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Abstract
Fish removal has been used to restore temperate lakes, and positive effects on ecological state and water clarity have frequently been recorded in many lakes. Recently, a supplementary measure, transplantation of submerged macrophytes after fish removal, has been applied to restore warm Chinese shallow lakes in order to compensate for the expected lack of increasing grazing control of phytoplankton after the biomanipulation. These measures have successfully shifted turbid warm lakes to a clear water state, but little is known about the responses to restoration of key physico-chemical variables. We analyzed the seasonal variation in nutrient concentrations in two subtropical and one tropical biomanipulated shallow Chinese lakes subjected to restoration. In all three lakes, a marked decline occurred in the concentrations of lake total nitrogen (TN), total phosphorus (TP), total suspended solids (TSS), and chlorophyll a (Chl a), while the transparency (SD:WD ratio, Secchi depth to water depth ratio) increased. A clear water state was established, lasting so far for 7 to 23 months, and TN, TP, Chl a, and TSS levels in the three restored lakes decreased to, on average, 49%, 58%, 41%, and 18% of the level prior to restoration and/or the level in a reference lake, respectively, while the annual mean SD:WD ratio exhibited a 1.5–4 fold increase. In conclusion, lake restoration by transplantation of submerged macrophytes after fish removal had major positive effects on the physico-chemical variables in our study lakes. However, continuous control of omnivorous and herbivorous fish biomass is recommended as the fish typically present in warm, shallow lakes to some extent feed on submerged macrophytes, when available.
Highlights
In many countries world-wide, food web biomanipulation, such as removal of planktivorous and benthivorous-omnivorous fish and transplantation of submerged macrophytes, has been considered as an important and useful approach to restore eutrophic shallow lakes
The average concentrations of total nitrogen (TN), total phosphorus (TP), chlorophyll a (Chl a), and total suspended solids (TSS) were lower in the restored parts of
Lake Wuli and Lake Qinhu than in the unrestored areas (Table 1), and lower concentrations were found in South Lake at Jinan University after restoration (Table 1)
Summary
In many countries world-wide, food web biomanipulation, such as removal of planktivorous and benthivorous-omnivorous fish and transplantation of submerged macrophytes, has been considered as an important and useful approach to restore eutrophic shallow lakes. Benthivorous fish may induce resuspension of sediment, resulting in reduced water transparency, and increase the nutrient exchange between sediment and water [4,5,6]. Removal of planktivorous and benthivorous fish may produce strong cascading effects on phytoplankton, nutrients, as well as on water clarity [7,8], and the method has been used to restore temperate lakes for many years [3,4,8,9]. Submerged macrophytes affect the physico-chemical environment and biota in shallow lakes [11,12] and play a key role in maintaining a clear water state [13,14] through a number of different mechanisms such as reduced resuspension of bottom sediments [15] and suppression of algal growth through competition with phytoplankton for nutrients [16,17]. Submerged macrophytes provide a refuge for zooplankton, which avoid predation by planktivorous fish [20], leading to a higher grazing pressure on phytoplankton [21]
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