Submerged macrophytes benefit from lanthanum modified bentonite treatment under juvenile omni-benthivorous fish disturbance: Implications for shallow lake restoration

The re-establishment of submerged macrophytes is crucial in shallow lake restoration transforming a turbid into a clear water system, but it might be inhibited by juvenile omni-benthivorous fish. These fishes disturb the sediment, and even uproot new sprouts, creating turbid water through sediment resuspension and release of nutrients stored in the sediment. Lanthanum modified bentonite (LMB), an in-situ capping material, can immobilise phosphate and consolidate sediments, potentially weakening the negative effects of juvenile omni-benthivorous fish. We hypothesised that applying LMB would improve water quality and promote the growth of submerged macrophytes by eliminating or weakening the increase of suspended solids, nutrient concentration, and algal reproduction due to juvenile omni-benthivorous fish disturbance. To test our hypothesis, a two-by-two factorial mesocosm experiment with two densities of juvenile crucian carp (absence and 10 g/m³) and two doses of LMB (absence and 800 g/m²) was conducted. Juvenile crucian carp significantly inhibited the growth and reproduction of the macrophyte Vallisneria natans by reducing the underwater light and direct effects such as uprooting. No significant inhibition or promotion by LMB on the growth of V. natans in the absence of juvenile crucian carp was observed. However, LMB significantly inhibited the fish-induced sediment resuspension and nutrients increases. Consequently, algal production was reduced and underwater light conditions were improved, which promoted the growth and reproduction of V. natans, e.g., more ramets, biomass, and leaves of V. natans. Our results indicated that submerged macrophytes could benefit from LMB application under high intensity disturbance by juvenile omni-benthivorous fish, thereby conducive to the maintenance of a clear water state in shallow lakes.