Ponds are neither dimictic nor polymictic: A variable transition season between stratified summers and mixed autumns

Autumn is an important transition time for seasonally stratified lakes where summertime stratification shifts to cooler, mixed periods. While ponds can also experience prolonged summertime stratification, their relatively low water volume accelerates mixing, suggesting they could have distinctly different autumn mixing dynamics compared to lakes. This study examined the mixing dynamics of mostly temperate, small, shallow, and summertime stratified waterbodies. We show that, in contrast to most lakes, ponds have long and distinctive stratified to mixing transition seasons, characterized by repeated mixing and restratification, which can occur for days to months, before sustained isothermal conditions. Transition season mixing starts earlier in shallower and low‐nutrient ponds, with most ponds having at least one continuous week of restratification in autumn. Ultimately, describing pond transition seasons provides an important update to established turnover paradigms for lakes and indicates that these periods have the potential for high biogeochemical and ecological activity.