Changes in cyanoprokaryote populations, Microcystis morphology, and microcystin concentrations in Lake Elphinstone (Central Queensland, Australia)

Lake Elphinstone is a tropical inland water body in the far north of the Fitzroy Catchment in Central Queensland, Australia, and has experienced recurrent toxic cyanoprokaryote blooms since 1997. This article reports on an examination of the environmental conditions of the lake and the concurrent cyanoprokaryote species together with their toxicity. The lake was sampled three times during periods of high cyanoprokaryote cell concentrations. Successive changes in the dominant Microcystis species were accompanied by variation in the concentration of the hepatotoxin microcystin. Environmental parameters recorded during dominance by both the highly toxic species Microcystis panniformis and the nontoxic M. botrys are provided. Nutrient status, temperature, and light conditions were associated with species change within the blooms. Variation of microcystin concentrations coincided with speciation change (i.e., morphological variation) within the blooms. Also discussed is the environmental impact of toxin production by M. panniformis with respect to influence on cell division, energy states, and toxin photodegradation. Lake Elphinstone is the first Australian location reported to have M. panniformis.     

Recreational exposure to microcystins during algal blooms in two California lakes

We conducted a study of recreational exposure to microcystins among 81 children and adults planning recreational activities on either of three California reservoirs, two with significant, ongoing blooms of toxin-producing cyanobacteria, including Microcystis aeruginosa (Bloom Lakes), and one without a toxin-producing algal bloom (Control Lake). We analyzed water samples for algal taxonomy, microcystin concentrations, and potential respiratory viruses (adenoviruses and enteroviruses). We measured microcystins in personal air samples, nasal swabs, and blood samples. We interviewed study participants for demographic and health symptoms information. We found highly variable microcystin concentrations in Bloom Lakes (<10 μg/L to >500 μg/L); microcystin was not detected in the Control Lake. We did not detect adenoviruses or enteroviruses in any of the lakes. Low microcystin concentrations were found in personal air samples (<0.1 ng/m³ [limit of detection]-2.89 ng/m³) and nasal swabs (<0.1 ng [limit of detection]-5 ng). Microcystin concentrations in the water-soluble fraction of all plasma samples were below the limit of detection (1.0 μg/L). Our findings indicate that recreational activities in water bodies that experience toxin-producing cyanobacterial blooms can generate aerosolized cyanotoxins, making inhalation a potential route of exposure. Future studies should include collecting nasal swabs to assess upper respiratory tract deposition of toxin-containing aerosols droplets.     

Microcystins associated with Microcystis dominated blooms in the Southwest wetlands, Western Australia

Potentially toxic cyanobacterial blooms are becoming common in the freshwater wetlands on the Swan Coastal Plain, Western Australia. During summer the dominant bloom-causing species belong to the genera Microcystis and Anabaena and to a lesser extent Aphanizomenon and Nodularia. Although toxic cyanobacteria have been recorded in the Swan-Canning and Peel-Harvey estuaries in Western Australia, very little is known about the blooms in the surrounding freshwater lakes. In this study, a total of 32 natural bloom samples representing 13 lakes were analyzed by HPLC for microcystin (MC)-LR, -RR, and -YR. Twenty-eight samples proved to be toxic. The highest total microcystin concentration ranged from 1645 to 8428.6 microg L(-1), and the lowest concentrations were less than 10 microg L(-1) with some below the detection limit (< 0.05 microg L(-1)). MC-LR (100%) was the predominant microcystin, followed by MC-YR (71.4%) and MC-RR (60.7%). The presence of a Nodularia spumigena bloom in the freshwater Lake Yangebup was associated with the detection of nodularins (1664 microg L(-1)). This is the first study to demonstrate the presence of microcystins and nodularins in urban lakes on the Swan Coastal Plain, Western Australia.     

