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.