnacles among treatments were due to differences in the availability of free space or to manipulation. Where the density of settlers was high, it was positively related to the availability of free space suggesting that settlement was a simple function of the availability of free space. However, the pattern of settlement varied from the onset to the end of settlement: quadrats from which only barnacles had been removed were occupied early in the settlement period and later-arriving larvae were restricted to less favourable sites where free space was available. Thus, settlement preferences may be masked when the supply of larvae is saturating or the duration of the selction experiment is too long. Where the density of settlers was low and free space was non-limiting, there was no relationship between the density of settlers and the availability of free space. In the high intertidal zone at Site A, the density of settlers was greater in treatments with ephemeral algae (wetter quadrats) than in those without (drier quadrats). In general, early post-settlement mortality in treatments where algae had been removed increased with intertidal height, whereas in treatments where algae were present it remained relatively constant among heights. Post-recruitment mortality did not differ significantly among treatments in the high intertidal zone, suggesting that factors which influence selection of the substratum by cyprid larvae and promote early post-settlement survival may be particularly important in determining subsequent population structure of barnacles in this zone. Post-recruitment mortality, mainly due to predation by whelks, was highest in the low intertidal zone at both sites and did not differ significantly between treatments. In the mid intertidal zone at Site B, whelk foraging appeared to be constrained by desiccation stress and post-recruitment mortality was highest in treatments with Fucus spp. Where predation is intense, initial selection of the substratum by cyprid larvae may have little effect on the subsequent population structure of barnacles.