Detection and monitoring toxigenicity of cyanobacteria by application of molecular methods

The aim of this study was early genetic identification of microcystin-producing cyanobacteria and monitoring their toxigenicity by determining toxin concentrations in three Polish lakes throughout the summer of 2004. The assessment of cyanobacterial blooms was carried out in shallow, eutrophic water bodies: Lake Jeziorak, Lake Bninskie, and Sulejow Reservoir. Samples for DNA, phycological, and toxin analyses were collected from July till October. Molecular analysis of the 16S rRNA region was used to detect cyanobacteria in water samples. The microscopic analysis was performed to investigate seasonal variation of phytoplankton. Cyanobacteria, with domination by Microcystis, Planktothrix, and Planktolyngbya were detected during the whole monitoring period in Sulejow Reservoir, Lake Bninskie, and Lake Jeziorak, respectively. The presence and identification of toxic strains in water bodies was studied by PCR amplification of mcy genes in the microcystis synthesis pathway. The presence of the mcyA, mcyB, mcyD, and mcyE genes in water samples indicated the genetic potential to produce microcystins. Toxicity of water samples and microcystin concentrations were established by PPIA and HPLC, respectively. The maximum concentration of microcystins was 11.13 microg/L and 4.67 microg/L in samples dominated by P. agardhii and M. aeruginosa, respectively. Molecular analysis showed that toxigenic strains of cyanobacteria occurred in the three lakes throughout the summer season.     

Toxic cyanobacterial blooms in reservoirs under a semiarid mediterranean climate: the magnification of a problem

Sicilian reservoirs constitute the most important water resources available on the island. During summer 2001, the intense water utilization of Lake Arancio reservoir reduced the water level significantly, which coincided with the formation of intense blooms formed by the microcystin (MC)-producing cyanobacterium Microcystis aeruginosa. During summer 2003, Lake Arancio was continuously filled and the vertical stratification of the water column was maintained resulting in five to sixfold lower cell numbers of M. aeruginosa. For both years, a significant relationship between MC net production and Microcysytis cell growth was observed, implying that Microcystis cell numbers can be used to infer MC concentrations in water. Unexpectedly, dense blooms of the MC-producing cyanobacterium Planktothrix rubescens occurred during winter 2005/2006 in the reservoirs Lake Pozzillo, Prizzi, Nicoletti, and Garcia but have not been reported earlier. In this season, MC concentrations higher than those recorded in summer were measured, implying that monitoring of Mediterranean drinking water reservoirs needs to be intensified during winter, a season usually considered to be less prone to the formation of cyanobacterial blooms.     

Selective control of toxic Microcystis water blooms using lysine and malonic acid: an enclosure experiment

Three enclosures (10 x 10 x 1.5-1.3 m in depth) were set beside Dianch Lake, Kunming, People's Republic of China, for the period from July 28 to August 26, 2002. The enclosures were filled with cyanobacterial (Microcystis aeruginosa) water bloom-containing lake water. Lake sediment that contained macrophytes and water chestnut seeds was spread over the entire bottom of each enclosure. Initially, 10 g/m(2) of lysine was sprayed in Enclosure B, and 10 g/m(2) each of lysine and malonic acid were sprayed together in Enclosure C. Enclosure A remained untreated and was used as a control. The concentrations of lysine, malonic acid, chlorophyll a, and microcystin as well as the cell numbers of phytoplankton such as cyanobacteria, diatom, and euglena were monitored. On day 1 of the treatment, formation of cyanobacterial blooms almost ceased in Enclosures B and C, although Microcystis cells in the control still formed blooms. On day 7 Microcystis cells in Enclosure B that had been treated with lysine started growing again, whereas growth was not observed in Microcystis cells in Enclosure C, which had been treated with lysine and malonic acid. On day 28 the surface of Enclosure B was covered with water chestnut (Trapa spp.) and the Microcystis blooms again increased. In contrast, growth of macrophytes (Myriophllum spicatum and Potamogeton crispus) was observed in Enclosure C; however, no cyanobacterial blooms were observed. Lysine and malonic acid had completely decomposed. The microcystin concentration on day 28 decreased to 25% of the initial value, and the pH shifted from the initial value of 9.2 to 7.8. We concluded that combined treatment with lysine and malonic acid selectively controlled toxic Microcystis water blooms and induced the growth of macrophytes.     

