Identification of Fine-Scale Marine Benthic Ecoclines by Multiple Parallel Ordination

The species-environment relationship is a fundamental structural property of natural ecosystems. Marine sedimentary macrofauna is known to be structured by a range of environmental variables; however, the mechanisms by which environmental variables covary to form complex-gradients (i.e., groups of intercorrelated environmental variables), and how these are related to coenoclines (i.e., gradients in species composition), remain poorly understood. We classified our study area into geomorphological features that were used for stratified sampling of macrofaunal polychaetes, molluscs, and echinoderms. The resulting species-by-site matrix was subjected to indirect gradient analysis by a multiple parallel ordination strategy, using detrended correspondence analysis and nonmetric multidimensional scaling. One major and one minor coenocline were identified. Based on the correlation between complex-gradients and the main coenocline we hypothesise the existence of two ecoclines that we have termed Periodic hypoxia and Periodic physical forcing. We conclude that a combination of recurrent (periodical) and extreme events is likely to determine the variation found in the species composition of marine sedimentary ecosystems. Based on the results of our study, we conclude that indirect gradient analysis is a useful tool for enhancement of our basic mechanistic understanding of the processes governing the compositional structure of marine sediment communities.