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Recovery of submerged macrophytes has been considered a key factor in the restoration of shallow eutrophic lakes. However, in some subtropical restored lakes, small omnivorous fish dominate the fish assemblages and feed in part on submerged macrophytes. Knowledge of the effects of small omnivores on the growth of submerged macrophytes is scarce and their responses are potentially species-specific, i.e. the growth of some species may be hampered by fish grazing while growth of others may be promoted by the nutrients becoming available by fish excretion. We conducted mesocosm experiments to examine the effects of the small omnivorous bitterling Acheilognathus macropterus, a common species in restored subtropical lakes in China, on nutrient concentrations and the growth of four species of submerged macrophytes (Hydrilla verticillata, Vallisneria denseserrulata, Ceratophyllum demersum and Myriophyllum spicatum). We found that the bitterling significantly increased nutrient concentrations via excretion and thereby enhanced the net growth of the less grazed nuisance macrophyte M. spicatum. In contrast, the net growth of C. demersum was reduced by the bitterling, most likely due to grazing as indicated by gut content analyses. Dominance by bitterling may, therefore, pose a threat to the long-term success of lake restoration by provoking a shift in the submerged macrophyte community towards nuisance species through selective grazing. Nutrient excretion may potentially also stimulate the growth of phytoplankton and periphyton, hampering the growth of submerged macrophyte. (C) 2020 Elsevier B.V. All rights reserved.

Omnivorous fishes are prevalent in warm waters and may have strong impacts on water quality by excreting nutrients and reducing periphyton biomass. However, most studies have focused on large-sized species and overlooked the role of small omnivores. Filter-feeding mussels may modulate the negative effects of small omnivorous fishes on water quality, and stocking of mussels has been frequently used in shallow eutrophic freshwaters in China to improve the water clarity. However, the mechanisms behind such management practices are poorly studied. We conducted a mesocosm experiment to examine the ecosystem effects of the bitterlingAcheilognathus macropterusas modulated by the musselSinanodonta woodiana, one of the mussels upon which it relies for breeding. We hypothesized that bitterling would exert negative effects on the lake environment, specifically higher phytoplankton biomass and lower water clarity, but that these effects might be alleviated by the filter-feeding activities ofS. woodiana. In a 56-d mesocosm experiment with and without bitterling in the presence and absence of mussels, we found interactive effects of bitterling and mussels. In mesocosms with bitterling, nutrient concentrations, phytoplankton biomass, and total suspended solids (TSS) increased, but there were no changes in periphyton biomass in the mussel-free treatments. In contrast to the effects of large-sized omnivorous fishes reported from the literature, bitterling mainly affected TSS levels by increasing organic suspended solids rather than inorganic solids, indicating weak effects on sediment resuspension. However, the presence of mussels alleviated the negative effects of bitterling by decreasing nutrient levels, phytoplankton biomass, and TSS concentrations. Mussels alone had no effects on periphyton biomass, but the mussel-bitterling interactions boosted the growth of periphyton. Our study suggests that the negative effects of bitterling on water quality (e.g., increased nutrient concentrations and phytoplankton biomass) are alleviated by the presence of filter-feeding mussels, but the stimulatory interactive effects of mussels and bitterling on periphyton may impair the recovery of submerged macrophytes.

Long-chain n-alkenes showing a predominance of n-C25:1 and n-C27:1 were detected in recent sediment of Lake Lugu, an oligotrophic alpine lake in Yunnan Province, SW China. The abundances of n-C25:1 and n-C27:1 alkenes varied considerably in the sediment core, most obviously in showing higher values during the period of 1821–1982, followed by a significant decrease between 1982 and 2012. Such variations were similar to those of long-chain 1,15-alkyl diols, biomarkers of eustigmatophytes, and a significant correlation was observed between n-C25:1 (or n-C27:1) alkene and C32 1,15-alkyl diol in the sediment core. This correlation and the fact that long-chain n-alkenes in some eustigmatophytes (e.g., Nannochloropsis sp.) were dominated by C25 and C27 compounds indicates that eustigmatophytes were the most likely contributor of the long-chain n-alkenes (especially the n-C25:1 and n-C27:1 alkenes) in Lake Lugu sediment. Chlorophytes, on the other hand, cannot be excluded as a possible contributor of the long-chain n-alkenes because these algae are common in Lake Lugu and they are known to biosynthesize n-C27:1 alkene. Productivities for some species of eustigmatophytes and chlorophytes considerably decreased when the lake water was heavily influenced by catchment soil erosion and anthropogenic sewage inputs.

Organic geochemical analysis of a sediment core from Lake Fuxian was carried out to reconstruct the progression of eutrophication in this lake from ca. 1857 to 2012. The results indicate that two episodes can be identified during the eutrophication of the lake water. Both bulk geochemical proxies and sediment lipid biomarkers recorded an enhanced eutrophication in this lake since 1986. Total organic carbon (TOC), total nitrogen (TN) and phytoplankton biomarkers were most abundant in this period, indicating high phytoplankton productivity. Phytoplankton composition showed an elevated proportion of dinoflagellate and green algae relative to diatoms, in response to the anthropogenic nutrient inputs (e.g., sewage, fertilizers) since the 1980s. On the other hand, only the sediment lipid biomarker detected a minor eutrophication of the lake water between 1951 and 1986. During this period, phytoplankton biomarkers, regardless of their labile or refractory nature, exhibited a little enhanced concentration in the sediment, indicating a minor increase of phytoplankton productivity. Phytoplankton composition was characterized by a slight increase of diatoms relative to dinoflagellates and green algae. Variations of algal productivity and community structure in this period could be attributed to the nutrients (e.g., soil Si) input from catchment erosion. A causal relationship might be present between the eutrophication process of Lake Fuxian and the anthropogenic activities within the catchment.

A reduction in runoff and higher evaporation rates are expected to occur towards 2050 in arid and semiarid regions of the world, resulting in a reduction of water level and salinization of inland waters. Besides the natural process of catchment erosion, human activities such as irrigation of crops may also increase salinization. Reduced biodiversity in freshwater systems is the most commonly reported effect of salinization, which may have implications for food web structure and likely for ecosystem functioning as well. The objective of the study was to analyze the effects of salinity on community and food web structure in 24 lakes along a wide salinity gradient, from freshwater (0.5 g L-1) to hypersaline lakes (115 g L-1), in a semiarid region in North West China. Fish, zooplankton and macroinvertebrate communities were sampled during July 2014 for determination of taxonomy and size structure and stable isotopes analysis. Based on the stable isotope data we calculated the food web Layman metrics: total area, nitrogen range, carbon range, centroid distance and also trophic position for each community and for the entire food web. We found a significant decline in the number of taxa of all communities analyzed and a reduction of food web complexity with enhanced salinity. Accordingly, the metrics used to describe the entire food web were all negatively related with salinity; however, when examining the food web of each single community the effect was not significant, implying that the species reduction at community level did not affect the trophic diversity. Our results suggest a negative effect of salinity not only on the structure but also on the functioning considering the entire food web of the studied lakes.