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The patterns of occurrence of the peptide hepatotoxin microcystin‐LR (MC‐LR) was studied in three hypereu‐trophic hardwater lakes (Coal, Driedmeat, and Little Beaver) in central Alberta, Canada, over three open‐water seasons. MC‐LR concentration was based on high‐performance liquid chromatography detection and expressed as μg.g−1 of total plankton biomass, ng.L−1 of lake water, and μg.g−1 of Microcystis aeruginosa Kuetz. emend. Elenkin. MC‐LR was highly variable temporally (differences up to 3 orders of magnitude) within each lake over an individual year, between years in an individual lake, and between lakes in any year. Seasonal (within‐year) changes in MC‐LR concentration (expressed in the preceding units) were positively correlated to the abundance and biomass Of the cyanobacterium M. aeruginosa (r =0.60–0.77), total and total dissolved phosphorus concentration (r =0.46–0.59), pH (r=0.38–0.58), and chlorophyll a (r=0.25–0.59). Surprisingly, there was no relationship between MC‐LR concentration and water temperature (range: 7°‐24°C, r =‐0.13 to 0.02) and a negative correlation with nitrate concentration (r =–0.27 to ‐0.34). In two synoptic surveys examining spatial variability, MC‐LR concentrations were quite variable (CV of 185 and 36% between sampling sites for Coal and Little Beaver lakes, respectively). Spatial distribution of MC‐LR on any one day was correlated with the abundance and biomass of M. aeruginosa. Over a 24‐h period, MC‐LR concentration in M. aeruginosa decreased more than 6‐fold at night relative to daytime concentrations. In general, analytical and within‐site variation of MC‐LR was relatively small (CV < 4 and 9%, respectively) but greatest both within and between years in a lake followed by diel and spatial variation.

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]

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.

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