Geosmin synthase (geoA) and 2-MIB cyclase (mic) are key biosynthetic genes responsible for the production of taste and odour (T&O) compounds, geosmin and 2-MIB. These T&O compounds are becoming an increasing global problem for drinking water supplies. It is thought that geosmin and 2-MIB may be linked to, or exacerbated by, a variety of different environmental and nutrient triggers. However, to the best of our knowledge, no studies to date have evaluated the combined effects of seasonality, temperature, and nutrient concentrations on geoA and mic copy numbers in conjunction with T&O concentrations. In this study, environmental triggers behind geosmin and 2-MIB production were investigated in nine reservoirs across Wales, U.K. between July 2019 - August 2020. The abundance of geoA and mic were quantified through quantitative Polymerase Chain Reaction (qPCR). Temporal changes in geoA and geosmin concentrations revealed geoA to be an indicator of monthly geosmin concentrations, although only when geosmin concentrations exceeded 100 ng L^-1. Model analysis of a reservoir with elevated geosmin concentrations revealed geoA to be significantly associated with mean temperature (p < 0.001) and the nutrients dissolved reactive silicate (p < 0.001), dissolved iron (p < 0.001), total inorganic nitrogen to phosphorous ratio (TIN:TP) (p < 0.001) and ammonium to nitrate ratio (NH₄⁺:NO₃^-) (p < 0.001). Sulphate also demonstrated a significant positive linear relationship with geoA (p < 0.001). For mic analysis, NH₄⁺:NO₃^- was significantly associated with mic (p < 0.05) and an association with dissolved reactive silicate was also observed (p = 0.084). Within this study we also report extreme variance in gene copy numbers between the study seasons. No consistent relationship could be determined for mic copy numbers mL^-1 and 2-MIB (ng L^-1). The findings from this study indicate that TIN:TP and NH₄⁺:NO₃^- serve as good predictors for elevated geoA and mic, along with negative linear relationships observed for mean temperature and dissolved reactive silicate. Overall, our findings demonstrate the importance of nutrient concentrations, nutrient ratios and temperature for evidence based predictive capacity of taste and odour events in drinking water reservoirs.

Control of algal blooms and associated biologically-induced water quality risks in drinking reservoirs is problematic. Copper sulphate (CuSO₄) treatment is one intervention that has been utilised for >100 years. Evidence indicates a favourable short-term reduction in Cyanobacterial biomass (e.g. bloom termination), but here we indicate that it may also increase longer-term water quality risk. In 2022, we investigated the impacts of CuSO₄ spraying on Cyanobacterial communities and nutrient levels within a drinking water supply reservoir using environmental DNA (eDNA) to assess community shifts, alongside monitoring nutrient fractions, orthophosphate (OP) and total phosphate (TP), post-treatment. CuSO₄ application successfully reduced Cyanobacterial abundance, however elimination of Cyanobacteria resulted in a shift in bacterial dominance favouring Planctomycetota throughout the summer and a combination of Actinobacteriota and Verrucomicrobiota, throughout autumn. As Cyanobacterial abundance recovered post-treatment, Cyanobacterial genera demonstrated greater diversity compared to only three Cyanobacterial genera present across samples pre-treatment, and included taxa associated with water quality risk (e.g. taste and odour (T&O) metabolite and toxin producers). The increase in Cyanobacteria post-treatment was attributed to an increase in biologically available nutrients, primarily a significant increase in OP. Overall, findings suggest that the significant shift in biodiversity likely induces a less stable ecosystem with greater plasticity of response to changing environmental and biogeochemical variables. Legacy implications of CuSO₄ spraying, in terms of shifts in ecosystem and nutrient balance over time, may have implications for drinking water quality, but importantly also for reservoir management options. As such, the effects of CuSO₄ spraying should be considered carefully before consideration as a contender for in-reservoir biological control.

Geosmin and 2-methylisoborneol (2-MIB) are secondary bacterial metabolites that create an earthy-musty taste and odour (T&O) in drinking water. Both compounds exhibit low odour thresholds and are the leading causes of customer complaints to water companies worldwide. Water companies must predict spikes in T&O concentrations early to intervene before these compounds reach the treatment works. Cyanobacteria are key producers of T&O in open waters, yet the influence of broader microbial and algal communities on cyanobacterial T&O events remains unclear. This study identified T&O risk indicator taxa using next-generation sequencing of bacterial (16S rRNA) and algal (rbcL) communities in three reservoirs in Wales, UK. Ordination analysis of these communities revealed clustering according to assigned T&O concentration levels, identifying T&O signature communities. Random Forest (RF) analyses accurately classified samples for high and low concentrations of geosmin and 2-MIB, demonstrating the biological consortium’s predictive power. Based on shared ecological traits of bacterial and algal taxa, we propose five categories corresponding to different magnitudes of T&O risk. Indicator taxa in T&O risk categories can then be used to predict T&O events, empowering water companies first to optimise treatment response and, importantly, to determine triggers before an event to evidence preventative intervention management.

Generating rapid, easy-to-interpret community data for drinking reservoirs as a means of tackling water quality management is of increasing demand within the water industry. Taste and odour (T&O) is one of many increasing concerns to water companies worldwide, incurring huge costs as customer complaints accumulate and additional treatment and resource management are required. However, there remains a two-fold issue in addressing T&O management: firstly, predicting the initial onset of a T&O event relies on a highly complex understanding of environmental considerations and their interaction with T&O-related taxa, and secondly, there remains a lag between the notification of a T&O event and the resolution of the issue by reservoir management staff. This is partly due to slow, low-resolution methods of detecting and reliably identifying problem taxa in samples. These methods are unable to provide information on the huge plethora of taxa related to T&O metabolite production and often cannot provide data in a timely enough manner for an opportune management response. This means the water industry is often forced to use a reactive, rather than proactive, approach to water quality monitoring. Here, we present methods for implementing a high-throughput sequencing approach to monitoring drinking reservoirs for water quality and improving the sustainability of water supplies, as well as methods for presenting these data on easy-to-interpret data dashboards that can be updated rapidly as new data are generated. Our methods and dashboarding approaches are currently being trialled and tested within the UK water industry, and so here, we show anonymised examples of those data presentations. We propose that these methods can greatly aid reservoir management teams in their approach to T&O monitoring and can be used to implore more sustainable management pipelines, safeguarding future water sources.

Taste and odour (T&O) issues caused by geosmin and 2-MIB persist worldwide in drinking water. Though posing minimal health risks, their low odour thresholds lead to customer complaints and costly treatments. These compounds in freshwater reservoirs mainly stem from cyanobacterial activity, influenced by interactions among microbes, nutrients, and physical processes like stratification and redox cycling. Conventional prediction methods rely on environmental factors or cyanobacterial biomass but don’t fully capture the biological mechanisms behind T&O production. This review synthesises current understanding of the environmental drivers of geosmin and 2-MIB and evaluates existing modelling approaches. A case study introduces an environmental DNA (eDNA)-informed modelling framework integrating biological indicators, chemical variables, and physical drivers within Generalised Additive Models (GAMs) to forecast geosmin concentrations in three UK drinking-water reservoirs. Integrating biological indicators into predictive models provides a promising pathway toward operational early-warning systems for proactive T&O management.