TOPOGRAPHIC HETEROGENEITY, HYDRODYNAMICS, AND BENTHIC COMMUNITY STRUCTURE - A SCALE-DEPENDENT CASCADE

The influence of boulder size on flow velocity and rocky shore interti dal macrobenthic diversity and biomass was examined. A 3.6 m radius ar ea around boulders of different size categories (50-75, 75-100, and 20 0-250 cm) and control sites (no boulders) was divided into six 60 degr ees sectors and 4 distance classes away from boulders (0-30, 30-90, 90 -180, and 180-360 cm), forming 24 sampling cells for each boulder and control site. Flow velocity patterns near boulders were examined with Marsh-McBirney current meters and plaster cylinders immersed in the ce nter of each sampling cell. Sessile organisms in 4 randomly positioned 10 x 10 cm quadrats from each sampling cell were sampled. The blotted mass of each species collected was determined for each quadrat (n = 1 440). Downstream of a large boulder, mean flow velocity was half that upstream. This was also the only orientation where flow velocity never reached >15 cm s(-1). Similarly, velocity indices obtained from the e rosion of plaster cylinders were significantly lower downstream compar ed to upstream of boulders, and differences were greater for large bou lders. Invertebrate biomass decreased significantly downstream of boul ders. This effect increased with boulder size, and was significant for the large boulder category (200-250 cm). Algal biomass was not signif icantly influenced by boulders, though distribution patterns were anal ogous to invertebrate biomass. Diversity (H') peaked along the distanc e gradient around larger boulders, and generally increased with boulde r size. The positive correlation between biomass and the velocity inde x was stronger with increasing boulder size. The correlation between i nvertebrate biomass and the now velocity index was not significant at control sites, but 27.4 % of the variation in invertebrate biomass aro und large boulders could be explained by the velocity index. These res ults support the hypotheses that (1) hydrodynamics is a vector linking topographic heterogeneity and community structure, and (2) this casca de (topographic heterogeneity --> hydrodynamics --> community structur e) is scale-dependent.