Risk Management of Cyanotoxins in Singapore

Abstract

Cyanotoxins produced by cyanobacteria pose significant challenges to water resource management due to the potential impacts they have on human health. Cylindrospermopsin (CYN) and microcystins (MC) are the more commonly detected cyanotoxins in Singapore’s reservoirs. Among the MC congeners monitored locally, the most frequently detected variants are MC-RR (37.6%), followed by MC-LR (25.6%). MC-LA and MC-YR are the least frequently detected variants (7.1%). No cyanotoxins have been detected in Singapore’s treated drinking water. Singapore’s National Water Agency (PUB) and the National Environment Agency (NEA) developed recreational water quality guidelines using Chl a concentrations of 50 μg/L. In local surface waters, long-term data showed that at 50 μg/L of Chl a , MC-LR concentrations ranged from <0.025 μg/L to 1 μg/L. In addition to using Chl a concentrations, Microcystis cell counts in reservoir water have also been used to manage cyanotoxin risk in drinking water. Specifically, routinely monitored data from all 17 Singapore reservoirs indicated that to keep MC-LR concentrations below the WHO provisional guideline of 1 μg/L in drinking water, Microcystis cell counts needed to be <10,000 cells/ml. Culture experiments using local Microcystis isolates showed M. aeruginosa produced the most MC compared to M . ichthyoblabe , M. flos-aquae , and M . viridis . Based on the maximum toxin cell quota equivalent to the WHO provisional guideline for MC-LR of 1 μg/L in drinking water, a 5000 cells/ml cell count guideline was derived for M . aeruginosa . This cell count has also been incorporated into Singapore’s cyanotoxin risk management framework for reservoirs.