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Macrophytes can be crucial for maintaining clear water conditions in temperate shallow lakes. However, their restorative potential and role in regulating phytoplankton remains uncertain in tropical lakes. We investigated the effects of emergent (Ludwigia adscendens and Persicaria barbata) and submerged (Vallisneria spiralis) macrophytes on the phytoplankton community of a turbid tropical reservoir. Through two in situ mesocosm experiments (~ 1000 l capacity) lasting 4 weeks, we (1) determined the effects of macrophyte density on phytoplankton biomass and composition, and (2) compared these effects between emergent and submerged macrophytes. In Experiment 1, macrophyte treatments reduced phytoplankton biomass and increased water clarity in a density-dependent manner. Only the ‘high density’ treatment (300 g/m2 emergent and 650 g/m2 submerged macrophytes) induced a taxonomic and functional shift from an initial community dominated by turbid water-adapted filamentous cyanobacteria to one dominated by clear water-adapted green algae and cryptophytes. In Experiment 2, emergent and submerged macrophytes reduced phytoplankton biomass and distinctly altered taxonomic and functional composition, with submerged macrophytes inhibiting Microcystis and stimulating cryptophyte taxa. Our results indicate that macrophytes can induce substantial phytoplankton community shifts in turbid tropical lakes, demonstrating the potential to assist in the reversal from turbid to clear water states during restoration efforts.

Interactions between macrophytes and phytoplankton have been studied extensively in temperate water bodies, but far less attention has been paid to those for emergent macrophytes in the tropics. We investigated the effects of six emergent macrophyte species (Brachiaria mutica, Ipomoea aquatica, Sphagneticola trilobata, Ludwigia adscendens, Pandanus amaryllifolius and Persicaria barbata) on the phytoplankton community of a tropical reservoir using ex situ experiments with multiple planting densities (Treatment 1: 0.108, Treatment 2: 0.24, Treatment 3: 0.40, Treatment 4: 0.58 g dry weight L−1) in 100-L mesocosms. Chlorophyll a (as a proxy of total phytoplankton biomass) and phytoplankton community structure were examined. The initial phytoplankton community (average chlorophyll a: 20.36 ± 0.69 µg L−1) was dominated by cyanobacteria: Planktolyngybya, Pseudanabaena and Cylindrospermopsis (average relative biovolume 0.68 ± 0.03). Over four weeks, L. adscendens (Treatments 3 and 4) and P. barbata (Treatments 2 to 4) significantly reduced chlorophyll a concentrations by 3.29–6.69 µg L−1 compared to the controls. Comparing species effects over four weeks for Treatment 4, P. barbata significantly lowered chlorophyll a concentrations compared to I. aquatica while there were no significant differences between all other species. All treatments of L. adscendens and P. barbata significantly reduced the relative biovolume of cyanobacteria in the phytoplankton communities by 0.19–0.39 compared to the controls. The highest-density treatments of all six macrophyte species led to significant shifts in the phytoplankton community structure by reducing the relative abundance of filamentous cyanobacteria (Planktolyngbya, Pseudanabaena) and/or increasing the relative abundance of cryptomonads (Chroomonas, Rhodomonas). This study shows the potential of emergent macrophyte species, especially L. adscendens and P. barbata, to reduce total phytoplankton biomass and cyanobacterial dominance in tropical water bodies, and a range of plant densities which were effective.

Rising temperatures (1.4–6 °C) due to climate change have been predicted to increase cyanobacterial bloom occurrences in temperate water bodies; however, the impacts of warming on tropical cyanobacterial blooms are unknown. We examined the effects of four different temperatures on the growth rates and microcystin (MC) production of five tropical Microcystis isolates ( M. ichthyoblabe (two strains), M. viridis , M. flos-aquae , and M. aeruginosa ). The temperature treatments are based on current temperature range in Singapore's reservoirs (27 °C and 30 °C), as well as projected mean (33 °C) and maximum temperatures (36 °C) based on tropical climate change estimates of +6 °C in air temperature. Increasing temperatures did not significantly affect the maximum growth rates of most Microcystis strains. Higher growth rates were only observed in one M. ichthyoblabe strain at 33 °C and M. flos-aquae at 30 °C where both were isolated from the same reservoir. MC-RR and MC-LR were produced in varying amounts by all four species of Microcystis . Raised temperatures of 33 °C were found to boost total MC cell quota for three Microcystis strains although further increase to 36 °C led to a sharp decrease in total MC cell quota for all five Microcystis strains. Increasing temperature also led to higher MC-LR:MC-RR cell quota ratios in M. ichthyoblabe . Our study suggests that higher mean water temperatures resulting from climate change will generally not influence growth rates of Microcystis spp. in Singapore except for increases in M. ichthyoblabe strains. However, toxin cell quota may increase under moderate warming scenarios depending on the species.

