Eutrophication of the urban water bodies is one the biggest challenge causing severe ecological and economic loss. Urban ponds are more prone to eutrophication due to their small size and polluted catchment areas. Biomonitoring using phytoplankton provides cost-effective estimation of the level of eutrophication. Ten urban ponds in different areas of the Mumbai city were chosen to investigate the phytoplankton community structure, and level of eutrophication. We assessed the 3 algal indices viz. Shannon-Wiener indices, Palmer and Nygard's (Myxophycean and diatom) indices. Linear relationship of these indices was tested against Carlson trophic state indices in order to assess the effectiveness of these indices to measure the degree of eutrophication in urban lakes. All ten lakes were found to be eutrophic, of which two were very low eutrophic (TSI – 53.74-53.95), four were low-mid eutrophic (TSI – 55.18 – 57.5), and four lakes were mid eutrophic (TSI 61.4 – 62.2). Shannon-Wiener indices (r= -0.73) and Myxophycean indices (r= 0.77) showed strong correlation with TSI whereas Diatom indices (r= -0.12) and Palmer’s Algal Pollution Indices (r= - 0.47) showed weak correlation with TSI. Thus study found that Shannon-Wiener indices and Myxophycean indices are reliable and cost effective means to assess the eutrophication of urban ponds in Mumbai.

Eutrophic freshwater lake ecosystems are receiving increasing public attention due to a global increase in large-scale harmful cyanobacterial blooms in surface waters. However, the contribution of phytodetritus accumulation in benthic sediments post-bloom remains unclear. In this study, field investigations were performed using microsensors to evaluate benthic phytodetritus mats by measuring TOC/TN ratios, pigments, biodegradable compounds and odorants as descriptive parameters. Results show that the massive amount of phytodetritus trapped by aquatic plants gradually evolved into benthic cyanobacterial detritus mats, which were characterized as anoxic, reductive and low pH. It was confirmed that the occurrence of odorants is more serious in the detritus mats due to decay and decomposition of the accumulated phytodetritus. The mean odorant content in the vegetated zones was 3–52 times higher than that in the unvegetated zones. The dominant odorants were dimethyl trisulfide (DMTS), β-ionone and β-cyclocitral, with mean contents of 52.38 ng·(g·dw)-1, 162.20 ng·(g·dw)-1 and 307.51 ng·(g·dw)-1, respectively, in the sediment. In addition, odorant production appears to be associated with the distribution of biodegradable compounds in the sediment. This is supported by the marked correlation observed between biodegradable compounds and odorants. Multiple regression analysis showed that biodegradable compounds can be used as indicators to predict odorant content in the sediment. It is noteworthy that the odorant trend in the water column and sediment is symmetrical, indicating a risk of diffusion from the sediment to the water column. This study helps to clarifying the contributions of benthic cyanobacterial detritus mats to odorant production in shallow eutrophic lakes. The information provided herein may also be useful for future management of aquatic ecosystems.

Riverbed sediments are the interface layer in riverine ecosystems connecting the overlying medium of water and the vadose zone. The transport behavior of phosphorus (P), which has been recognized as the primary cause of freshwater eutrophication, in riverbed sediments remains unclear. Understanding the impact of riverbed sediments on P transport is a necessary prerequisite for the development of appropriate strategies to reduce potential groundwater pollution. In this study, riverbed sediments were collected from the upstream, midstream, and downstream sections of the Beiyun River, China, and packed into vertical soil columns to perform leaching experiments to quantify P transport characteristics. In addition, the impact mechanisms were further explored by conducting laboratory batch tests of P adsorption and desorption. The results demonstrated that approximately 80% of P can be adsorbed by riverbed sediments in soil column leaching experiment, and a tailing phenomenon was observed in its desorption. The hydraulic conductivity properties of riverbed sediments were evaluated by the advection-dispersion equation, showing a gradually decreasing adsorption capacity for P from upstream to downstream sections, which was supported by the results obtained from adsorption–desorption thermodynamic and kinetic batch tests. The estimated annual leaching masses of P increased from 60.72 g/(m² a) in the upstream section to 132.31 g/(m² a) in the downstream section. The role of riverbed sediments as a source or sink of P is possibly determined by their coarse sand particles content, and the mean equilibrium P concentration (EPC0). The competitive relationship between P and other forms of nutrients also has an important influence on its source-sink role. These findings suggest that the prevention of the potential P leaching is most needed in the downstream sections of Beiyun River, and corresponding control strategies should be developed to avoid groundwater pollution.

