Climate change stressors including warming and heatwaves can alter zooplankton composition and dominance patterns in shallow lakes, which can disrupt ecosystem function and curtail ecosystem services. To understand such changes, we performed a mesocosm experiment with controls reflecting the present temperature conditions and a treatment reflecting a future climate change scenario, including heatwaves of 0-8°C. In the future climate scenario, the predatory invertebrate, Mesostoma exerted a strong top-down control particularly on Daphnia, resulting in a switch in the herbivore dominance to Ceriodaphnia. Cyclopoid copepods were the least affected taxa but showed tendencies to sustain longer into the winter at elevated temperatures. A complementary predation experiment revealed that Mesostoma feed at a higher rate on Daphnia than on Ceriodaphnia and cyclopoid copepods. In addition to the food-chain alterations, the algal biomass and cyanobacteria increased with warming which has considerable implications for management of shallow lakes.

12 outdoor mesocosm (0.7 m diameter and 1 m height) were set up at Lund University in April 2020 and two treatments were applied (n = 6) from 14th of July to 13th July 2021. Each mesocosm contained 400 L water and 500g of homogenized sediments from lake Ringsjön, southern Sweden (55°51′43.2″N, 13°32′24″E). The controls were mirroring present environmental conditions (‘Present’) and the heated treatments, the future climate warming scenarios (‘Future’), which included heatwaves from 0°C to 8℃ above ambient temperatures (average 4℃) (IPCC, 2013). Temperature in each mesocosm was recorded every 10 sec using loggers. 

We sampled zooplankton populations every two weeks. Water was collected using a Plexiglas™ tube (length = 1 m, diameter = 70 mm, volume = 3 L) from three locations across the diameter of each mesocosm (2 closer to the sides and 1 in the middle). All 9 L of water were pooled, mixed in a bucket, and 5 L of that water was filtered using nylon mesh of 55 μm pore size. The organisms collected on the mesh filter were rinsed into a 100 ml transparent bottle, preserved with Lugol's acid solution, and stored at 4°C for later enumeration. Copepods were identified and counted to order level, whereas, cladocerans and other larger invertebrates were identified and counted to genus level at 32× magnification using a stereomicroscope (Olympus SZ‐40). We also took water samples directly from the surface of each mesocosm and then immediately analyzed them for total chlorophyll a (Chl a) on a fluorometer (AlgaeLabAnalyser, bbe moldaenke, Schwentinental, Germany). The total concentration of major phytoplankton groups, namely, green algae, cyanobacteria, cryptophytes, and diatoms were also measured.

Mesostoma predation rate on Daphnia magna, Ceriodaphnia spp., and cyclopoid copepods were determine following the procedure suggested by Lehman and Sandgren (1985). For each prey species, 500 ml tap water [temperature: 19 to 21°C] and 10 individuals of prey were placed into six glass jars. The number of Mesostoma individuals (3 to 4 mm) then added increased sequentially from 1 to 6 in each jar. The experiment lasted either for 24 hrs. or until there were only 1-2 individuals of prey remaining in each jar. After that, Lugol's acid solution was added to each jar to stop the experiment and the remaining prey individuals in each jar were counted.

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