AVENI-DEFORGE, K*; WETHEY, D.S.: Physical constraints on zonation patterns of the barnacle _Chthamalus fragilis_
Zonation patterns in the rocky intertidal are linked to microclimates. The upper distribution limits of many species in the intertidal are determined by intolerance to the microclimate conditions rather than absolute tidal height. The barnacle Chthamalus fragilis is common to the rock jetties of South Carolina, where its upper distribution limit changes as a function of slope and azimuth of its substrate. Small, temperature data loggers were glued to the rocks at the upper distribution limits of Chthamalus aggregations, and 10cm beneath the upper limit, amid dense populations. Temperature was recorded at 15 minute intervals for sites with a variety of slopes and azimuths. Adult survival was determined by comparing daily photographs taken of the regions surrounding the data loggers. Larval recruitment and survival was measured on rocks that had been cleared of adults by comparing daily photographs of permanent transects adjacent to temperature loggers. Temperatures as high as 53oC were observed at rock surfaces just above the upper distribution limit of Chthamalus several times over the course of the summer. Data loggers within the barnacle zones commonly recorded temperatures 5oC lower than those at the upper distribution limits. Mortality and growth of early recruits varied daily as a function of recent exposure to high temperature conditions. The mortality of adults was strongly affected by the most extreme rock temperatures, and repeated exposure to relative extreme temperatures.
