Barnacle settlement: field experiments on the influence of larval supply, tidal level, biofilm quality and age on Balanus amphitrite cyprids

A set of 3 field experiments lasting 24 h was conducted during April 1998 a t the Duke University Marine Laboratory (Beaufort, North Carolina, USA) to: (1) assess the influence of larval supply, intertidal height, quantity and quality of biofilm and age of the larvae on the settlement of Balanus amph itrite Darwin and (2) examine the correspondence between small-scale plankt onic distribution of larvae, the initial spatial pattern of newly settled l arvae and the vertical distribution of adult barnacles. Precolonized methac rylate (Plexiglas) disks, arranged within 3 blocks and established so as to eliminate edge effects within 3 large experimental panels, were placed at 3 predetermined tidal heights (High, Medium, Low) corresponding to the uppe r Limit, modal zone and the lower limit of adults of B. amphitn'te. Split-s plit-plot ANOVAs were performed on densities of newly attached larvae (meta morphosis not completed) to test their habitat selection behavior to surfac es which had been precolonized by microbiota (bacteria and diatoms) at 3 he ights (origin factor) for 0, 7, 14 or 21 d (age factor). The physical envir onment (salinity, temperature, current flow) was stable and comparable duri ng the 3 experiments. B. amphitrite cyprids were uniformly distributed in t he water column. Larval supply was poorly correlated with the intensity of settlement over the 1 wk experimental period. In fact, the same larval supp ly could induce either high (4 x) or low (Ix) settlement after 2 tidal cycl es, and, inversely, similar settlement intensities were associated with pla nktonic larval abundance varying significantly at 2 d intervals (109 to 171 cyprids 923 l(-1)). Settlement was homogeneous on each experimental unit ( no significant block effect). Tidal height, however, was a significant fact or in determining the vertical patterns of newly settled larvae during the first experiment where larvae were abundant but not during subsequent exper iments for which fewer larvae were collected. The degree of microbial preco lonization was the main parameter affecting the settlement of B, amphitrite . For the first 2 experiments, 'weighed cyprid settlement' significantly de creased as the age of the biofilm increased, revealing a strong preference of settlers for clean surfaces and avoidance of biofouled surfaces of all i ntertidal origins. Further analysis of biofilm samples showed that free-spa ce availability in the microbial film and bacterial densities were signific antly inversely correlated to settlement intensity. Moreover, settlement to 'favorable' substrata decreased by nearly 1/2 during our experimental peri od, suggesting changes in the selectivity of settling larvae. Our experimen ts confirm the role of larval supply in determining the vertical intertidal distribution of adults of B, amphitn'te, but the short-term variability in the larval supply/settlement coupling observed over a 1 wk period muse be integrated in models of recruitment dynamics of barnacles.