Spatial and Temporal Patterns in the Seasonal Distribution of Toxic Cyanobacteria in Western Lake Erie from 2002–2014

Lake Erie, the world’s tenth largest freshwater lake by area, has had recurring blooms of toxic cyanobacteria for the past two decades. These blooms pose potential health risks for recreation, and impact the treatment of drinking water. Understanding the timing and distribution of the blooms may aid in planning by local communities and resources managers. Satellite data provides a means of examining spatial patterns of the blooms. Data sets from MERIS (2002–2012) and MODIS (2012–2014) were analyzed to evaluate bloom patterns and frequencies. The blooms were identified using previously published algorithms to detect cyanobacteria (~25,000 cells mL⁻¹), as well as a variation of these algorithms to account for the saturation of the MODIS ocean color bands. Images were binned into 10-day composites to reduce cloud and mixing artifacts. The 13 years of composites were used to determine frequency of presence of both detectable cyanobacteria and high risk (>100,000 cells mL⁻¹) blooms. The bloom season according to the satellite observations falls within June 1 and October 31. Maps show the pattern of development and areas most commonly impacted during all years (with minor and severe blooms). Frequencies during years with just severe blooms (minor bloom years were not included in the analysis) were examined in the same fashion. With the annual forecasts of bloom severity, these frequency maps can provide public water suppliers and health departments with guidance on the timing of potential risk.     

Identification of potentially toxic environmental Microcystis by individual and multiple PCR amplification of specific microcystin synthetase gene regions

Reliable cyanotoxin monitoring in water reservoirs is difficult because of, among other reasons, unpredictable changes in cyanobacteria biomass, toxin production, and inadequate sampling frequency. Therefore, it would be useful to identify potentially microcystin-producing strains of cyanobacterial populations in field samples. With this aim, we developed a methodology to distinguish microcystin-producing from non-producing Microcystis strains by amplifying six characteristic segments of the microcystin synthetase mcy cluster, three corresponding to the nonribosomal peptide synthetase, genes mcyA, mcyB, and mcyC, and three to the polyketide synthase, genes mcyD, mcyE, and mcyG. For this purpose five new primer sets were designed and tested using purified DNA, cultured cells, and field colonies as DNA sources. Simultaneous amplification of several genes in multipex PCR reactions was performed in this study. The results obtained showed that: (i) the expected specific amplicons were obtained with all microcystin-producing strains but not with nonproducing strains; (ii) cells could be directly used as DNA templates, 2000 cells being a sufficient number in most cases; (iii) simultaneous amplification of several gene regions is feasible both with cultured cells and with field colonies. Our data support the idea that the presence of various mcy genes in Microcystis could be used as a criterion for ascribing potential toxigenicity to field strains, and the possibility of applying whole-cell assays for the simultaneous amplification of various genes may contribute significantly to simplifying toxigenicity testing.     

Dog Poisonings Associated with a Microcystis aeruginosa Bloom in the Netherlands

In early autumn 2011, three dogs died after they had been exposed to a Microcystis aeruginosa bloom on Lake Amstelmeer, The Netherlands. The cyanobacterial scum from the lake contained up to 5.27 × 10³μg g⁻¹ dry-weight microcystin, the vomit of one of the dogs contained on average 94 µg microcystin g⁻¹ dry-weight. In both cases, microcystin-LR was the most abundant variant. This is the first report of dog deaths associated with a Microcystis bloom and microcystin poisoning in The Netherlands.     

A new morphospecies of Microcystis sp. forming bloom in the Cheffia dam (Algeria): seasonal variation of microcystin concentrations in raw water and their removal in a full-scale treatment plant

Toxic cyanobacterial blooms are an increasing problem in Algeria. The production of cyanotoxins (microcystins) and their presence in drinking water represent growing hazards to human health. In this study, seasonal variations in the concentrations of total microcystins and physicochemical parameters (pH, temperature, dissolved oxygen, nitrate, orthophosphate, and chlorophyll-a) were analyzed in the Cheffia dam (Algeria), mainly used to supply drinking water. The removal of cyanobacterial cells and microcystins was also evaluated in full-scale plant associated with the Cheffia reservoir. The levels of microcystins (MCYSTs) in both raw and drinking water were evaluated using the protein phosphatase type 2A (PP2A) inhibition test as MCYST-LR equivalents. Identification of microcystin variants was achieved by LC/MS/MS. During the period of study (March-December 2004), microscopic observation showed the dominance in the autumn months (September-November) of a new morphospecies of Microcystis sp. The MCYST-LR equivalent concentrations in raw water varied between 50.8 and 28,886 ng L(-1). The highest level of toxins was observed in October 2004 and was significantly correlated with the chlorophyll-a. Three variants of microcystins assigned as microcystin-YR (MCYST-YR), microcystin-LR (MCYST-LR), and 6Z-Adda stereoisomer of MCYST-LR were observed in the crude extract of the Microcystis sp. bloom sample. During the bloom period, total elimination of Microcystis sp. and toxins were achieved through a classical treatment plant comprised of coagulation and flocculation, powdered activated carbon at 15 mg L(-1), slow sand filtration and chlorination before storage.     

