Scales of coastal heterogeneity: influence on intertidal community structure

The effect of 3 spatial heterogeneity scales on benthic community species richness, diversity, total biomass, biomass of dominant taxa (Fucaceae, littorinids and Mytilus edulis) and M. edulis recruitment was investigated in the intertidal zone over a 90 km portion of the south shore of the St. Lawrence Estuary, Quebec, Canada. Three scales of coastline heterogeneity and cascade (interaction) of these scales were examined using a nested and crossed design with replication. Large-scale heterogeneity (LSH) was determined with a shore heterogeneity index (SHI) which was the ratio of the length of the coastline, measured on a 1:20 000 map, over the 1 km Linear distance between end points. The SHI were classified as low, medium, and high. Within each of the 3 categories of LSH, 2 levels of medium-scale heterogeneity (MSH) were examined: linear coasts (100 m linear portions of the shore) and headlands (mean length 52 +/- 5 m). Within each of the MSH sections, small-scale heterogeneity (SSH) was investigated by sampling twenty 10 x 10 cm quadrats at positions upstream, apex and downstream. SSH was represented as 2 types of rock surfaces, flat exposed surfaces and crevices (n = 540). LSH did not influence the community characteristics considered, MSH had only a minor influence on them but SSH had a major influence on them. LSH explained 1% or less of biomass variance of taxa encountered; MSH explained at most 11% of biomass of littorinids; SSH explained 30% of variance in total biomass, 28% of biomass of M, edulis, and 21% of biomass of Fucaceae and littorinids. We also observed that some scale combinations affected the diversity and biomass of littorinids.