Abstract
Excessive anthropogenically-caused nutrient loading from both external and internal sources has promoted the growth of cyanobacteria in Lake Taihu from 2005 to 2014, suggesting increased production and release of cyanotoxins. In order to explain the spatial distribution and temporal variation of microcystins (MCs), the intracellular concentrations of MCs (MC-LR, -RR and -YR, L, R and Y are abbreviations of leucine, arginine and tyrosine) were monitored monthly from July 2013 to June 2014. Three MC variants are present simultaneously in Lake Taihu; the MC-LR and -RR variants were dominant (accounting for 40% and 39% of the total), followed by MC-YR (21%). However, MC-YR accounted for a higher proportion in colder months, especially in March. The highest concentrations of intracellular MCs were found in July and October when cyanobacteria cell density also reached the maximum. The average concentrations of MC-LR, -RR and -YR in July were 4.69, 4.23 and 2.01 μg/L, respectively. In terms of the entire lake, toxin concentrations in northern parts were significantly higher than the eastern part in summer, when MC concentrations were several times higher than the guideline value by WHO throughout much of Lake Taihu. Results from correlation and redundancy analysis (RDA) showed that total MCs, including all variants, were strongly and positively correlated with cyanobacteria cell density, water temperature, total phosphorus (TP) and pH, whereas each variant had different correlation coefficients with each of the considered environmental variables. MC-RR showed a stronger relationship with temperature, in contrast to MC-YR and -LR. Dissolved inorganic carbon (DIC) showed a negative relationship with each variant, suggesting that rising DIC concentrations may inhibit cyanobacterial growth and thereby reduce MC production in the future.
Highlights
The frequent occurrence of cyanobacterial harmful algal blooms (CHABs) poses a serious threat to animal and human health because of their ability to produce a wide variety of toxins throughout the world [1,2]
Sampling was conducted in the heavily-polluted areas, while quarterly sampling was conducted in the entire lake with 32 sampling sites (Figure 2)
Our study suggested that cyanobacteria cell density in Lake Taihu has exhibited a marked increase both in the northern areas and throughout the entire lake over the past ten years
Summary
The frequent occurrence of cyanobacterial harmful algal blooms (CHABs) poses a serious threat to animal and human health because of their ability to produce a wide variety of toxins throughout the world [1,2]. Climate change and global warming may increase the frequency and density of cyanobacterial blooms in freshwater systems [3,4]. Cyanotoxins, secondary metabolites produced by intensive CHABs, can threaten the supply of drinking water and fisheries-related food supplies [5,6]. The toxins can accumulate in organisms and be transferred via aquatic food webs, presenting potential risks to human health [7]. MCs are potent inhibitors of protein phosphatases 1 and 2A for both plants and animals and are produced predominantly by freshwater genera, including Microcystis, Planktothrix and Anabaena [9]. More than 90 variants of MCs with variable toxicity have been identified from cyanobacterial blooms and cultures [8]
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