Cyanobacteria possess many adaptations to develop population maxima or "blooms" in lakes and reservoirs. A potential consequence of freshwater blooms of many cyanobacterial species is the production of potent toxins, including the cyclic hepatotoxins, microcystins (MCs). Approximately 70 MC variants have been isolated. Their toxicity to humans and other animals is well studied, because of public health concerns. This review focuses instead on the production and degradation of MCs in freshwater environments and their effects on aquatic organisms. Genetic research has revealed the existence of MC-related genes, yet the expression of these genes seems to be regulated by complex mechanisms and is influenced by environmental factors. In natural water bodies, the species composition of cyanobacterial communities and the ratio of toxic to nontoxic species and strains are largely responsible for total toxin production. Cyanobacteria play vital roles in aquatic food webs, yet production, accumulation, and toxicity patterns of MCs within aquatic food webs remain obscure.

Although the cyanobacterial toxin microcystin has been detected in Canadian fresh waters, little is known about its prevalence on a national scale. Here, we report for the first time on microcystin in 246 water bodies across Canada based on 3474 analyses. Over the last 10 years, microcystins were detected in every province, often exceeding maximum guidelines for potable and recreational water quality. Microcystins were virtually absent from unproductive systems and were increasingly common in nutrient-rich waters. The probable risk of microcystin concentrations exceeding water quality guidelines was greatest when the ratio of nitrogen (N) to phosphorus (P) was low and rapidly decreased at higher N:P ratios. Maximum concentrations of microcystins occurred in hypereutrophic lakes at mass ratios of N:P below 23. Our models may prove to be useful screening tools for identifying potentially toxic “hotspots” or “hot times” of unacceptable microcystin levels. A future scientific challenge will be to determine whether there is any causal link between N:P ratios and microcystin concentrations, as this may have important implications for the management of eutrophied lakes and reservoirs.

Abstrsd: Micmcystin-LR (MC-LR) wncen&&ns were examined in water, phytoplankton, invertebrates, and two fishes for up to 3 years in four cmtral Alberta lakes spanning a tmphic gradient in total phosphorus from I5 to over 500 pgL-' in epilinmetic waters MC-LR was not detected by HPLC in phytoplankton tinm an oligo-mesotmphic lake, was less than 150 ng cellular totiml-' of lake water in a eutmphic-bypereatmphic lake, and was up to M)(H) and I I 000 ngL-' in two hypenubophic lakes. MC-LR in phytoplankton was strongly correlated with the abundance of the cyanobacterium Micm~stir oeruginosa, and with aqueous micmcystin concsntration. determined by protein phosphatase bioassay (r = 0.83). MC-LR was also detected in moplankton (up to 67 pgg-' ofbiomass) and MC-LR concenti~ in zooplankton and~phytoptankton were correlated (r = 0.69). Although nine groups of macroinvertebrates were analyzed, MC-LR was only detected in gastmpods (up to 120 pgg-'1. MC-LR appears te be bansferred to invertebrates through grazing activity. MC-LR was not detected in the liven ofnmthem pike (Esox luciw) and white sucker (CawfomuE mmmermni) ~llectcd from one lake containing toxinproducing phymplankton. Accumulation of MC-LR in aquatic fwd webs appears to -in the primary consumer with probable tmnsfer of the toxin to higher trophic levels. Rhm6: Pendant UM p&ode atteigmnt une d& de 3 ans, on a mesurl les concentrations de micrwystine-LX (MC-LR) dam l'eau et le phytoptancton ainsi que chez tes inv&btis et chek dew esp&s de poissons de 4 lacs du centre de I'Alber+a 06 le gradient tmphique de phosphors total Cpilim&tique allait de 15 B phrs de 500 pgL-'. La HPLC du phytoplancton du tat oIigom6sotmphe n'a pas mis en tvidence de MC-LR; la concentration de la toxine allulaire etait inf&ieae g 150 ng.L-' d'eau dam le lac eutrophe-hypereutmphe et elk atteignait 6000 et I1 000 ngL-' dam Ies deux hypereutmphes. Par aillew, on a not6 une forte cotitation entre la concentration de MC-LR phytoplmctcmique et t'abor&xe de la cyan&&de Micmcystir ~eruginma et la concenbaticnrde micnxystine aquewse, do&e avec une pmt6ine-phospbatas.c (r= 0,83). Dn a aussi d6tectC de la MC-LR chez le moplancton (jusqu? 67 pgg-' de biomassc) et observt une con&tion entre Ia concentrations de MC-LR zwptanctonique et phytoplmctonique (r = 0.69). On a anaIys& neufgmupes de macroiwe&&s. mais la MC-LR n'a 6tC detect6 que chez les gast6mpodes (jusqu'& I20 vgg-'). La MC-LR semble passeraux invert&b& par le tmutage. On "'en a pas d6tectt dam le foie du bmche @ox /ucim) et du meunier noir (Caxmmw commermnz]~r&v6s dam l'un des lacs oti le phytoplaneton est pmdwteti de toxine. La bioconcentation de MC-LR dans lea r6seaux tmphiques aquatiques sable survenir au niveau des consommatews primaires et il y a probablement tmnsfen de la toxine auxnivcaux tmphiques plus tleds. frmduit par la R&ction]

There is growing concern that harmful cyanobacterial blooms are increasing in frequency and occurrence around the world. Although nutrient enrichment is commonly identified as a key predictor of cyanobacterial abundance and dominance in freshwaters, several studies have shown that variables related to climate change can also play an important role. Based on our analysis of the literature, we hypothesized that temperature or water‐column stability will be the primary drivers of cyanobacterial abundance in stratified lakes whereas nutrients will be the stronger predictors in frequently mixing water bodies. To test this hypothesis, as well as quantify the drivers of cyanobacteria over different scales and identify interactions between nutrients and climate‐related variables, we applied linear and nonlinear mixed‐effect modeling techniques to seasonal time‐series data from multiple lakes. We first compared time series of cyanobacterial dominance to a published lake survey and found that the models were similar. Using time‐series data of cyanobacterial biomass, we identified important interactions among nutrients and climate‐related variables; dimictic basin experienced a heightened susceptibility to cyanobacterial blooms under stratified eutrophic conditions, whereas polymictic basins were less sensitive to changes in temperature or stratification. Overall, our results show that due to predictable interactions among nutrients and temperature, polymictic and dimictic lakes are expected to respond differently to future climate warming and eutrophication.

1. To assess the influence of lake trophic status on the occurrence of microcystin‐LR (MCLR) in the tissue of resident pulmonate snails, we sampled seven lakes of varying primary productivity in Alberta, Canada. Parameters associated with productivity were measured every 2 weeks from mid‐May through mid‐September 1995. Phytoplankton and resident pulmonate snails were collected and analysed for MCLR concentration via high‐performance liquid chromatography. 2. For all species of gastropod (i.e. Lymnaea stagnalis, Helisoma trivolvis and Physa gyrina), the concentration of MCLR in the tissue was correlated (P ≤ 0.03) with toxin in the phytoplankton, but not with extracellular aqueous microcystin (P > 0.28). The concentrations of toxin in the tissues of L. stagnalis and P. gyrina were also correlated with the relative abundance of Microcystis spp. (P < 0.01). 3. Given that Microcystis spp. abundance and MCLR concentration within phytoplankton are correlated with indicators of productivity, we conclude that trophic status is important in influencing the occurrence and concentration of MCLR in pulmonate snails.