Benthivorous fish cause a shift from a clear water state established by combing phosphorus immobilization and submerged macrophytes to a turbid state: A mesocosm study

Both chemical phosphorus (P) immobilization and submerged macrophytes are widely used in lake restorations to reduce internal phosphorus loading. Their combined effects seem much stronger than used alone. However, benthivorous fish populations recover fast and they may thus counteract the restoration effects. We conducted a mesocosm experiment with three densities of crucian carp (Carassius auratus gibelio) to study the effects of benthivorous fish on the combined effects of lanthanum (La) modified bentonite and submerged macrophyte Vallisneria denseserrulata in lake restoration: control (fish-free), low-density (45 g m(-2)) and high-density (83 g m(-2)). Crucian carp increased the concentrations of total phosphorus, total nitrogen, total suspended solids, and chlorophyll a and deteriorated the light climate in the water column compared with the controls. The porewater soluble reactive phosphorus concentrations in the deep sediment were two times higher in the high-density treatment than in the control group and the low-density treatment. Crucian carp increased the mobile phosphorus (mainly the NaOH-OP) in the 0-2 cm sediment, likely due to the dilution of La in the surface sediment as caused by accelerated downward transportation of La, and at the same time La-bound P (part of the HCl-P fraction) was mixed deeper into the sediment with increasing fish density. In the high-density mesocosms, plant biomass and density were reduced by 37% and 51%, respectively, which is another possible mechanism for the water quality deterioration. We conclude that it is important to control benthivorous fish when conducting lake restoration by combining phosphorus immobilization and submerged macrophytes. (C) 2022TheAuthor(s).Published by Elsevier B.V.