Poyang Lake is the largest freshwater lake in China and a globally important wetland with various functions. Exploring the multidecadal trend of water quality and hydroclimatic conditions is important for understanding the adaption of the lake system under the pressure from multiple anthropogenic and meteorological stressors. The present study applied the Mann–Kendall trend analysis and Pettitt test to detect the trend and breakpoints of hydroclimatic, and water quality parameters (from the 1980s to 2018) and the trend of monthly–seasonal ammonia (NH₄-N) and total phosphorus (TP)concentrations (from 2002 to 2018) in Poyang Lake. Results showed that Poyang Lake had undergone a highly significant warming trend from 1980 to 2018, with a warming rate of 0.44 °C/decade in terms of annual daily mean air temperature. The wind speed and water level of the lake presented a highly significant decreasing trend, whereas no notable trend was detected for precipitation variations. The annual mean total nitrogen (TN), NH₄-N, TP, and permanganate index (CODMₙ) concentrations showed significant upward trends from the 1980s to 2018. Remarkable abrupt shifts were detected for TN, NH₄-N, and CODMₙ in around 2003. They were in accordance with the water level breakpoint of the lake, thus implying the important role of hydrological conditions in water quality variations in floodplain lakes. A significant increasing trend has been detected for Chl-a variations during wet season from 2008 to 2018, which could be attributed to the increasing trend of nutrient concentration during the nutrient-limited phase of Poyang Lake. These hydroclimatic and water quality trends suggest a high risk of increasing phytoplankton growth in Poyang Lake. This study thus emphasizes the need for adaptive lake eutrophication management for floodplain lakes, particularly the consideration of the strong trade-off and synergies between hydroclimatic conditions and water quality variations.

Excessive nitrogen (N) losses from food production and consumption have resulted in noticeable environmental impacts, e.g., air pollution and climate change, saturation of soil N, and water eutrophication. In the present study, a rural-scale N flow model was constructed in Quzhou county, Hebei province to investigate the characteristics of the N flux, N use efficiency (NUE), and N loss and their driving factors in the food production and consumption system during 1997–2017. Our results show that the N fluxes of the crop-production subsystem (CPS), the livestock-breeding subsystem (LBS), and the household-consumption subsystem (HCS) all followed an upward trend. During 1997–2017, the N losses from the system were high (51.38%), and the CPS was a major source. When the N fertilizer application level was optimal (403–475 kg N ha⁻¹), the NUE in the CPS (NUEc) decreased sharply, resulting in a higher N cost than that observed at larger scales. For the LBS, the NUE of animal feed (NUEa) was high (46.37%); however, the waste utilization rate of the HCS was below 30%. The chemical fertilizer application level, feed input, animal-food demand, and livestock manure application level were closely related to the environmental N losses. Due to the lack of reasonable N treatment and utilization methods, the increasing N losses are expected to have a large future impact on environmental issues such as haze, soil acidification, and frequent algal blooms. Therefore, adjusting N management in the processes of food production and consumption is of great significance to the improvement of global NUE and reduction of environmental pollution.

In shallow eutrophic lakes, metal remobilization is closely related to the resuspension and eutrophication. An improved understanding of metal dynamics by biogeochemical processes is essential for effective management strategies. We measured concentrations of nine metals (Cr, Cu, Zn, Ni, Pb, Fe, Al, Mg, and Mn) in water and sediments during seven periods from 2014 to 2018 in northern Lake Taihu, to investigate the metal pollution status, spatial distributions, mineral constituents, and their interactions with P. Moreover, an automatic weather station and online multi-sensor systems were used to measure meteorological and physicochemical parameters. Combining these measurements, we analyzed the controlling factors of metal dynamics. Shallow and eutrophic northern Lake Taihu presents more serious metal pollution in sediments than the average of lakes in Jiangsu Province. We found chronic and acute toxicity levels of dissolved Pb and Zn (respectively), compared with US-EPA “National Recommended Water Quality Criteria”. Suspended particles and sediment have been polluted in different degrees from uncontaminated to extremely contaminated according to German pollution grade by LAWA (Bund/Länder-Arbeitsgemeinschaft Wasser). Polluted particles might pose a risk due to high resuspension rate and intense algal activity in shallow eutrophic lakes. Suspended particles have similar mineral constituents to sediments and increased with increasing wind velocity. Al, Fe, Mg, and Mn in the sediment were rarely affected by anthropogenic pollution according to the geoaccumulation index. Among them, Mn dynamics is very likely associated with algae. Micronutrient uptake by algal will affect the migration of metals and intensifies their remobilization. Intensive pollution of most particulate metals were in the industrialized and down-wind area, where algae form mats and decompose. Moreover, algal decomposition induced low-oxygen might stimulate the release of metals from sediment. Improving the eutrophication status, dredging sediment, and salvaging cyanobacteria biomass are possible ways to remove or reduce metal contaminations.

Samples from two reservoirs with eutrophication problems, located in Pontevedra and Ourense (Northwestern Spain), were cultured, along with a third crop from a reservoir with no problems detected in Ourense (Northwestern Spain). The samples were grown under the same conditions (with an average temperature of 21 ± 2 °C, and a 3000 lux light intensity) in triplicate, and their growth, absorbance and pH were studied. High correlation values were obtained for pH and cellular growth (R² ≥ 95%). The water from Salas showed the greatest microalgal growth (0.15 × 10⁶ cells/ml to 31.70 × 10⁶ cells/ml of Microcystis sp. for the last day of culturing) and the greatest increase in pH (5.72–9.02). In all the cultures studied here, the main species that reproduced was Microcystis sp., which can produce neurotoxins and hepatotoxins. In addition, water samples were cultured with sediments of their own reservoir and with others to observe their evolution. The sediments studied in this case were rich in biotites, which can lead phosphate to be a limiting factor for phytoplankton due to the formation and sedimentation of insoluble salts of ferric phosphate. In crops grown with sediments from the Salas reservoir, actinobacteria developed which can inhibit microalgal growth. The study of the growth of cyanobacteria and possible methods of inhibiting them directly concerns the quality of water and its ecosystems, avoiding pollution and impact on ecosystems.

