Testing the efficacy of an estuarine eutrophic condition index: Does it account for shifts in flow conditions?

Abstract

Eutrophication of estuaries via anthropogenic nutrient enrichment is an issue being addressed extensively on a global scale, however it remains a topic that garners further attention due to its complexity. The aim of this study was to test the efficacy of a recently proposed eutrophic condition index in five permanently open estuaries, subsequent to flow variations. Further, in order to elucidate their potential role in augmenting eutrophic conditions, the influence of vegetated habitats on associated microalgal communities was investigated. The study took place in the summer of 2014 and 2015. On each sampling occasion, all of the proposed ‘state’ indicators (nutrients, oxygen and microalgal communities) were assessed and supplemented with a heuristic ‘pressure’ component (flow conditions). Overall, the proposed index was shown to be sensitive to environmental perturbations providing shifts in classification ratings illustrating the dilution (Swartkops Estuary) or augmenting (Kromme, Gamtoos, and Kariega estuaries) effect of freshwater pulse events. Additionally, the sensitivity of selected epiphyte and microphytobenthos (MPB) parameters to water quality variations was verified, thus supporting their inclusion as indicators in the proposed index. Regarding the role of microhabitats in promoting microalgal growth, it was found that MPB biomass was higher (5–290%) and benthic diatom diversity generally lower (5–50%) in vegetated compared to unvegetated habitats – an important consideration when applying the index. The habitat complexity and stabilisation provided by estuarine macrophytes (Phragmites australis and Zostera capensis) supported notable autotroph colonisation. This highlights their importance in structuring trophic pathways, whilst also potentially providing a corridor for the intensification of eutrophic symptoms in estuaries. Overall, this study provides an important step towards the verification of a proposed assessment methodology which may serve to provide a baseline from which the eutrophic status of estuaries can be monitored – particularly in countries where such assessment frameworks are lacking.