We explored patterns of change in bacterioplankton and phytoplankton community composition in response to typhoons in a subtropical, subalpine freshwater humic lake in Taiwan. Thermal profiles with depth, nearsurface dissolved oxygen, and meteorological conditions were measured at high temporal resolution using an instrumented buoy. We collected samples across a time series spanning two typhoon seasons and assessed bacterial community composition at each time point using a fingerprinting technique targeting the internal transcribed spacer region of the ribosomal RNA operon. Wind and rain associated with the typhoons mixed the otherwise stably stratified water column. After each of six typhoon-induced mixing events, the bacterial community composition was reset to similar conditions, revealing a surprisingly deterministic pattern of recovery that was reminiscent of secondary succession following large infrequent disturbances in terrestrial ecosystems. Distinct bacterial assemblages were associated with epilimnion and hypolimnion samples collected more than 80 d after a mixing event. These assemblages were also distinct from the communities observed in the generally unstratified water column within 7 d after a typhoon. Our results were surprising since bacterial communities are not expected to respond in such predictable ways because of their immense complexity and diversity. In contrast, phytoplankton communities did not recover in a predictable way after typhoons.

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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.

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