Several biological and physical factors change the rocky shore com-munities. The desiccation time and the tolerance of the intertidal species produce the vertical zonation. In many studies around the world, a temporal change in this zonation is presented. In Costa Rica, only studies that include temporal trends were carried out in Punta Mala and Montezuma, Pacific coast in 80`s. The rocky intertidal of the Cocos Island National Park, Costa Rica were surveyed photographically. The Chatham bay was sampled in three expeditions (January 2007, October 2007 and April 2008). Photos corresponding to 25x25cm quadrats were taken with the goal to determine diversity and composition differences in rocky shore organisms between sampling dates. The Wafer bay was sampled in January and October 2007. The intertidal of Chatham consists of basaltic rock, while Wafer has basaltic and ignimbrite boulders. The main difference between sites were the higher algae cover (erect-frondose forms) and number of organism bands at Chatham bay. Temporal change was not found in the total cover of sessile fauna and autotrophs. The barnacle Tetraclita stalactifera, that occurs above the algal fringe (lower intertidal), was the invertebrate with the highest coverage. The mobile fauna bio-diversity presented no significant trend between sampled months. However, the identity of species, their cover and their abundance showed a moderate temporal change. In October 2007, when the sea surface temperature was 23ºC the infralittoral zone had an increase in green algae cover. The red algae (crust and erect-frondose forms) were dominant in January and April. The pulmonate limpet, Siphonaria gigas and a bacterial biofilm at mid littoral showed a negative association. The snails of the high littoral and the supralittoral zone showed a temporal change in their abundance, but with contrasting patterns between sites. The temporal variation in the assemblages increased from the supralittoral to the infralittoral possibly due to changes in the water temperature and climatic conditions, that could influence the intertidal zone during the high and low tide, respectively.
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