Population dynamics of harmful algal blooms in Lake Champlain: A tale of two phases

Understanding the dynamics of harmful algal blooms (HABs) in lakes can inform management strategies to reduce their economic and health impacts. Previous studies have analyzed spatially replicated samples from a single time or have fit phenomenological models to time series data. We fit mechanistic population models to test the effects of critical nutrient concentrations and the density of potential algal competitors on population growth parameters in HABs in Lake Champlain, U.S.A. We fit models to five years (2003-2006, 2008) of weekly cyanobacteria counts. Plankton dynamics exhibited two phases of population growth: an initial “bloom phase” of rapid population growth and a subsequent “post-bloom phase” of stochastic decline. Population growth rates in the bloom phase were strongly density dependent and increased with increasing TN:TP ratios. The post-bloom phase was largely stochastic and was not obviously related to nutrient concentrations. Because TN:TP was important only in the initial phase of population growth, correlative analyses of the relationship between cyanobacteria blooms and nutrient concentrations may be especially sensitive to when snapshot data are collected. Limiting nutrient inputs early in the season could be an effective management strategy for suppressing or reducing the bloom phase of cyanobacteria population growth.