Seasonal and episodic lake mixing stimulate differential planktonic bacterial dynamics.

Yuan Yang Lake (YYL), Taiwan, experiences both winter and typhoon-initiated mixing, and each type of mixing event is characterized by contrasting environmental conditions. Previous work suggested that after typhoon mixing, bacterial communities in YYL reset to a pioneer composition and then follow a predictable trajectory of change until the next typhoon. Our goal was to continue this investigation by observing bacterial community change after a range of mixing intensities, including seasonal winter mixing. We fingerprinted aquatic bacterial communities in the epilimnion and hypolimnion using automated ribosomal intergenic spacer analysis and then assessed community response using multivariate statistics. We found a significant linear relationship between water column stability and the epilimnion to hypolimnion divergences. In comparison to the summer, we found the winter community had a distinct composition and less variation. We divided the bacterial community into population subsets according to abundance (rare, common, or dominant) and occurrence (transient or persistent) and further explored the contribution of these subsets to the overall community patterns. We found that transient taxa did not drive bacterial community patterns following weak typhoon mixing events, but contributed substantially to patterns observed following strong events. Common taxa generally did not follow the community trajectory after weak or strong events. Our results suggest intensity, frequency, and seasonality jointly contribute to aquatic bacterial response to mixing disturbance.