Risk factors for potentially toxic blue-green cyanobacterial blooms of Microcystis aeruginosa in recycled wastewater intended for agricultural irrigation

Abstract

The sudden and often unpredictable occurrence of potentially toxic cyanobacterial blooms in agricultural irrigation water presents a human and animal health hazard internationally. The cyanobacterium Microcystis aeruginosa produces a range of protein-phosphatase inhibitory microcystins which are the only cyanotoxins for which the World Health Organisation currently produces action thresholds. Contact with irrigation water and irrigated food containing cyanobacterial toxins has caused human illness and livestock deaths, and a suspect relationship with chronic neurodegenerative human diseases including Parkinson's and Alzheimer's has been proposed. Environmental risk factors for sudden cyanobacterial blooms are, however, poorly researched, motivating the current study which took place in the main storage dam of the Hawkesbury Water Recycling Scheme in North-Western Sydney. The main aim of this research was to examine interaction of the key chemical, physico-chemical and climatological factors affecting stored, recycled wastewater intended for agricultural irrigation and relate them to blooms of Microcystis aeruginosa over a one-year period (March 2016- March 2017). Multiple regression analysis results showed statistically significant correlations between potentially toxic cyanophyte (M. aeruginosa) abundance and DO (p = 0.002), and Total Organic Carbon (TOC) (p = 0.017) in a highly significant model (p = 0.006), with nine degrees of freedom. Significant correlations were not, however, obtained with algal-nutrient phosphate and nitrate in a range of redox states, suggesting that these were not growth limiting for toxin-producing bacteria in recycled water. This echoed an earlier research finding for enterococci in the same dam. Bivariate, linear regression supported these findings, in addition atmospheric temperature being identified as the most important climatological growth factor (r = +0.47). These findings are highly important in Australian dams due to an engineering strategy applied at sewage treatment plants (STP) to control both algae and cyanobacteria in water intended for discharge to receiving waters and for agricultural irrigation. The strategy involves encouraging denitrification by the addition of a high-carbon source such as soluble sludge organics or methanol in an anaerobic treatment environment. These research findings may explain the persistence of toxic blooms in algal-free environments following implementation of this practice. A number of recommendations are made future research to aid in planning interventions to avoid blooms and preserve the integrity of stored, recycled water supplies in Australia.

Date of Award	2019
Original language	English