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Abstract Aquatic heatwaves are increasing in frequency, intensity, and duration worldwide. While increases in mean water temperatures are linked to enhanced phytoplankton biomass, it is unclear how heatwaves alter phytoplankton dynamics in lakes at an ecosystem scale. We investigated changes in surface chlorophyll during 29 summer heatwaves between 2008 and 2019 in 3 north temperate lakes. These lakes vary in staining and were either references or manipulated with nutrients and top predator additions. The manipulations provided a variety of nutrient, grazing, and light conditions during heatwave and non‐heatwave conditions. Surface chlorophyll concentrations increased during 24 out of 29 heatwaves. In the low‐nutrient reference lake the mean increase in chlorophyll was 57% while in the two experimental lakes the mean increases were 127% and 183%. Overall, the effects of the whole‐lake experiments were variable but still provided context for possible patterns amid a diverse set of food web and nutrient conditions. 

Abstract Resilience, measured by the distribution of passage times between alternate states, indicates persistence of a state in stochastic dynamic systems such as blooms of cyanobacteria in lakes. We used high‐frequency datasets to compare the resilience of low and high states of phycocyanin, a pigment indicator of cyanobacteria, in Lake Mendota, Wisconsin, USA, for three growing seasons that ranged sevenfold in external phosphorus (P) load. Each year we observed 139–265 passage times across the unstable threshold that separated the low‐ from high‐phycocyanin states. Each sample of passage times is highly skewed with low median, larger mean, much larger SD, and wide tails extending to long lifetimes of a state. About 25% of events, whether low or high phycocyanin, lasted a day or more. Among these 3 years of contrasting external P load, there were no discernible differences in the resilience of either ecosystem state. We attribute this lack of contrast to the sustained recycling of P from sediments and the high stochasticity of phycocyanin in this lake.