Role of settlement in determining the distribution and abundance of barnacles in a temperate mangrove forest

An increased understanding of the role of early life history stages in dete rmining the distributions of benthic invertebrates require studies covering a wide range of habitats, and preferably multiple scales. For one of the m ore prominent invertebrate groups, intertidal barnacles, much of our existi ng knowledge is drawn from studies of rocky shores, with few studies from o ther habitats. Here, we describe the distribution of adults of the temperat e barnacle Elminius covertus in mangrove (Avicennia marina) forests of sout heastern Australia. We sampled E. covertus on pneumatophores over a large-s cale (50-100 m) across the forests from the landward to seaward edges. We a lso described large scale patterns along the shore (also a scale of 100 m). At the seaward edge of the forest, on a small spatial scale, the vertical distribution of E. covertus on pneumatophores over 15 cm was also investiga ted. Settlement and recruitment were also estimated by allowing barnacles t o settle on artificial substrata. The processes influencing the patterns of adults of E. covertus were very different over the two major scales in thi s study. At the large-scale, adult Elminius covertus were abundant at the s eaward edges of forests, declining through the forest, and were absent at t he landward sections of forests. Recruitment, measured over 1 month, and 1 week, matched this pattern, and settlement, measured over a single high tid e, also matched the adult distributions. Along the shore, differences in ab undance of adult barnacles were matched by variation in settlement and recr uitment. Post-settlement mortality had little influence on this pattern. In contrast, on the small, vertical scale, barnacles were most abundant on th e upper 5 cm of pneumatophores, and least common near the substratum, This vertical pattern reflected the pattern of recruitment after 1 month. Settle ment, however, was different, with more settlement on the 5 cm of pneumatop hores closest to the substratum. The distribution of recruits after 1 week was intermediate between settlement and recruitment after 1 month. The vert ical pattern of adults is therefore determined by post-settlement mortality occurring during the first month after settlement. (C) 1999 Elsevier Scien ce B.V. All rights reserved.