Use of Stream Biofilm Bacterial Communities and Associated Metals as Indicators of Urban Runoff Impact on Freshwater Ecosystems

Abstract

The main focus of this project was to investigate how anthropogenic activities affect stream biofilm microbial communities and to assess the potential uses of biofilms as indicators of freshwater ecosystem health. The work improves our understanding of the impact of urbanisation on stream ecosystems and provides innovative techniques to assess this impact more accurately. Stream biofilms are the complex aggregation of microorganisms, covering almost every surface in freshwater environments. They play key roles in stream ecosystems and are grazed by many other organisms. Therefore, the potential impact of urbanisation on biofilm microbial communities could alter the functions of biofilms and affect the whole stream ecosystem. A wide range of experiments was conducted in both flow chamber microcosms and natural stream environments to investigate the impact of urban runoff on microbial community structure and composition. Using community fingerprinting techniques such as Terminal-Restriction Fragment Length Polymorphism and Automated Ribosomal Intergenic Spacer Analysis, as well as sequencing of bacterial 16S rRNA genes, we investigated changes occurring in biofilm bacterial and ciliate protozoan communities under exposure to urban contaminants. Flow chamber experiments revealed that significant differences in the structure of the microbial community could be detected within only a few days of exposure to urban runoff contaminants and remained detectable weeks after transfer to uncontaminated water. The rapid and persistent changes highlight the sensitivity of microbial communities and suggest that natural biofilm communities frequently exposed to urban runoff are significantly altered. Experiments carried out in natural streams confirmed the differences in composition and tolerance to contaminants between biofilm communities from preserved and impacted streams, suggesting critical implications for the whole stream ecosystem. The structure and composition of microbial communities associated with biofilms result from the influence of past and present environmental conditions and therefore constitute potential integrative indicators of stream health. Rapid changes in bacterial and ciliate community structure during exposure to metal contaminants demonstrated the sensitivity of these communities and their potential use as indicators of the influence of urban areas on stream ecosystem. Additional investigation conducted in natural streams indicated that bacterial and ciliate communities can reveal changes occurring along an urbanisation gradient and confirmed their reliable use as ecological bio-indicators. These novel indicators were implemented successfully to investigate the efficiency of an enclosed stormwater treatment system where traditional biological indicators such as benthic macroinvertebrates are not available. Results revealed the successful improvement of water through the treatment train and the minimal impact on the receiving creek. As biofilms are sessile structures, they may also accumulate urban contaminants and could form a critical link in the movement of urban contaminants from abiotic to biotic components of the stream. Therefore, the absorption and release rates of metals (Zn, Cu and Pb) in biofilms were investigated. Results revealed the fast accumulation of metals in the matrix during the first few days. High enrichment factors were recorded between the biofilm wet weight and the water, reaching 500:1 for zinc, 1500:1 for copper and 6000:1 for lead after 21 days of exposure. During recovery in uncontaminated water, metals were retained in the biofilm and 10 to 16 % of zinc and copper accumulated during exposure still remained in the biofilm after 14 days of recovery. The release of lead was even slower and more than 35 % of accumulated lead remained in the biofilm after 14 days of recovery. Investigation in natural streams confirmed the accumulation of metals in biofilms from urban streams and highlighted the potential risk for organisms at a higher trophic level. These results suggested that metals associated with biofilms could provide a highly relevant indicator of the presence of metals in freshwater systems at concentrations detrimental to aquatic biota. Using biofilm bacterial, ciliate and macro-invertebrate communities as bio-indicators, the relevance of biofilm associated metals (As, Cd, Cr, Cu, Pb, Ni and Zn) was investigated and compared to sediment associated metals. A greater proportion of the changes occurring in each of the communities could be explained by metal concentrations in biofilm than in sediments confirming the ecological relevance of biofilm associated metals. This project provides an insight into the effect of urban contaminants on stream microorganisms and contributes to the understanding of how urbanisation affects the entire stream ecosystem. Novel tools emerging from the use of biofilm as an indicator of stream health will assist land planners, aquatic resource managers and decision makers by facilitating better assessment of urban influences on aquatic environments.