PARENT SESSION
Contributed Oral Session 23: Aquatic Ecology: Plankton Communities
Monday, August 8, 1:30 PM - 5:00 PM, Meeting Room 515 A, Level 5, Palais des congrès de Montréal
Fish reintroductions reveal smooth transitions between lake community states.
Garcia, Erica*,1, Mittelbach, Gary1, Taniguchi, Yoshinori2, 1 Kellogg Biological Station/MSU, Hickory Corners, MI2 Yamaguchi Prefectural University, Yamaguchi, Japan
ABSTRACT- The extent to which ecological communities respond smoothly versus discontinuously to changing environmental conditions has important consequences for the preservation and restoration of ecosystems. Recent theory suggests that changes in fish abundance may have dramatic, discontinuous effects on plankton community structure, resulting in abrupt transitions between alternative ecosystem states. However, there have been few opportunities to look for such transitions in natural lake systems. Here, we examine the impact of a two-order of magnitude decrease and then increase in planktivore abundance in Wintergreen lake (Michigan), caused by the extinction and reintroduction of two dominant fish species (largemouth bass, Micropterus salmoides and bluegill sunfish, Lepomis macrochirus). Over a 16+ yr period of slow change from high planktivory to low planktivory back to high planktivory, the zooplankton community changed smoothly and predictably between states. In years of low planktivory, the zooplankton community was dominated by a single, large cladoceran species, Daphnia pulicaria, whereas in years of high planktivory, D. pulicaria disappeared and was replaced by a suite of small-bodied cladocerans. Intermediate planktivory was characterized by a seasonal succession in dominance from D. pulicaria to D. dentifera. Planktivore density accounted for 
80% of the year-to-year variance in D. pulicaria abundance and seasonal duration in the water column. Other measures of pelagic community structure, including zooplankton diversity, zooplankton size-structure, and water clarity, also changed gradually and predictability with planktivore density. These results show little evidence of a hysteretic pattern or alternative stable states in this system where the transition between pelagic community states is strongly driven by planktivory.
Key words: alternative stable states, zooplankton community structure, planktivory, species introductions