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.

How fish communities change with eutrophication in temperate lakes is well documented, while only a few studies are available from subtropical lakes. We investigate the fish community structure in 36 lakes located in the Yangtze River basin, covering a wide nutrient gradient. We found that fish species richness and total fish catch per unit effort (CPUE) increased significantly with chlorophyll a (Chla). Among the different feeding types, the proportion of zooplanktivores increased significantly with Chla, while the percentage of omnibenthivores showed no obvious changes; the CPUE of piscivorous Culter spp. increased with Chla, while their proportion of total catch decreased pronouncedly. Based on the index of relative importance (IRI), the most important and dominant fish species was the zooplanktivorous Sijiao (Toxabramis swinhonis), followed by the omniplanktivorous sharpbelly (Hemiculter leucisculus) and the omnibenthivorous crucian carp (Carassius carassius), a small-sized species belonging to the Cyprinidae family. The CPUE of these three species increased significantly with Chla. The focus has, so far, been directed at large fish, but as emphasized by our results, the abundant small fish species were dominant in our subtropical study lakes even in terms of biomass, and, accordingly, we recommend that more attention be paid to the population dynamics of these species in the future.

The genetic diversity of picoeukaryotes (0.2–5.0 µm) was investigated in 8 lakes differing in trophic status in Nanjing, China. Denaturing gradient gel electrophoresis (DGGE) and cloning and sequencing of 18S rRNA genes were applied to analyze the picoeukaryotic communities. DGGE analysis showed that among the 8 lakes, the diversity of picoeukaryotes was greatest in the mesotrophic Lake Nan (24 bands) and least in the oligotrophic Lake Qian (12 bands). Cluster analysis of DGGE profiles revealed that the 8 lakes were grouped into 2 distinct clusters. Cluster 1 contained lakes Mochou, Zixia, Huashen, Nan, Pipa, and Qian, while cluster 2 contained lakes Xuanwu and Baijia. Clone libraries were constructed from the mesotrophic Lake Xuanwu and the oligotrophic Lake Zixia, and the 2 libraries were compared using the program LIBSHUFF. This analysis indicated that the picoeukaryotic community composition differed significantly between the 2 lakes (p = 0.001). A total of 25 operational taxonomic units were detected; 18 (62 clones) were related to known eukaryotic groups, while 7 (30 clones) were not affiliated with any known eukaryotic group. Alveolates and stramenopiles were the dominant groups in Lake Xuanwu, while alveolates and chlorophyta predominated in Lake Zixia. Multivariate statistical analysis indicated that the differences in the picoeukaryotic community composition of the 8 lakes might be related to trophic status and top-down regulation by metazooplankton.