Toxicity of microcystin-LR, a cyanobacterial toxin, to multiple life stages of the burrowing mayfly, Hexagenia, and possible implications for recruitment

Burrowing mayflies, genus Hexagenia, were extirpated from the major water bodies of North America in the early 1950s, paralleling an increase in eutrophication and organic pollution, and a decrease in dissolved oxygen concentrations. Burrowing mayflies recolonized the western basin of Lake Erie, but remain absent in other former habitats such as Oneida Lake, New York. Eutrophication is commonly associated with a shift in the phytoplankton community toward dominance by cyanobacteria, and therefore, one class of cyanobacterial toxins, microcystins, were investigated as a contributing factor to Hexagenia's eradication or as an impediment to recolonization. Laboratory experiments were conducted to determine if microcystin-LR (MC-LR) produced negative effects on Hexagenia at three points within its life cycle: egg, hatchling nymph (<24-h old, <1 mm total length), and pre-emergence nymph (>17 mm). Treatment concentrations ranged from the guideline set by the World Health Organization for drinking water (0.001 microg mL(-1)) to 0.1 microg mL(-1) for the egg experiment and 10 microg mL(-1) for the nymph trials. Eggs showed a delay in hatching and an altered distribution of hatching over the study period when submerged in 0.1 microg mL(-1) MC-LR (an elevated concentration representative of bloom scum). The 72-h (1.1 microg mL(-1)) and 96-h (0.049 microg mL(-1)) LC(50) values for hatchling nymphs exceeded typical bloom concentrations of North American lakes, (0.01 microg mL(-1)). Large nymphs were more tolerant of the toxin, as indicated by 100% survival over seven days exposure to 10 microg mL(-1), suggesting older larvae can withstand brief encounters with high microcystin levels for at least short periods of time. The sensitivity of young nymphs and eggs to MC-LR may have implications for the recruitment of the genus in water bodies with persistent summer cyanobacterial blooms.     

Effects of cyanobacteria bloom extract on some parameters of immune function in mice

Microcystis aeruginosa is a common cyanobacterium in water blooms that appear world widely in eutrophic freshwaters, and its toxic blooms have caused many death and illness cases. This paper presents the first data on the immunotoxicity of a microcystin (MC) extract of cyanobacteria bloom collected from Taihu Lake, China to BALB/c mice. The cyanobacteria bloom extract (CBE) containing MCs was administered by i.p. injection for 14 days at three sublethal doses of 16, 32, 64 mg lyophilized algae cells/kg body weight. Exposure to CBE decreased body weights dose-dependently. Meanwhile, liver body weight ratios were markedly increased. The significant differences were also observed in spleen and thymus body ratios upon the elevation of treatment dose comparing to control. CBE was also found to reduce the phagocytosis evaluated using phagocytic index of peritoneal phagocyte; this suppression was not evident in percentage phagocytosis. Treatment of CBE produced the inhibition of lipopolysaccharide-induced lymphoproliferation and the dose-dependent decrease of the numbers of antibody-forming cells in mice that were immunized by using T-dependent antigen sheep red blood cells. However, CBE did not affect concanavalin A-induced T cell proliferation. Our results demonstrate that exposure to CBE resulted in immunosuppression in mice.     

Biochemical and ultrastructural changes of the liver and kidney of the phytoplanktivorous silver carp feeding naturally on toxic Microcystis blooms in Taihu Lake, China.

Many experimental studies have documented the impact of microcystins (MC) on fish based on either intraperitoneal injection, or oral gavaging via the diet, but few experiments were conducted by MC exposure through natural food uptake in lakes. In this study, the phytoplanktivorous silver carp were stocked in a large pen set in Meiliang Bay of Taihu Lake where toxic Microcystis blooms occurred in the warm seasons. Fish samples were collected monthly and MC concentrations in liver and kidney of the fish were determined by LC-MS. The maximum MC concentrations in liver and kidney were present in July when damages in ultrastructures of the liver and kidney were revealed by electron microscope. In comparison with previous studies on common carp, silver carp showed less damage and presence of lysosome proliferation in liver and kidney. Silver carp might eliminate or lessen cell damage caused by MC through lysosome activation. Recovery in the ultrastructures of liver and kidney after Microcystis blooms was companied with a significant decrease or even disappearance of MC. Catalase and glutathione S-transferase in liver and kidney of silver carp during Microcystis blooms were significantly higher than before and after Microcystis blooms. The high glutathione pool in liver and kidney of silver carp suggests their high resistance to MC exposure. The efficient antioxidant defence may be an important mechanism of phytoplanktivorous fish like silver carp to counteract toxic Microcystis blooms.     

