Experimental setup
The mesocosm are situated at
Huazhong Agricultural University
in Wuhan City, Central China (30°29′N, 114°22′ E). 10 cm of sediment was
added which was collected from the top few cm of sediment in Lake
Liangzi (N 30°11′3″, E 114°37′59″; sediment TN, 5.5 ± 0.4 mg
g-1 and TP, 0.42 ± 0.08 mg g-1, dry
weight, n = 5)), and was thoroughly mixed in a clean container before
transferring it into the experimental containers. On top of this a layer
35 cm of tap water was added (TN, 2.438 ± 0.249 mg
L-1; TP, 0.022 ± 0.006 mg L-1, n =
5). Each container received 12 V. spinulosa (shoot length:12.1 ±
2.0 cm, mean ± SD, n = 480) and 16 P. crispus turions (length:5.0
± 0.5 cm, mean ± SD, n = 640) which were planted evenly distributed
within each container.
The treatment with shading of 75% of the incoming sunlight (S1) was
achieved by covering the containers with one layer of black sun-shading
net. The nutrient solution used for the nutrient loading treatment (E1)
was made by dissolving NaNO3 and
KH2PO4 salts in demineralized water.
Nutrients were added once a month to these E1 containers, simulating a
high-level nutrient loading of 2 mg L-1 N and 0.2 mg
L-1 P, a eutrophic state, in the range of
concentration and ratio of earlier experiments by
Coppens et al. (2016) and
Jeppesen et al. (2007). For the herbivory
treatment, adult snails (n = 5, size ranged from 1 to 3 cm) were added
to each herbivory treatment container (H1), a density of snails similar
to natural lakes around the Yangtze River
(H. J. Wang et al., 2010), and no extra
snails were added to the rest of the containers (H0). Due to snail
mortality and reproduction, and some snails even recovered from the H0
treatment, snail biomass changed largely over time in the treatments. We
thus used the final snail biomass achieved as a continuous variable to
predict the response parameters in our experiment.