Microcystins (MCs) are a group of toxins produced by certain cyanobacteria that occur naturally in fresh waters and can cause acute poisoning in humans and animals. Because cyanobacteria have been found in the freshwater reservoirs of Singapore, a comprehensive survey for cyanotoxins was undertaken over a 12-month period in 17 reservoirs from November 2012 to October 2013. For the quantitative analysis of the reservoir samples, an liquid chromatography–tandem mass spectrometry (LC-MS/MS) method was developed that targeted the following hepatotoxins: MC-LR, MC-RR, MC-LW, MC-LF, MC-WR and MC-LY. The most prevalent MC variants identified were MC-LR and MC-RR. Results showed that the level of intracellular MC-LR in the raw or untreated water was close to the World Health Organisation (WHO) provisional MC-LR drinking water guideline of 1μgL–1 for 1 of 200 samples tested (0.8μgL–1), and that intracellular MC concentrations were above 0.3μgL–1 in samples collected from four reservoirs. Based on random forest analysis, total monthly rainfall and total nitrogen concentrations were found to be the most important factors affecting intracellular MC concentrations for these four reservoirs. The toxin levels for the other reservoirs were relatively low compared with the WHO provisional MC-LR guideline limit of 1.0μgL–1.

Cyanobacterial blue–green algal toxins are produced by harmful algal blooms (HABs). Most species of phytoplankton are not harmful, but excessive amounts of certain HAB taxa can cause harm to human and animal health, aquatic ecosystems and local economies. To investigate the prevalence of cylindrospermopsin (CYN) and anatoxin-a (ANA) in Singapore’s reservoirs, a hazard analysis was initiated to profile the CYN and ANA levels present. Water samples from 17 reservoirs were monitored monthly over a 12-month period (November 2012–October 2013). Analyses were conducted by liquid chromatography–tandem mass spectrometry (LC-MS/MS) using a triple-stage quadrupole mass spectrometer with a turbo-assisted ion spray source. CYN was more prevalent than ANA. Intracellular CYN concentrations exceeded 0.4μgL–1 in 6 of 17 man-made reservoirs surveyed, and slightly exceeded the provisional CYN drinking water guidelines of 1μgL–1 (National Health and Medical Research Council and National Resource Management Ministerial Council 2011) on one occasion (1.1μgL–1, July 2013) in one reservoir. The dominant cyanobacteria genera during that period were Cylindrospermopsis, Planktolyngbya, Pseudanabaena and Microcystis. For ANA, all 17 reservoirs had concentrations below 0.1μgL–1. Based on random forest analysis, the most important environmental factors affecting CYN concentrations were total nitrogen (most important), nitrate, total phosphorus and Cylindrospermopsis counts (least important). The findings of this study indicate that reducing total nitrogen concentrations may be useful in minimising CYN concentrations in tropical reservoirs.

In experiments investigating nutrient effects on tropical Microcystis, increasing nitrogen and phosphorus concentrations were found to have a significant positive effect on maximum cell yields of two strains of Microcystis ichthyoblabe (from Lower Peirce and Tengeh Reservoirs) and one strain of Microcystis flos-aquae isolated (Lower Peirce Reservoir) from Singapore. However, only increasing nitrogen concentration had a positive effect on growth rates of M. ichthyoblabe and M. flos-aquae from Lower Peirce Reservoir. MC-RR and MC-LR were produced by all three strains with MC-RR being the dominant variant. Phosphorus played an important role in MC production with increases in phosphorus from medium to high concentrations leading to decreases in MC-RR cell quotas for all three strains at the two highest nitrogen levels tested. The different growth and toxin production responses between M. ichthyoblabe strains could be due to location-specific differences.

The authors regret that there were some errors in their manuscript. Below are corrections (underlined) to the Results section (sub-heading 3.1 Temperature effects on growth) and the correct version of Fig. 1.

Toxic cyanobacterial blooms are a major issue in freshwater systems in many countries. The potentially toxic species and their ecological causes are likely to be different in tropical zones from those in temperate water bodies; however, studies on tropical toxic cyanobacterial blooms are sporadic and currently there is no global synthesis. In this review, we examined published information on tropical cyanobacterial bloom occurrence and toxin production to investigate patterns in their growth and distribution. Microcystis was the most frequently occurring bloom genus throughout tropical Asia, Africa and Central America, while Cylindrospermopsis and Anabaena blooms occurred in various locations in tropical Australia, America and Africa. Microcystis blooms were more prevalent during the wet season while Cylindrospermopsis blooms were more prevalent during the dry period. Microcystin was the most encountered toxin throughout the tropics. A meta-analysis of tropical cyanobacterial blooms showed that Microcystis blooms were more associated with higher total nitrogen concentrations, while Cylindrospermopsis blooms were more associated with higher maximum temperatures. Meta-analysis also showed a positive linear relationship between levels of microcystin and N:P (nitrate:phosphate) ratio. Tropical African Microcystis blooms were found to have the lowest microcystin levels in relation to biomass and N:P (nitrate:phosphate) compared to tropical Asian, Australian and American blooms. There was also no significant correlation between microcystin concentration and cell concentration for tropical African blooms as opposed to tropical Asian and American blooms. Our review illustrates that some cyanobacteria and toxins are more prevalent in tropical areas. While some tropical countries have considerable information regarding toxic blooms, others have few or no reported studies.