Ascendency as an ecological indicator: a case study of estuarine pulse eutrophication

Abstract

Increasingly, management agencies require that the remediation of eutrophic waters be addressed at the level of the whole ecosystem. One whole-system approach to quantify ecosystems is called ecological network analysis. Ascendency theory, the branch of the field that deals with the quantification of whole-system status, specifically addresses the definition of eutrophication. This definition has been applied to data taken over a gradient of eutrophication. Three separate areas were observed: a non-eutrophic area (with Zostera noltii meadows), an intermediate eutrophic area ( Z. noltii absent and macroalgae abundant at times) and a strongly eutrophic area (where Enteromorpha spp. blooms occur with regularity). Pulse eutrophication was considered as the major driving force behind a gradual shift in primary producers from a community dominated by rooted macrophytes ( Z. noltii) to a community dominated by green macroalgae. The measures associated with the intermediate eutrophic region turned out not to be intermediate to those at the gradient extremes. The most likely explanation appears to be the highly unstable nature of this area. Conditions along the spatial gradient are discussed as representing various stages in the temporal evolution of the system, and analysed in the framework of the Intermediate Disturbance Hypothesis, Bifurcation, Chaos, and Catastrophe theories.