Mesocosm experiment
The mesocosm experiment was conducted to simulate three-level food
chains (algae-zooplankton-shrimp) at the Taihu Laboratory for Lake
Ecosystem Research, located at the eastern side of Taihu Lake (31︒03’N,
120︒42’E). Lake seston was used as basal resources to allow a regional
assortment of phytoplankton species representative of local water
bodies. We implemented a factorial design by crossing three levels of
nutrient supply (HP, total phosphorus at 0.08 mg L-1;
MP, total
phosphorus
at 0.058 mg L-1; LP, total phosphorus at 0.028 mg
L-1, Fig. S1) with three types of zooplankton
communities (Daphnia magna + Daphnia brachyurum ,Sinocalanus dorrii + D. brachyurum , and D. magna +S. dorrii + D. brachyurum ). Nutrient manipulation was
achieved by adding K2HPO4 solutions (0.5
mol L-1) to 150 L tanks that contained mixtures of
lake water and aerated tap water (at the proportion of 1:1). Tanks
receiving no P additions represents the LP treatments with total
phosphorus concentration at 0.028 mg L-1 (Fig. S1c).
In addition, KNO3 solutions were added to all tanks to
compensate for the dilution effects of tap water (Fig. S1d-f).
Zooplankton communities were manipulated by manually adding pre-cultured
zooplankton species (see details in SI Appendix 1 ) to each tank
at a total density of around 30 ind. L-1. Among these
species, D. magna possesses larger body size, slower locomotion
(thus lower anti-predation ability), and higher nutrient concentrations
than the other two species (Sterner et al. 1992). We therefore
expected D. magna to be higher-quality food for predators. The
inclusion of D. brachyurum in all treatments was to prevent the
extinction of zooplankton communities due to predator exploitation.
Before filling the tanks, animals originally contained in lake water
were eliminated via repeatedly screening through a 15 µm mesh. For each
of the nine treatments (3 nutrient levels×3 zooplankton communities), we
created six mesocosms, half with and the other half without predators
(i.e., two individuals of juvenile shrimp Exopalaemon Modestus )
in order to quantify the strength of trophic cascades (He et al.2021). Therefore, a total of 54 experimental units (i.e., 3 nutrient
levels × 3 zooplankton community compositions × 2 predator treatments ×
3 replicates) were created. All the mesocosms were fixed in a pond to
ensure a relatively constant ambient temperature and checked three times
a day to remove unexpected allochthonous materials. The experiment
lasted for 6 weeks from 14 July to 25 August, 2021.