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 termedPeriodic hypoxiaandPeriodic 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.

An investigation of periodic hypoxia at Ardbear Salt Lake

The factors potentially leading to the development of periodic hypoxia were investigated based on field studies performed over a series of two years between November, 1997 and November, 1999. This was attempted by investigating both hard and soft seafloor. Descriptive and predictive models were developed that incorporated physical, chemical and biological data derived from the present study and previous work and were used to anticipate future possible crashes of the faunal communities due to hypoxia/anoxia. It was concluded that periodic hypoxia at Ardbear is a natural event caused by several factors to varying degrees. The results of the present study indicate that the major controlling factors for oxygen depletion are the shallow depth of the system, coupled with the development of thermal stratification during periods of calm ambient meteorological conditions of the warmer summer months. In addition, a soft, muddy bottom and highly productive plankton blooms exacerbate conditions in a system already predisposed to hypoxia.