SPATIAL-DISTRIBUTION OF ALGAE AND INVERTEBRATES IN THE ROCKY INTERTIDAL ZONE OF THE STRAIT OF MAGELLAN - ARE PATTERNS GENERAL

Spatial variability in the distribution of macroalgae and invertebrate s was examined at a number of sites in the Strait of Magellan. Two mai n predictions of models of zonation were tested in this study: (1) con sistency in patterns of vertical distribution among sites one to tens of kilometers apart, and (2) homogeneity between areas at the same lev el on the shore, tens to hundreds of meters apart. Two types of habita t were considered: continuous rocky shores and blocks of rock in grave l beaches. In the former habitat, percent cover estimates of sessile o rganisms (algae and mussels) were obtained from three replicated plots (0.5 x 0.5 m) sampled non-destructively in each of three random areas (stretches of shore about 20 m long) at each of three levels on the s hore: high, middle and low intertidal zone. This procedure was repeate d at six different sites along the Chilean coast of the Strait of Mage llan. Blocks of rock were sampled at two sites about 1 km apart. Estim ates of the abundance of sessile (barnacles and mussels) and mobile (l impets) organisms were obtained for six blocks at each of two levels o n the shore (high and low intertidal zone) at each site. At each level on the shore, three blocks were sampled on the top and three on the v ertical sides. A single plot was sampled on each block. This design al lowed a test of the null hypothesis of no interactive effects between position on the blocks and level on the shore. Both the predictions we re falsified. (1) there were large between-site differences in the ver tical structure of assemblages and (2) variability between areas at th e same level on the shore was large for some of the most common algae. In contrast, the null hypothesis of no interactive effects of positio n on the blocks and level on the shore was retained. The results of th is study show that vertical position on the shore alone is not a good predictor of the structure of assemblages of benthic organisms in the rocky intertidal of the Strait of Magellan.