Zooplankton provide early warning of a regime shift in a whole lake manipulation

Background/Question/Methods

Regime shifts are rapid and dramatic transitions to alternate ecosystem states.  While regime shifts are difficult to predict, statistical indicators may provide early warnings.  Specifically as critical thresholds are approached, the variance and autocorrelation of time series often indicate “critical slowing down” (i.e. slower return rate to baseline conditions).  We conducted a four year lake manipulation to test for early warnings prior to a food web transition by adding largemouth bass (Micropterus salmoides) to a planktivore-dominated lake.  We measured zooplankton biomass daily in a reference and manipulated lake.  We calculated variance and autocorrelation of the zooplankton time series using 28-day rolling windows.  We asked if the variance in the manipulated lake (relative to the reference lake) increased and if autocorrelation approached one prior to the food web regime shift.

Results/Conclusions

Additions of largemouth bass to the manipulated lake moved the system toward piscivore dominance with cascading effects on the food web.  The variance and autocorrelation of zooplankton biomass was similar between the two lakes in the first two years of the manipulation.  During the third year, variance was much higher and autocorrelation approached one for sustained periods in the manipulated but not in the reference lake.  The statistical indicators were similar between the two lakes during the fourth year as the food web transition moved to completion.  The joint response of variance and autocorrelation in year 3 provided an early warning of the food web transition consistent with the theoretical expectation of critical slowing down prior to regime shift.  This result supports theory that early warnings are detectable prior to regime shifts and that simple indicators like zooplankton biomass might be useful indicators for ecosystem management.