Aquaculture can affect the polyunsaturated fatty acids (PUFA) and mercury (Hg) in fish by altering their diet. Here, planktivorous (silver carp and bighead carp), omnivorous and carnivorous fish with different dietary strategies were selected from two reservoirs, one with on-going aquaculture (WJD) and another without aquaculture (HF) in Southwest China. We compared the total mercury (THg), methylmercury (MeHg) contents and PUFA profiles of fish and their potential diets in these two reservoirs. THg and MeHg contents in omnivorous and carnivorous fish were lower from the WJD Reservoir, which is related to the lower THg and MeHg contents in the artificial fish food. THg and MeHg contents in silver carp from the WJD Reservoir were lower than those from the HF Reservoir, while they were similar in bighead carps from the two reservoirs. The Hg variation in planktivorous fish were inconsistent with that in plankton. THg contents in phyto- and zooplankton from the HF Reservoir were higher than those from the WJD Reservoir, yet their MeHg contents were similar. Artificial fish food which contained higher total PUFA eicosapentaenoic (EPA; 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3), significantly increased the total PUFA and EPA + DHA contents in carnivorous fish, but had less effect on that in omnivorous fish from the WJD Reservoir. Eutrophication caused by aquaculture reduced total PUFA and EPA + DHA contents of plankton in WJD, yet did not reduce those in planktivorous fish. The impacts of aquaculture on Hg and PUFA accumulated in fish were varied among different fish species, and the mechanism needs further exploration.

Biomanipulation is an efficient tool to control eutrophication and cyanobacterial blooms in temperate lakes. However, the effects of this technique are still unclear for tropical ecosystems. Herein, we evaluated the effects of the biomanipulation on cyanobacterial biomass in a tropical shallow reservoir in Northeast Brazil. A mesocosm experiment was conducted in Tapacurá reservoir (Pernambuco) with eight treatments, in which we factorially manipulated the presence of submerged macrophytes (Ceratophyllum demersum), large herbivorous zooplankton (Sarsilatona serricauda), and nutrients (0.4 mg L⁻¹ of nitrogen and 0.5 mg L⁻¹ of phosphorus). On the first, fifth, and tenth days, we analyzed the total biomass of cyanobacteria, and the morphotypes coccoid, heterocyted filamentous, and non-heterocyted filamentous cyanobacteria; these components were compared through a three-way ANOVA. The bloom was composed mainly of five Microcystis morphospecies (coccoids) and Raphidiopsis raciborskii (heterocyted filaments). On the fifth day of the experiment, the combined addition of macrophytes and zooplankton was more efficient at controlling cyanobacterial biomass. On the tenth day, all macrophyte treatments showed significant cyanobacterial biomass reduction, decreasing up to 84.8%. On the other hand, nutrients and zooplankton, both isolated and combined, had no significant effect. Macrophytes also reduced the biomass of coccoids, heterocyted filaments, and non-heterocyted filaments when analyzed separately on the tenth day. Ceratophyllum demersum was more efficient at controlling the bloom than the addition of large herbivorous zooplankton, which could be related to allelopathy since cyanobacterial biomass was also reduced when nutrients were added. The addition of submerged macrophytes with allelopathic potential, associated with the increase of large herbivorous zooplankton, proved to be an efficient technique for controlling tropical cyanobacterial blooms.

Lake eutrophication has become a serious environmental problem in China. Manipulations covering more elements of trophic pyramid are methods for lakes to obtain clear water state and should be studied in detail. In the present study, Meishan Dongpo Lake was divided into two parts, and a combined biomanipulation project was conducted in one part (RLake), and the other part was used as a control (CLake). Species of submerged-plant, fish, macrobenthos, and zooplankton were screened and a certain number of them were added to adjust the eco-chain in RLake. After restoration, the coverage of submerged macrophytes reached >85%; zooplankton greater than 0.6 mm in size increased in number, and the ratio of zooplankton biomass to chlorophyll-a (Chl-a) concentration increased. The dominant fish species changed, and disturbance of the sediment was reduced. The average density of mollusks in RLake was 111.5 ± 19.8 ind m⁻², which was much higher than that in CLake (36.7 ± 2.1 ind m⁻²). Water quality and clarity were substantially improved, and nutrient concentrations, particularly total phosphorus, total nitrogen, and Chl-a were significantly reduced. The aquatic community parameters were negatively correlated with the nutrient parameters and Chl-a. The ecological restoration have adjusted the aquatic ecosystem in RLake, and many positive feedback effects among the aquatic communities made them remove internal nutrients and Chl-a more efficiently.