Barnacle settlement versus recruitment as indicators of larval delivery. II. Time-series analysis and hypothesized delivery mechanisms

Abstract

Larval delivery to benthic habitats can strongly influence populations and community structure. Censusing of individuals has been used to infer mechanisms of delivery with measurements made daily of settlement and weekly to monthly of recruitment. Conclusions from these studies are quite different: daily settlement studies find that delivery varies with the spring-to-neap tidal cycle, suggesting shoreward transport by the internal tides, while weekly to monthly recruitment studies find variations related to upwelling and downwelling with delivery due to upwelling relaxation. To resolve this dichotomy, the 2 types of observations were replicated simultaneously. 'Safety Walk' settlement plates were placed in the intertidal zone at Bastendorff Beach, Oregon. Plates were divided into quarters, which were censused daily, weekly, biweekly, and every 4 wk. Settlement was highly pulsed; pulses occurred between the neap and spring tide, with ∼74 % of the settlement variability attributable to variation in daily tidal range. Much smaller secondary pulses occurred at the onset of upwelling. The recruitment time series did not correlate with any physical variable, probably due to high cyprid mortality, density-dependent settlement, averaging of physical variables over the sample intervals, and aliasing. Replicating settlement and recruitment studies at the same place and over the same time frame demonstrated that time series of daily settlement provide detailed insight into the possible mechanism of delivery of cyprids to the shore, while recruitment time series did not. © Inter-Research 2009.