The vertical zonation of epifauna and algae on rocky substrates of the Gulf of Nicoya, Costa Rica. The vertical distribution of epifaunal and algal species was studied at five intertidal rocky shore sites on the Gulf de Nicoya estuary, Pacific coast of Costa Rica. Each site was visited once during low tide, between April and July 2004. Two sampling sites were located on each side of the mouth of the estuary (Montezuma and Punta Agujas), one in the middle zone (Caldera), and two in the inner zone (Cocorocas, and Yuca island), the latter close to the mouth of the Tempisque river. Sites were selected to measure variation in the vertical distribution of intertidal organisms in relation to the estuarine gradient. Nine horizontal strata were sampled at each locality. Each stratum had 5 quadrats of 25 x 25 cm. Percent live coverage on the rock surface was estimated by digital image analysis. Classification (cluster analysis) and ordination (multidimensional scaling, and principal component analysis) techniques were applied. A total of 42 species was found: 16 mollusks, 10 algae, eight barnacles, three anemones, two polychaetes, two crustaceans, and one sponge. The vertical zonation is caused by exposure during low tide. The high, mid and low intertidal belts were dominated by the snails Nodilittorina aspera, and N. modesta (high); the barnacle, Chthamalus panamensis (mid), and an algal assemblage (low). At the Agujas site the infralittoral zone was bare rock. Multiple Discriminant Analysis (MDA) was also applied and showed that the supralittoral zone had a high similarity in percent live cover and identity with other localities. The mid section had moderate differences in barnacle cover. The coverage of C. panamensis was high in Caldera and Cocorocas, and decreased abruptly at Yuca island, where Balanus inexpectatus was important in the infralittoral zone. The species diversity increased from the high intertidal zone to the lowest zones. Different species were found on the infralittoral zones of the localities. The possible main causes of this spatial trend are the low salinity, high turbidity, and sedimentation of the estuarine inner zone (Yuca island). Tidal mixing and river discharges, increased the density of suspended matter, and this might explain the high coverage of suspension feeders (Cocorocas and Caldera). The combination of strong wave force and rock type may cause the presence (Montezuma) or absence (Agujas) of certain species. Rev. Biol. Trop. 54 (Suppl. 1): 49-67. Epub 2006 Sept. 30.
Keywords: epifauna, zonation, rocky intertidal, estuary, Golfo de Nicoya, digital image analysis, multivariate statistics