Experimental design
The experiment was conducted at the LEAP (Large Experimental Array of
Ponds) facility at Gault Nature Reserve of McGill University in Quebec,
Canada (45° 32’ N, 73° 08’ W), (Fugère et al. 2020) (Fig. 1). 48 large
mesocosms (surface area=2.43m2) were filled each with
500L of water piped from Lac Hertel, a mesotrophic lake on the reserve,
1km upstream of the experiment. The water was sieved to remove fish,
tadpoles, macroinvertebrates, and macrophytes, but contained intact
communities of zooplankton and phytoplankton. Material from the source
community was used to assemble four community types defined by the
initial relative abundance of each species (10%, 20%, 30% or 40%)
(Table 1). Relative abundance was calculated as mass-weighted
frequencies using an average value of individual mass for each species.
Each community was seeded with a total of 1g wet mass of community
biomass, which works out to between ~2,000-3,000
individuals, depending on the community type (Table 1). Abundances of
the larger species were determined by manual counting, while Wc, only
~0.5mm wide, was weighed and added in bulk, using an
estimate of mean frond mass. Each of the four community types were
replicated in 12 mesocosms (total number of mesocosms = 48) which were
arranged in six blocks of eight mesocosms, each block containing two
replicates of each community type, with community type randomized within
block. All mesocosms received a one-time initial addition of inorganic
Nitrogen and Phosphorus (KNO3 and
H2KPO4), to obtain initial dissolved
concentrations of these nutrients in the mesocosms comparable to those
of the pond from which the source community was taken (800
µgL-1 N and 40 µgL-1 P). The
mesocosms were covered with 70% shade cloth to mimic canopy cover.
Although this minimized the input of wind-carried debris like leaf
litter, rainwater could pass through the mesh cloth, which roughly
balanced water lost due to evaporation. Communities were then left to
grow for 12 weeks, from the beginning of July to the end of September,
ending shortly before the first frost. All mesocosms were randomly
sampled every two weeks to estimate species relative abundances. This
was done by first mixing the communities to break up species clustering,
then removing a fixed percentage of the surface area
(~5%) with a net. Although mixing modified the spatial
structure of the community, it allowed us to efficiently obtain
representative samples. These samples were exhaustively counted before
being returned to the mesocosm.