LAUSR

Concentrations of phycocyanin, a pigment of Cyanobacteria, were measured at 1-min intervals during the icefree seasons of 2008–2018 by automated sensors suspended from a buoy at a central station in Lake Mendota, Wisconsin, U.S.A. In each year, stochastic-dynamic models fitted to time series of log-transformed phycocyanin concentration revealed two alternative stable states and random factors that were much larger than the difference between the alternate stable states. Transitions between low and high states were abrupt and apparently driven by stochasticity. Variation in annual magnitudes of the alternate states and the stochastic factors were not correlated with annual phosphorus input to the lake. At daily time scales, however, phycocyanin concentration was correlated with phosphorus input, precipitation, and wind velocity for time lags of 1–15 d. Multiple years of high-frequency data were needed to discern these patterns in the noise-dominated dynamics of Cyanobacteria. Blooms of harmful Cyanobacteria have adverse effects on water quality, survival of aquatic animals, and human health in lakes, reservoirs, and coastal oceans (Carmichael and Boyer 2016; Huisman et al. 2018). Blooms are associated with high inputs of nutrients interacting with climate change (Michalak et al. 2013). Growing demand for food and a shift toward more *Correspondence: [email protected] Associate editor: Monika Winder Author Contribution Statement: The study was conceived by S.R.C., E.H.S., and P.C.H., the automated sensor system was designed by P.C.H., modeling was conceived and conducted by S.R.C., B.M.S.A., M.S., and E.V.N., and the paper was written by all authors. Data Availability Statement: Data are available in the North Temperate Lakes Long-Term Ecological Research database (http://lter.limnology. wisc.edu). Additional Supporting Information may be found in the online version of this article. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.