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.

The influence of filter-feeding bivalves on plankton communities, nutrients, and water quality in a given aquatic ecosystem is so profound that they can be considered ecosystem engineers. In a 70-day mesocosm experiment, we tested the hypothesis that Corbicula fluminea would change plankton community structure by reducing small zooplankton and large phytoplankton and improve water quality by reducing nutrients. We monitored levels of nitrogen and phosphorus, organic suspended solids (OSS), and light at the sediment surface. Within the plankton, phytoplankton biomass (as Chl a, >0.45 μm), the biomass of microphytoplankton (>20 μm), nanophytoplankton (2–20 μm), picophytoplankton (0.2–2 μm), and zooplankton were determined. Compared with the controls, C. fluminea reduced the abundance of rotifers and the biomass of phytoplankton, and picophytoplankton, thereby modifying the plankton community structure. We did not observe reductions in TN and TP concentration, but OSS concentrations were reduced, and light intensity at the sediment surface was increased as a result of the improved water transparency. Our research shows that colonization by C. fluminea may modify plankton community structure and improve water quality of eutrophic shallow lakes, shedding further light on the ecological roles of filter-feeding bivalves in aquatic ecosystems.

Terrestrial carbon resources are generally regarded as inferior‐quality food for zooplankton due to a lack of essential biomolecules (e.g., polyunsaturated fatty acids). However, recent stable isotopic studies have suggested terrestrial carbon to be substantial resource for zooplankton, though the mechanisms of assimilation are not well understood. Here, we consider the possibility that heterotrophic microbes can upgrade the nutritional value of terrestrial carbon and thereby support the somatic growth and reproduction of zooplankton. To test this hypothesis, a simplified experimental food chain was established. Aquatic microbes were raised under dark laboratory conditions on a terrestrial carbon source, decaying corn straw (Zea mays L.), and the resulting degradation products were supplied as food to Daphnia magna. Significant increases in microbial abundance and concomitant increases in polyunsaturated fatty acid were observed during the degradation of corn straw. Furthermore, experiments indicated that the somatic growth and reproduction of D. magna was supported exclusively by the products of corn straw decay. Our results indicate a pathway for the assimilation of terrestrial carbon by zooplankton, in which heterotrophic microbes serve as trophic links.

Nile tilapia is a highly invasive fish species, deliberately introduced into many lakes and reservoirs worldwide, sometimes resulting in significant ecosystem alterations. A short-term mesocosm experiment with and without Nile tilapia (Oreochromis niloticus) was designed to test the hypotheses that the presence of tilapia may affect phytoplankton community structure, increase nutrients availability in water column and deteriorate water quality. Nutrients, total suspended solids (TSS) and biomass of phytoplankton in different size classes (as Chla) were measured. We found that tilapia increased the total nitrogen (TN), total dissolved nitrogen (TDN), NH₄⁺and TSS concentrations, deteriorating the water quality. In addition, under tilapia presence, the biomass of phytoplankton, as well as that of micro- and nano-phytoplankton, increased leading to a change in the structure of the phytoplankton assemblage. Moreover, a reduction in the biomass of periphyton was observed. Omnivorous tilapia is often dominant in tropical and subtropical waters, and removal of this fish may represent an effective management tool to improve the water quality.