The intertidal rocky shore of Bahía Salinas (seasonal upwelling area), northern Pacific coast of Costa Rica, was studied to determine the vertical zonation of algae and invertebrates. In two sheltered and two exposed sites the communities were sampled twice, once during the upwelling (April) and again during the nonupwelling season (October). At each site five strata were sampled with 25x25cm quadrats. Each stratum had five quadrats and digital photographs of each quadrat were taken. Digital image analysis was used to estimate the percent cover of sessile species and abundance of mobile fauna. A typical vertical zonation was found using Principal Component Analysis (PCA) biplot and Multiple Discriminant Analysis (MDA). The analysis of similarities (ANOSIM) detected a seasonal change in the biological composition of the inferior strata. This change in the percent cover of the algal assemblage and their herbivores were related to the seasonal upwelling. Sessile invertebrates had a constant percent cover between April and October, but the difference between exposed and sheltered sites was significant. Invertebrates showed a patchy structure in sheltered sites while in exposed sites organisms formed horizontal bands. Interactions between species indicated that predation and space monopolization were possible causes of lower cover of barnacles in sheltered sites and lower abundance of mollusks in the exposed area. Regional difference of intensity in the upwelling event and larval recruitment may explain the high cover of sessile invertebrates of Bahía Salinas in comparison to the Bay of Panama, another seasonal upwelling system. Rev. Biol. Trop. 56 (Suppl. 4): 91-104. Epub 2009 June 30.
Keywords: tropical, rocky intertidal, zonation, upwelling, seasonality, digital image analysis, PCA, MDA, eastern Pacific, Costa Rica