Temporal variabilities of the concentrations of intra- and extracellular microcystin and toxic Microcystis species in a hypertrophic lake,Lake Suwa,Japan (1991–1994)

Temporal variability in the concentration of toxic heptapeptide microcystin was studied during the warm season of four years (1991–1994) in a hypertrophic lake (Lake Suwa) in central Honshu, Japan. Lake water samples (ca. 5 L) were filtered to separate intracellular microcystin (cell fraction) from extracellular microcystin (filtered lake water fraction). These fractions were analyzed to measure the total quantity of microcystin in lake water. Total amounts of extra- and intracellular microcystin were measured with high performance liquid chromatography. Concentrations of intracellular microcystin usually exceeded concentrations of extracellular microcystin (24 out of 26 times). High concentrations of intracellular microcystin were found during the exponential growth phase of the blooms, whereas concentrations of extracellular microcystin were highest at the end of the blooms. However, concentrations of extracellular microcystin remained very small (<4μg/L) compared to the levels of intracellular microcystin. The relatively higher percentages of microcystin in filtered lake water ( > 20%) at the end of blooms suggests that release of microcystin from cells occurs during senescence and the decomposition period of Microcystis cells. © 1998 John Wiley & Sons, Inc. Environ Toxicol Water Qual 13 : 61–72, 1998     

Effects of arsenate on microcystin content and leakage of Microcystis strain PCC7806 under various phosphate regimes

Both arsenic pollution and eutrophication are prominent environmental issues when considering the problem of global water pollution. It is important to reveal the effects of arsenic species on cyanobacterial growth and toxin yields to assess ecological risk of arsenic pollution or at least understand naturally occurring blooms. The sensitivity of cyanobacteria to arsenate has often been linked to the structural similarities of arsenate and phosphate. Thus, we approached the effect of arsenate with concentrations from 10(-8) to 10(-4) M on Microcystis strain PCC7806 under various phosphate regimes. The present study showed that Microcystis strain PCC7806 was arsenate tolerant up to 10(-4) M. And such tolerance was without reference to both content of intra- and extra-cellular phosphate. It seems that arsenate involved the regulation of microcystin synthesis and cellular polyphosphate contributed to microcystin production of Microcystis responding to arsenate, since there was a positive linear correlation of the cellular microcystin quota with the exposure concentration of arsenate when the cells were not preconditioned to phosphate starvation. It is presumed that arsenate could help to actively export microcystins from living Microcystis cells when preconditioned to phosphate starvation and incubated with the medium containing 1 microM phosphate. This study firstly provided evidence that microcystin content and/or release of Microcystis might be impacted by arsenate if it exists in harmful algal blooms.     

Genetic diversity of the toxic cyanobacterium Microcystis in Lake Mikata

The aim of the present study was to clarify the bloom dynamics and community composition of hepatotoxin microcystin-producing and non-microcystin-producing Microcystis genotypes in the environment. In Lake Mikata (Fukui, Japan) from April 2003 to January 2004, seasonal variation in the number of cells with microcystin (mcy) genotypes and the genetic diversity of the total population were investigated using quantitative competitive PCR and a 16S rDNA clone library, respectively. Using competitive PCR, cells with mcyA genotypes were quantified in August and October, and the ratio of the number of these mcyA genotypes to colony-forming Microcystis cells was 0.37 and 2.37, respectively. The 16S rDNA clones obtained could be divided into 12 ribotypes: a-l. Sixty-one Microcystis strains isolated from Lake Mikata during the sampling period were subjected to toxicity tests using HPLC and ELISA, PCR-based detection of the mcyA gene, and sequence analysis of the 16S rDNA. All isolates could be differentiated into 11 ribotypes (a, b, d, f, h, i, and m-q). Ribotypes b, f, i, m, n, and p had at least one strain that was a microcystin producer. In natural communities ribotypes b and f accounted for 85% of the 16S rDNA clones in August, and ribotypes b and i accounted for 24% of the clones in October. Thus, in some bloom stages the presence of microcystin genotypes identified using the 16S rDNA clone library correlated with that of mcy genotypes determined using competitive PCR.     

