Freshwater Cyanobacteria within Extreme Environments

Abstract

The proliferation of undesirable cyanobacteria within eutrophic freshwaters is now reported on a global scale, however the factors triggering cyanobacteria bloom formation were found to be especially complex. Even within a single water body, physical, chemical and biological factors all influence the assemblage of phytoplankton found. Notwithstanding the body of research conducted over the past 50 years, where concerted efforts have been applied to observing bloom patterns and the drivers of cyanobacteria blooms, substantial knowledge gaps were identified. An examination of field methods, the factors promoting the growth of cyanobacteria blooms in modified inland aquatic ecosystems, modelling to predict the onset of a bloom using a 7 day positive model, and consideration of the likely effects increasing atmospheric CO2 could have on the development of surface dwelling species were key themes addressed. The thesis represents a combination of field based and controlled experimental laboratory research. A three-year case study was undertaken at an inland freshwater impoundment where cyanobacteria blooms had become a burgeoning issue. Standard surface monitoring and in situ multi-parameter instruments were used to determine environmental stressors. Importantly the water body was characterised as a well-mixed system. A hypertrophic classification was attributed to the water body due to nutrient super saturation, high water temperatures and suspended sediment measured in all years. A discrepancy between the available analysis methods was found, as the molecular method detected a diversity of microorganisms including several cyanobacteria species, whereas microscopy detected only one species. Notably harmful colonies of picocyanobacteria were not observed using standard microscopy. The molecular data also showed cyanobacteria represented 30% of the total bacterial community yet; molecular analyses may be limited when cell counts are required for a comparison with the alert level framework. The case study found the adoption of multi-monitoring and analysis methods constitutes best practice, and should therefore be integral to early detection, risk mitigation and system characterisation. A controlled laboratory study was undertaken to measure the efficacy of emerging technology as a water quality-monitoring stratagem, with emphasis placed on the accuracy of fluorometry in turbid waters above 50 nephelometric turbidity units (NTUs). The results found in vivo fluorometry using a Manta 2 probe was useful for detecting Microcystis aeruginosa at concentrations consistent with the National Health and Medical Research Council’s alert framework (Green, Amber and Red). Phycocyanin was found to be a more reliable measure of cyanobacteria than chlorophyll a, and by using a calibrated instrument it was possible to accurately detect cyanobacteria in turbid waters up to 220 NTUs. Cyanobacteria are highly effective at CO2 assimilation, with a remarkable capacity to adapt to different CO2 conditions, however scant research had previously focused on understanding how freshwater species may respond to increasing atmospheric CO2. Controlled laboratory experiments were performed and diazotrophic and non-diazotrophic cultures were exposed to past and projected atmospheric CO2 concentrations. Both experimental species adapted to the ambient low and high CO2 conditions, however, significantly higher bio volume was measured in the elevated CO2 chambers. The ability of freshwater ecosystems to maintain steady state water chemistry or base-neutralizing capacity if atmospheric CO2 concentrations reach levels projected for the latter part of this century is questionable, and furthermore, the study provided empirical evidence to support the theoretical position that increasing CO2 may lead to changes in water chemistry, particularly a decrease in pH values.