S2 A detailed description of methods for zooplankton enumeration and biomass calculation.
For crustacean identification and enumeration, successive sub-samples representing between
0.5% and 5% of the total sample (depending on sample density) were removed with a wide-bore
pipette and examined under a stereo microscope at ~40 magnification. When necessary,
specimens were dissected and examined further under a compound microscope at 100-400
magnification. Crustacean zooplankton were identified to species when possible, using keys in
Brooks (1959), Smith and Fernando (1978), Czaika (1982), De Melo and Hebert (1994), and
Hebert (1995), but following taxonomy of Taylor et al. (2002) for Bosminidae and Hudson et al.
(1998) for Cyclopoida. Immature copepodite stages could usually only be identified to order, but
because all samples were dominated by one cyclopoid and one calanoid species (Diacyclops
thomasi and Leptodiaptomus minutus), immature copepodite stages of the appropriate order were
assigned to these species for purposes of biodiversity calculations, unless assignment to another
species was possible (e.g., late-stage cyclopoid copepodites; Epischura lacustris copepodites).
Copepod nauplii were enumerated but not identified. Each sub-sample was counted in its
entirety, but after at least 50 individuals of any given taxon (named species or immature stage)
had been counted, that taxon was not counted in subsequent sub-samples. A total of at least 250
individuals (never more than 265) were counted in each sample, with the restriction that no
individual taxon could count for more than 40 of the 250 total (regardless of how many
individuals of that taxon were actually enumerated). This restriction reduced the apparent
abundance of dominant species in the counted sample, thus increasing the chances of
encountering and identifying rare species, but did not distort calculations of actual relative
abundance or density.
For each crustacean taxon in each sample, the first 15 individuals encountered (only the first
ten for adult copepod species) were measured using an eye-piece micrometer. Biomass was
calculated based on these measured lengths and published length-weight regressions (McCauley
1984; Culver et al. 1985; Lawrence et al. 1987; Yan and Mackie 1987, in order of increasing
preference). The length-weight regression for Senecella calanoides was provided by W.G.
Sprules (personal communication).
Rotifers were counted following a similar protocol to that used for crustaceans. Using a
Sedgwick-Rafter cell under a compound microscope at 100 magnification, we enumerated
successive sub-samples representing between 0.2% and 2% of the total sample, until a total of at
least 200 individuals had been counted (never more than 220), with no single taxon accounting
for more than 40 of the 200 total. Rotifers were identified to species when possible (but
frequently only to genus) following Edmondson (1959) and Stemberger (1979), but were not
measured. Rotifer biomass was calculated based on published values (Stemberger and Gilbert
1987; Yan et al. 1991) or by estimating volume from dimensions given by Stemberger (1979)
and formulae of Ruttner-Kolisko (cited in McCauley 1984) and assuming a specific gravity of 1
and a dry:wet weight ratio of 0.1.
Preserved colonist aliquots were counted based on a fixed volume, rather than a fixed
number of individuals. We counted crustaceans from a sample volume equivalent to 1% of the
enclosure volume (79 L), and rotifers from a volume equivalent to 0.25% of the enclosure
volume (20 L). This greater counting effort was intended to give a more accurate picture of
colonist numbers and identities and to compensate for the lack of replicate colonist samples. We
also scanned the enclosure samples from the final week of the experiment in their entirety to
better detect rare species.
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