Shihoudian Lake is one of the ecological restoration engineering pilot sites of Baiyangdian Lake, China. To evaluate the phytoplankton characteristics and eutrophication status in Shihoudian Lake, we investigated the community structure of phytoplankton, including the species composition, density, biomass dominance, biodiversity and water quality parameters, in autumn 2018 and spring and summer 2019. The relationships between the community structure and the main environmental factors were analysed using a multivariate statistical method. A total of 143 species of phytoplankton were identified, belonging to 53 genera and eight phyla, and Cyanophyta and Prochlorophyta were the most dominant phyla. Both the density and the biomass were the highest in the summer. A redundancy analysis showed that total phosphorus and chemical oxygen demand were the primary influencing factors of the community distribution of Cyanophyta. Evaluation of the comprehensive diversity index and water quality index revealed that the water of Shihoudian Lake was lightly to moderately polluted, providing scientific evidence for eco-environmental protection and remediation.

Salinity is one of the important environmental factors affecting survival and growth of aquatic animals. However, the impact of low-salinity stress on M. rosenbergii post-larvae at different development stages remains elusive. Therefore, the aim of this study was to explore the underlying mechanisms of hypotonic stress at different development stages of M. rosenbergii post-larvae through transcriptome analysis and antioxidant parameters detection. The salinity of the control group was 15 psu (S15) and the hypotonic stress group was 6 psu (S6). Samples were collected at 7 days-post-hatch (dph), 14 dph and 21 dph larvae. The results showed that hypotonic stress caused oxidative damage in post-larvae evidenced by decreased glutathione peroxidase (GSH-Px); superoxide dismutase (SOD); anti-superoxide anion free radical (ASAFR); and increased malondialdehyde (MDA); nitric oxide (NO); and inducible nitric oxide synthase (iNOS) levels. Transcriptome analysis showed that there were 1428, 1187, 132 DEGs including 301, 366, 4 up-regulated genes and 1127, 821, 128 down-regulated genes at 7 dph, 14 dph and 21 dph larvae under hypotonic stress, respectively. Furthermore, GO and KEGG enrichment indicated that hypotonic stress led to dysregulation of immune signals including lysosome and autophagy in the 7 dph larvae. The autophagy-related genes including beclin 1-associated autophagy-related key regulator (Barkor); ubiquitin-like modifier-activating enzyme ATG7 (ATG7); Beclin; autophagy-related protein 13 (ATG13); nuclear receptor-binding factor 2 (Nrbf2); ubiquitin-like-conjugating enzyme ATG3 (ATG3); vacuole membrane protein 1 (VMP1); and autophagy-related protein 2 (ATG2) decreased at 7 dph, and 14 dph larvae, and then increased at 21 dph larvae under hypotonic stress. In the 14 dph and 21 dph larvae, the renin-angiotensin system was activated. In conclusion, our data indicated that hypotonic stress reduced the antioxidant capacity and impaired the immune system in post-larvae, but as development progresses, the adaptability of post-larvae to hypotonic stress gradually increased, and might reach a new homeostasis through the RAS signaling pathway.

The habitat and feeding environment of freshwater fish in freshwater lakes have been destroyed, with the problem of miniaturization and simplification of catches being serious. An artificial reef is an effective technical measure to protect and proliferate offshore fishery resources, but little research has been conducted on its application in freshwater lakes. A small artificial reef for freshwater lakes was designed according to the water depth of the lake and the habits of benthic fish. The artificial reef is composed of biomass modules, each of which is 900 × 120 mm. The community structure of phytoplankton around the artificial reef and its adjacent waters was studied. The results showed that 77 species from seven phyla were identified, with a high number of species from the Chlorophyceae. In terms of density composition, the density of cyanobacteria decreased month by month, while the phylum Chlorophyta and Cryptophyta increased first and then decreased. As for biomass composition, Chlorophyta and Cryptophyta increased first and then decreased. RDA analysis showed that water temperature, dissolved oxygen, and total phosphorus were the main influencing factors. To sum up, the artificial reef can improve the algae phase in the surrounding water column, inhibit the growth and reproduction of cyanobacteria to a certain extent, and have a significant enrichment and promotion effect on diatoms. Artificial fish reef affects the phytoplankton community structure of the surrounding water bodies mainly through the absorption of phosphorus nutrients. Artificial reefs can be popularized and applied in freshwater lakes to provide foraging and shelter for benthic fish in the lake.

To address the issues of submerged vegetation restoration in the deep-water area of Baiyangdian Lake, a type of suspended ecological bed was developed, which was constructed with aquatic plants, ropes and supporting poles. The depth of a suspended ecological bed could be adjusted based on the water level. From July to November 2019, the phytoplankton community structure in the water around the suspended beds was continually monitored, and the effects of the submerged plants present in the suspended beds on the phytoplankton community structure in the deep-water area of the lake were studied. The results showed that Myriophyllum spicatum L. grew well in the suspended beds, with 95% survival after 60 d. The maximum biomass was reached in 55 d, and the water surface coverage reached 23%. There were 71 species of phytoplankton belonging to seven phyla in the water surrounding the suspended ecological beds. Over half (36 species) were members of Prochlorophyta, and the average density and biomass of phytoplankton gradually decreased month by month. The proportion of cyanobacteria in the community showed a downwards trend, while the proportions of other phytoplankton phyla showed upwards trends. The annual average Shannon–Wiener diversity index was 0.88, the annual average Pielou evenness index was 0.32, and the annual average Margalef richness index was 1.21. The Shannon–Wiener diversity index and Pielou's evenness index both showed increasing trends. This comprehensive analysis shows that planting submerged plants in suspended beds can improve the phytoplankton community structure of the surrounding water and increase the biodiversity of phytoplankton, making this an effective measure for restoring submerged vegetation in algae-dominated northern lakes.

The aim of this study was to investigate the effects of an effective microorganism (EM)‐based microbial inoculum on the growth and expression of related genes in 1‐year‐old healthy Megalobrama amblycephala . The bacterial strains included Lactobacillus plantarum , Lactobacillus fermentum , Enterococcus faecium , Bacillus velezensis , Paenibacillus polymyxa and Lactococcus sp. The fish were randomly divided into five groups: one group was fed a basal diet (control group), and the other groups were provided feed containing EMs at levels of 5, 10, 20 or 40 g/kg. The growth performance, gastrointestinal digestive enzyme activity and expression of the growth hormone (GH), GH receptor A (GHRA), GH receptor B (GHRB) and insulin‐like growth factor‐1 (IGF‐1) genes in the liver of M. amblycephala after 60 days of outdoor cage culture were analysed. The weight gain rate for M. amblycephala significantly increased when EM was added to the feed, and the 10 g/kg EM group had the greatest weight gain, which was 7.34% higher than that of the control group; this group also presented the highest intestinal protease and lipase activities. Regarding hormone levels, GH, GHRA, GHRB and IGF‐1 expression levels were highest in the 10 g/kg EM group. Additionally, EM addition significantly improved lipoprotein metabolism in the liver, resulting in a decrease in low‐density lipoprotein levels and an increase in high‐density lipoprotein levels. The results of this study suggest that EM supplementation in feed improves the growth performance of M. amblycephala , reduces the risk of fatty liver and affects the expression of growth genes.