Quantifying biodiversity in aquatic ecosystems: evaluating the causes for congruent patterns across trophic levels

Understanding the drivers and patterns of biological diversity has been a central tenet for ecologists over the past century. Aquatic ecosystems appear to be particularly sensitive to biodiversity declines and thus comprehending the causes of these biodiversity losses has become a pressing issue. The objectives of this thesis are twofold. First, I wanted to investigate the effect of phosphorus, a surrogate for productivity in aquatic ecosystems, on the diversity of two taxonomic groups across broad spatial and temporal scales. Secondly, I wanted to quantitatively assess the use of biodiversity indicators in aquatic ecosystems, with particular emphasis on the role of body size.;The second chapter of this thesis investigated the efficiency of cross-taxon congruence as a method for estimating biodiversity in aquatic ecosystems. I approached this subject by performing a spatial field study coupled to a meta-analysis of the published literature. The field survey and meta-analysis show that organisms of similar size tended to exhibit more congruent diversity patterns. I argue that this pattern arises because organisms of similar body size have similar life history traits and thus similar biodiversity responses to environmental gradients. I demonstrate that cross-taxon biodiversity indicators are most effective when estimating the diversity of communities of most similar body size.;Overall, this research has expanded our understanding of patterns, drivers and similarities of biodiversity across trophic levels in aquatic ecosystems.;In the first chapter of this thesis, I took a palaeolimnological approach to investigate how the species richness of subfossil diatoms and cladocera varies in response to phosphorus. From both our spatial and temporal analyses, a significant decline in species richness of diatoms and cladocera was observed as phosphorus increased. When subdivided according to habitat preference, only the littoral species richness showed a decline in species richness. We attribute this decline in species richness in the littoral taxa to the interplay of nutrients on littoral habitat heterogeneity. At low levels of phosphorus, the littoral zone is thought to be more productive and harbor a greater abundance of submerged macrophytes and is thus able to maintain a more diverse community.