Diversity, structure and functioning of freshwater biofilms in response to herbicides

• Main Author: Rosenkranz, Helen
• Published: University of Bristol 2014
• Subjects: 363.738
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.665172

Various approaches have been published aimed at assessing the impact of herbicides on river biofilms, by conducting field and laboratory experiments. The vast majority focusses on laboratory standardised growth inhibition tests conducted on isolated single species, rather than on whole communities. In this thesis three different consecutive experimental approaches were conducted integrating different ecological complexities to determine the impact of herbicides on phototrophic river biofilms. Chlorotoluron and the glyphosate formulation Roundup®, two herbicides with different modes of action, were used to investigate single, as well as combined, impacts on the structure (biomass, biodiversity and architecture) and functioning (photosynthetic rates and light utilization efficiency) of biofilms. Furthermore, the combined impact of herbicides and freshwater viruses on these biofilm communities was investigated. In field experiments on biofilms, single glyphosate (Roundup®) and the combined herbicide treatment (glyphosate + chlorotoluron) significantly reduced the total biomass (dry weight) as well as the photosynthetic biomass (total chlorophyll) of biofilms compared to controls. In contrast, both herbicides (single and combined) significantly increased the biomass-specific rate of photosynthesis. Both effects resulted in the observation of a shift in community composition towards green algal dominance at the expense of diatoms, along with a reduction in taxonomic diversity. In single species cultures, isolated from the field, diatoms showed the greatest resistance to chlorotoluron and glyphosate (Roundup®), while cyanobacteria showed the highest sensitivity during toxicity tests. Both green algae cultures were negatively impacted by chlorotoluron, but only at concentrations greater than those applied in the field. When treated with Roundup® a significant increase in biomass was measured. Biofilms were exposed to viruses, either singly or in combination with herbicides. A significant increase in the ratios of diatoms to green algae and a decrease in taxonomic diversity was found upon virus application, and a partial cancellation of these effects was observed when combined with herbicides. These three consecutive studies not only broaden our knowledge on the impacts of herbicides on freshwater biofilm communities, but also illustrate the key importance of multilayered approaches. The research has shown that a specie's sensitivity may vary depending on its location in the biofilm, a factor not realised in single species toxicity testing. Combined interactions between viruses and herbicides were found to play a role in structuring biofilm communities and therefore underline the requirement for further research on the interaction of herbicides with other environmental factors. Using community assessments as described in this study in combination with traditional single species tests, a more reliable prediction of the environmental toxicity of herbicides can be obtained.