Collapse of a Planktothrix agardhii perennial bloom and microcystin dynamics in response to reduced phosphate concentrations in a temperate lake.

Planktothrix agardhii dynamics, microcystin concentration and limnological variables were monitored every 2 weeks for 2 years (2004-2006) in a shallow hypereutrophic artificial lake (BNV, Viry-Châtillon, France). Time-series analysis identified two components in the P. agardhii biomass dynamics: (1) a significant decreasing trend in P. agardhii biomass (65% of the overall variance) and (2) a residual component without significant seasonal periodicity. A path-analysis model was built to determine the main factors controlling the P. agardhii dynamics over the period studied. The model explained 66% of P. agardhii biomass changes. The decreasing trend in P. agardhii biomass was significantly related to a decrease in the PO4(3-) concentration resulting from an improved treatment of the incoming watershed surface water. The residual component was related to zooplankton dynamics (cyclopoid abundances), supporting the hypothesis of a top-down control of P. agardhii, but only when the biomass was low. Forty-nine percent of the variability in the microcystin (MC) concentration (min:<0.1 microg equivalent MC-LR L(-1); max: 7.4 microg equivalent MC-LR L(-1)) could be explained by changes in the P. agardhii biomass. The highest toxin content was observed when P. agardhii biomass was the lowest, which suggests changes in the proportion of microcystin-producing and -nonproducing subpopulations and/or the physiological status of cells.