Risk Management of Cyanotoxins in Singapore

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.


Introduction
The excessive production of cyanotoxins, produced by cyanobacteria, is increasingly recognised as a major challenge facing water resource management worldwide [1] [2]. Cyanobacteria are natural components of phytoplankton communities in freshwater systems. High water column stability allowing access to light, increased temperatures and nutrient enrichment (eutrophication) are some environmental conditions that can favour cyanobacterial proliferation (i.e. algal blooms) [2] [3]. Cyanobacterial blooms can lead to the excessive production of secondary metabolites, including taste and odour compounds (2-Methylisoborneol (2-MIB) and Geosmin) and cyanotoxins. High cyanotoxin loads from blooms have been linked to human and animal illness and death in over 50 countries [1].
Human exposure to cyanotoxins can occur through skin contact and ingestion or inhalation of water droplets during recreational activities [1] [4]. Commonly known cyanotoxins can be grouped according to their impacts on human health.
For example, Microcystins (MC) are hepatotoxic, while Anatoxin-a (ATX) and Saxitoxin (STX) are neurotoxic. In contrast, the toxicity of Cylindrospermopsin (CYN) is non-organ specific [5]. As cyanobacterial blooms are increasing in frequency, severity, extent and duration on a global scale [3], it is beneficial to understand the strategies different countries are undertaking to manage the risks of bloom-associated cyanotoxin loads in freshwater systems.
Two of the most internationally recognized sets of guidance levels used to manage cyanotoxin risk in drinking water have been developed by the World Health Organisation (WHO) and the U.S. Environmental Protection Agency (USEPA). Of all the cyanotoxins, the WHO guideline focuses on MC because it is produced by Microcystis, the most common bloom-forming genus of cyanobacteria in freshwater [6]. MC is known to have >80 variants [7]. Among these, MC-LR is one of the most toxic and well-studied, and is therefore used as a surrogate for all other MC variants [7] [8]. The WHO provisional guideline for MC-LR in drinking water is 1.0 μg/L. This MC-LR concentration is not expected to pose any significant risk to human health over a lifetime of exposure [9]. The USEPA 10-day Health Advisory (HA) value of MC in drinking water for bottle-fed infants and young children of pre-school age is 0.3 μg/L, while that for school-age children through adults is 1.6 μg/L. These HA values describe MC concentrations at or below which adverse health effects are not anticipated to occur over a 10-day duration of exposure [10]. Because adequate data on health effects are also available for CYN, the USEPA developed 10-day HA values for this cyanotoxin in drinking water; specifically 0.7 μg/L for bottle-fed infants and young children of pre-school age and 3 μg/L for school-age children through adults [10]. These USEPA and WHO guidelines form an important basis upon which individual countries develop their own locally-relevant standards.
At the national level, many countries have adapted the WHO guidance values on cyanotoxin concentrations to local conditions by using cyanobacterial biomass as a surrogate for cyanotoxin load [11]. Specifically, local cyanobacterial chlorophyll-a (Chl a) or other cyanobacterial pigments detected by fluorometry) [11]. In most national frameworks, breaching any alert level of cyanobacterial biomass triggers a set of pre-defined responses or intervention plans aimed at reducing direct human exposure to the increasing cyanotoxin load. Measures aimed at reducing cyanotoxin load in source waters can ultimately help reduce cyanotoxin levels in drinking water (i.e. end product) as well [12]. Thus, understanding frameworks used to manage cyanotoxin risk management in raw water sources helps inform cyanotoxin risk management in drinking water supplies.
Cyanotoxin production and associated cyanobacterial ecology and growth have been extensively documented in temperate regions, but similar information from tropical regions is lacking [13] [14]. The differing climate and ecological conditions in temperate and tropical regions can give rise to distinct cyanobacterial bloom and associated cyanotoxin production dynamics. Because cyanobacterial blooms typically occur during periods of higher water temperatures and access to light [2] [3], seasonal transitions in temperate regions allow for prediction of bloom occurrence [15]. Existence of a bloom season enables pre-emptive measures to be enacted before the season to mitigate subsequent bloom occurrence. In contrast, high temperatures and light conditions usually characterise tropical regions year-round [13]. Consequently, cyanobacterial blooms can occur at any time of the year and last for an extended period of time [16] [17]. In this article, we discuss the risk management framework pertaining to cyanotoxins in tropical Singapore fresh water bodies with respect to the relationship between Chl a and MC concentrations, and the approach we use to manage cyanotoxin levels in source water to safeguard drinking water safety.
Specifically, we present cyanobacterial biomass, cyanotoxins and water quality data collected from 17 reservoirs in Singapore over approximately seven years.