Effects on human health of some toxic cyanobacteria (blue-green algae) in reservoirs,lakes, and rivers

The public water supply has been implicated in gastrointestinal disorders in the United States, East Africa, and Australia, associated with the lysis of heavy blooms of cyanobacteria. The causative organisms include Microcystis, Anabaena, Aphanizomenon, and Oscillatoria. Using epidemiological techniques, we have shown significant increase in liver enzymes in blood, particularly γ-glutamyl transpeptidase, during a toxic bloom in the reservoir of a city. Therefore, public health awareness of the risks, and consequent action to control water blooms and remove harmful compounds from drinking water supplies, is required.     

Health of herring gulls (Larus argentatus) in relation to breeding location in the early 1990s. I. Biochemical measures

Tissues of 156 adult herring gulls (Larus argentatus) were sampled in the early 1990s from 11 colonies throughout the Laurentian Great Lakes and 2 reference colonies in Lake Winnipeg and the Bay of Fundy. Gulls from 1 or more Great Lakes differed from Lake Winnipeg or the Bay of Fundy for 17 of 19 clinical biochemical measures, whereas the freshwater and marine reference sites differed in only 3. Three differed with sex. There was little evidence to suggest that these differences reflect genotypic differences. Plasma thyroxine, albumin, calcium, magnesium, inorganic phosphorus, triglyceride, bile acids, total protein, uric acid, and urea nitrogen concentrations and aspartate aminotransferase (AST) activity were lower in gulls from one or more Great Lakes than for gulls from one or both reference sites, while those for globulins and glucose were higher. Highly carboxylated porphyrins accumulated in the livers of Great Lakes gulls and ethoxyresorufin O-deethylase (EROD) activity was induced. There was resistance to PCB/TCDD-induced EROD induction in the Lake Erie colonies. Gulls from five colonies were unable to obtain adequate food to maintain average body condition. Body condition was associated with seven biochemical measures. Colonies in designated Areas of Concern as well as those with high liver polychlorinated biphenyl (PCB) concentrations both differed for 50% of the biochemical measures. Associations between biochemical measures and delta15N-derived trophic position and/or contaminant levels in tissues suggest the effects may be toxicopathic responses. Associations were most frequently with PCBs and dioxin-like contaminants. The health of adult herring gulls varied with breeding location and "lifestyle" in the early 1990s, and Great Lakes gulls suffered from chemical and nutritional stressors that modulated physiological processes and endocrine function.     

Changes in concentrations of microcystins in rainbow trout, freshwater mussels, and cyanobacteria in Lakes Rotoiti and Rotoehu

Microcystin concentrations in cyanobacteria and their accumulation in rainbow trout (Oncorhynchus mykiss) and freshwater mussels (Hyridella menziesi) in Lakes Rotoiti and Rotoehu (New Zealand) were investigated. Hatchery rainbow trout were added to an enclosure in Lake Rotoiti where concentrations of microcystins in the phytoplankton and cyanobacterial cell concentrations could be closely monitored. Rainbow trout that were free to roam in the entire area of each lake were also included in the study. Freshwater mussels were suspended subsurface in cages in the enclosure. Phytoplankton samples, rainbow trout liver and muscle tissue, and the tissues of mussels were analyzed for microcystins using the ADDA-ELISA method, and selected samples were analyzed using LC-MS. A maximum concentration of microcystins in the phytoplankton samples of 760 microg L(-1) was recorded in Te Weta Bay, Lake Rotoiti, in March 2004. ELISA results confirmed microcystin immunoreactivity in rainbow trout liver and muscle tissues and in freshwater mussels. The microcystin congeners LR, YR, RR, AR, FR, LA, and WR were detected by LC-MS in caged freshwater mussels in Lake Rotoiti but were not detected in either muscle or liver tissue of rainbow trout. The daily tolerable intake limit of microcystins for human consumption recommended by the World Health Organisation is 0.04 microg kg(-1) day(-1). Modeling was carried out for the human intake of microcystin compounds from rainbow trout muscle tissue, and the potential health risks were estimated, assuming the ADDA-ELISA was determining compounds of toxicity equivalent to microcystin-LR.