Appendix C. Mesocosm experiment methods.
Mackey (1992) conducted experiments on the dynamics of freshwater herbivorous zooplankton (Daphnia pulex and rotifer species), using mesocosms housed in a greenhouse and techniques developed by McCauley and Murdoch (1990). Six, 750 L tanks were filled with filtered pond water. The pond water was passed through a 70 um mesh filter, then WaterPure filters containing Rainfresh CF3 coarse grain and CF1 fine grain (< 20 um) cartridges. Three tanks were enriched with phosphorus and nitrogen following McCauley and Murdoch (1990). Tanks containing phytoplankton and rotifers were inoculated with a single clone of Daphnia pulex, and sampled twice weekly for the duration of the experiment. Four, 1 L samples were taken for zooplankton, filtered through 35 μm mesh, anaesthetized with carbonated water and preserved in a 4% formalin solution. Rotifers were identified to species where possible, and genus otherwise. A minimum of 400 individual rotifers were counted in each sample, at least 200 of which belonged to the taxon most abundant in the sample. This provides a counting precision of +/- 10% with 95% confidence (Lund et al. 1958).
To remove periods in which many taxa were at zero relative abundance (which created numerical problems for model fitting), we merged species into two groups for all time series. The first group was the most abundant species across all time series, and the second group was the sum of all remaining species. The same rotifer species was used for the first group in all time series. Dynamics were fit using the model in Eq. A7. The smoothing parameter (λ) was set at λ=10-3, which ensured uncorrelated residuals. Net and fluctuating selection were calculated from Eq. 6 (Fig. C1).
FIG. C1. Observed zooplankton proportions (points) in the mesocosm experiment of Mackey (1992), and model fits (lines). Red, most abundant rotifer species; black, sum of the rare species. Each panel shows dynamics from one representative replicate. (a) Low enrichment, (b) high enrichment. |
LITERATURE CITED
Lund, J. W. G., C. Kipling, and E. D. Le Cren. 1958. The inverted microscope method of estimating algal numbers and the statistical basis of estimations by counting. Hydrobiologia 11:143–170.
Mackey, R. L. 1992. Dynamics of competing herbivores: a test of theory using freshwater zooplankton. M.Sc. thesis, 153 pp. University of Calgary, Calgary, Canada.
McCauley, E., and W. W. Murdoch. 1990. Predator-prey dynamics in environments rich and poor in nutrients. Nature 343:455–457.