MICROBIOLOGY OF AQUATIC SYSTEMS Seasonal and Episodic Lake Mixing Stimulate Differential Planktonic Bacterial Dynamics Ashley Shade & Chih-Yu Chiu & Katherine D. McMahon Received: 21 May 2009 / Accepted: 1 September 2009 / Published online: 18 September 2009 # Springer Science + Business Media, LLC 2009 Abstract 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 commu- nities 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. Introduction Disturbance remains one of the most simultaneously provocative and extensively published subject matters in ecology. Historically, disturbance has been debated as both a stimulant and repressor of diversity [1], a mechanism of community resetting and an initiator of succession [2], and a passive accomplice of regional immigrants or nonnative species for community invasion [3]. These few examples are but a subset of possible disturbance roles in communities or ecosystems. The importance of disturbance in structuring communities or ecosystems is well established. A common disturbance definition is the periodic disruption of ecosystem, community, or population structure, or changes in resource availability or the physical environment [4, 5]. Disturbances are characterized by intensity, frequency, or periodicity. In this study, our focus was on the temporal components of disturbance, and more specifically, the differences in bacterial community response to seasonal disturbances versus those that are episodic and across disturbance intensities. Seasonal and climatic disturbances A. Shade (*) Microbiology Doctoral Training Program, University of Wisconsin-Madison, Microbial Sciences Building, 1550 Linden Drive, Madison, WI 53706, USA e-mail: shade@wisc.edu C.-Y. Chiu Research Center for Biodiversity, Academia Sinica, Taipei, Taiwan 11529 K. D. McMahon Department of Bacteriology, University of Wisconsin-Madison, 3204 Engineering Hall, 1415 Engineering Drive, Madison, WI 53706-1691, USA K. D. McMahon Department of Civil and Environmental Engineering, University of Wisconsin-Madison, 3204 Engineering Hall, 1415 Engineering Drive, Madison, WI 53706-1691, USA Microb Ecol (2010) 59:546–554 DOI 10.1007/s00248-009-9589-6