Field Studies and Investigations Regarding the Prevalence of Cyanotoxins in Singapore
Singapore is an island located at the tip of peninsular Malaysia (1˚N of the equator). It has an average population density of 7953 inhabitants/km 2 [18] and is characterised by a tropical climate with Northeast and Southwest monsoons and inter-monsoon periods. Temperatures vary little year-round, ranging from 23˚C to 33˚C and long-term (1981 to 2010) mean annual rainfall total is 2166 mm [19].

Cyanotoxin Risk Management Approach in Singapore
In Singapore, two government agencies are involved in cyanotoxin risk management; PUB, Singapore's National Water Agency and the National Environ-  concern, control limits of these contaminants and corrective actions to be taken at each stage of the water loop to ensure water safety throughout the supply chain. One of the groups of contaminants monitored as part of the WSP is cyanotoxins produced by cyanobacterial blooms in the reservoirs, and the associated corrective actions involve catchment and reservoir management strategies to minimize bloom occurrence.
In 2007, PUB and NEA first used a risk-based assessment approach to establish recreational water quality guidelines. These guidelines consist of an alert level of Chl a at which concentrations of cyanotoxins in local waters were likely to be a concern for human health. The function of these guidelines is to determine the suitability of fresh water bodies for primary contact activities (e.g. skiing and wakeboarding) where the whole body or face and trunk are frequently immersed or it is likely that some water will be swallowed. By using available Chl a and cyanotoxin concentration data from the reservoirs, it was determined that 50 μg/L of Chl a corresponded to <0.025 to 1 μg/L of MC-LR. As a result, 50 μg/L of Chl a was established as the level beyond which there would be a higher risk of cyanotoxin production in concentrations that could have moderate or severe human health impacts. PUB and NEA regularly review this recreational guideline value using routinely monitored data (Figure 1). Analyses of an ex-   Microcystis cell counts used in Australian drinking water quality guidelines are similar to the Microcystis cell counts PUB uses. In Australia, the MC notifi-Journal of Water Resource and Protection cation level (30% of density equivalent to MC: 1.3 μg/L) and MC alert level (1.3 μg/L) in drinking water are based on MC production from a toxic strain of M. aeruginosa. These notification and alert levels corresponded to 2000 and 6500 Microcystis cells/ml respectively [23]. The alert level guideline for M. aeruginosa from Australia (6500 cells/ml) is close to the mean MC cell quota drinking water cell count guideline for M. aeruginosa from Singapore (<7337 cells/ml, Table 2).
PUB further investigated the relationship between Microcystis cell counts and MC-LR concentrations from all 17 Singapore reservoirs. The data showed that Microcystis cell counts < 10,000 cells/ml corresponded to MC-LR concentrations < 1 μg/L; the provisional WHO guideline for drinking water (Figure 3). These data corroborate results presented in Table 2 that suggest keeping MC-LR concentrations below 1 μg/L necessitate keeping mean and maximum cell counts of the highly toxic M. aeruginosa below 7337 cells/ml and 5095 cells/ml, respectively. In Table 2 Table 3.
A key component of reducing cyanotoxin risk in urban reservoirs is nutrient control. Unlike ambient temperature and light conditions which are beyond practical control in waterways and reservoirs, the magnitude of nutrient loading into water bodies can be managed and thereby used to prevent or minimize  Long-term monitoring data have enabled PUB and NEA to have a better understanding of the dynamics of cyanotoxin production in Singapore's water bodies, and thereby developed locally-relevant guideline values to manage cyanotoxin risk in Singapore. Regular review and update of the cyanotoxin risk management plan and associated nutrient control measures are needed to cope with changing circumstances. This is especially important given the impacts climate change is having on the frequency, extent and duration of algae bloom occurrence.

Conclusion
Understanding