Patterns in rocky intertidal community structure on the northern and eastern shores of Nova Scotia

This thesis examines the rocky intertidal community structure on the northern and eastern shores of Nova Scotia. I tested the predictions of an environmental stress model of community organization whereby species richness and diversity depend primarily on the level of environmental stress. In rocky intertidal habitats, there is a vertical gradient in intertidal elevation and horizontal gradients in wave exposure and, for polar and subpolar habitats, ice scour. I presumed that the highest stress occurs at the high elevation and in the wave/ice exposed area. Furthermore, I presumed that the rocky intertidal habitats in Nova Scotia, having harsh winters relative to other cold-temperate shores, are at a higher range of environmental stress; thus, I concentrated on a portion of the model, which predicts that richness and diversity are low at high levels of stress, and increase with decreasing levels of stress.;I investigated the spatial patterns in the abundance of seaweeds, Ascophyllum nodosum, Chondrus crispus, and Fucus spp., the barnacle Semibalanus balanoides, mussels Mytilus sp., snails Littorina spp., and the dogwhelk Nucella lapillus across the elevation and exposure gradients. I included those species that are large contributors to structuring the community on the rocky shores at Sea Spray and Tor Bay, and I also included species that play prominent roles in the rocky intertidal community in New England. Overall, I found that the abundances of these species generally decrease with increasing environmental stress. Differences in the abundances of these species between Sea Spray and Tor Bay again seem likely to be the result of ice scour at Sea Spray. The most prominent difference between sites was the overwhelmingly higher abundance of barnacles at Sea Spray. Potential interaction webs, based on species abundance trends across environmental stress gradients, were constructed for both intertidal communities.;Future research should test for differences in the nutrient supply between the northern and the eastern shores of Nova Scotia to further explain differences in species abundances. Experimentally testing for species interactions is recommended to validate these proposed interaction webs, and tests for the effects of these interactions on richness and diversity should be conducted on both shores. The demographic rates of rocky intertidal species should be measured because these could be another source of variation. Furthermore, determining species recruitment rates will allow for full testing of the environmental stress model predictions on the rocky intertidal communities in Nova Scotia.;To test these model predictions, I measured richness and diversity in 25 cm x 25 cm plots across gradients of intertidal elevation, wave exposure, and ice scour. My northern shore study site (Sea Spray) borders the Gulf of St. Lawrence, which is subject to seasonal ice scour, and my eastern shore study site (Tor Bay) borders the Atlantic Ocean. I found differences in richness and diversity across environmental stress gradients for each shore, with lowest values at higher levels of stress and increasing values with less stress, in compliance with the environmental stress model. In concordance with my presumptions on the direction of increasing environmental stress, both richness and diversity at Sea Spray were lower at the high elevation and lower in the exposed area. Similarly, richness at Tor Bay was lower at the high elevation and lower in the exposed area. Diversity at Tor Bay was also lower at the high elevation, but was lower in the sheltered area. This apparent reversal in the direction of increasing environmental stress along the exposure gradient suggests that sheltered areas are more stressful than exposed areas when ice scour is not a factor. This trend is likely attributable to the higher intertidal stresses (i.e., temperature, desiccation, irradiance) in sheltered habitats because of the limited wave action, which coincides with more of the data variation being explained by elevation than by wave exposure for both shores. I also found differences in the community structure, specifically in richness, diversity, and evenness, between Sea Spray and Tor Bay. Sea surface temperature and salinity were measured throughout the sampling period on both shores, although no differences were found; thus, it seems that it was likely the presence of sea ice at Sea Spray that distinguished the community structure there from that of Tor Bay.