SEASONAL-CHANGES IN DIVERSITY AND ABUNDANCE OF OSTRACODS ON 4 SPECIESOF INTERTIDAL ALGAE WITH DIFFERING STRUCTURAL COMPLEXITY

Monthly replicate samples of 4 species of common rockpool algae (Ceram ium nodulosum, Chondrus crispus, Cladophora rupestris, and Corallina o fficinalis) were collected in order to investigate the changes in ostr acod species abundance, richness and diversity both between the algae species and over a 12 mo period. In order to determine if algal struct ural complexity had an effect on ostracod abundance and diversity, str uctural complexity was assessed by counting the number of branches cm( -1) frond length. There were significant differences in complexity bet ween the 4 algal species. Ceramium had significantly more branches cm( -1) than the remaining 3 algal species, and Cladophora had significant ly more branches than Chondrus and Corallina. The abundance and divers ity of ostracod assemblages demonstrated pronounced seasonal variation , both within and between the 4 algal species. Ceramium supported a hi gher abundance, greater species richness and greater diversity of ostr acods throughout the year than did Chondrus, which had a less complex structure. The Chondrus samples formed a separate cluster during class ification and ordination, reflecting the relative paucity of species a nd low densities found on that algal species throughout the year. A to tal of 18 species of ostracod were found on the algae, but only 8 occu rred at densities sufficient for determination of their life-cycles. C ythere lutea reproduced in early spring and demonstrated positive cova riation in population density with Hirschmannia viridis. H. viridis an d Heterocythereis albomaculata demonstrated positive covariation in po pulation density and had similar life-cycles. All 3 species were found at low densities during the winter months, possibly as the result of seasonal migration to sublittoral areas. The remaining 5 species all d emonstrated positive covariation in population densities, but reproduc ed at different times of the year. Semicytherura nigrescens and Xestol eberis aurantia underwent reproduction during summer/autumn. Both Para doxostoma variabile and P. ensiforme appeared to be reproductively act ive for most of the year, but P. hibernicum appeared to be an annual. All 8 species occurred in significantly higher densities on Ceramium t han on Chondrus, and during the peak reproductive period as compared t o other times. The seasonal changes in ostracod density within the roc kpool can be primarily attributed to the reproductive cycles of the co mponent species within the assemblage, and secondarily to the supposed seasonal migration of certain species between intertidal and sublitto ral regions.