MEPS 171:59-70 (1998)  -  doi:10.3354/meps171059

Topographic heterogeneity, hydrodynamics, and benthic community structure: a scale-dependent cascade

Frédéric Guichard*, Edwin Bourget

GIROQ, Département de Biologie, Université Laval, Québec, Québec G1K 7P4, Canada

ABSTRACT: The influence of boulder size on flow velocity and rocky shore intertidal macrobenthic diversity and biomass was examined. A 3.6 m radius area around boulders of different size categories (50-75, 75-100, and 200-250 cm) and control sites (no boulders) was divided into six 60° 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 center 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 = 1440). 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 erosion of plaster cylinders were significantly lower downstream compared to upstream of boulders, and differences were greater for large boulders. Invertebrate biomass decreased significantly downstream of boulders. This effect increased with boulder size, and was significant for the large boulder category (200-250 cm). Algal biomass was not significantly influenced by boulders, though distribution patterns were analogous to invertebrate biomass. Diversity (H') peaked along the distance gradient around larger boulders, and generally increased with boulder size. The positive correlation between biomass and the velocity index was stronger with increasing boulder size. The correlation between invertebrate biomass and the flow velocity index was not significant at control sites, but 27.4% of the variation in invertebrate biomass around large boulders could be explained by the velocity index. These results support the hypotheses that (1) hydrodynamics is a vector linking topographic heterogeneity and community structure, and (2) this cascade (topographic heterogeneity - hydrodynamics - community structure) is scale-dependent.

KEY WORDS: Topographic heterogeneity · Spatial scales · Benthos · Hydrodynamics · Community structure

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