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). 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, and macroinvertebrates, 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.
Communities were then left to grow for 12 weeks. 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. These samples were exhaustively
counted before being returned to the mesocosm.