Calibration of the zooplankton community size spectrum as an indicator of change in Canadian Shield lakes

Developing the crustacean zooplankton community size spectrum into an indicator of change in lakes requires quantification of the natural variability in the size spectrum related to broad-scale seasonal, annual, and spatial factors. We used a random effects model to measure monthly, annual, and inter-lake variability in the slope and centered height of the crustacean zooplankton normalized abundance size spectrum (NASS) from 1981 – 2011 among eight Canadian Shield lakes. The slope was a relatively stable characteristic of the zooplankton community compared with the height, which varied significantly among lakes. We identified a seasonal signal in height and slope and used a mixed effects model to characterize the linear rate of change from May to October: there was an overall decline in height and an overall increase in slope. Seasonal variance was greater than annual variance for both the height and the slope, suggesting that long-term monitoring of lakes and inter-lake comparisons using zooplankton size spectra should be based on temporally standardized sampling protocols that minimize the effects of seasonal processes.