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Long-term decreases in the incident total radiation and water clarity might substantially affect the underwater light environment in aquatic ecosystems. However, the underlying mechanism and relative contributions of radiation dimming and decreasing water clarity to the underwater light environment on a national or global scale remains largely unknown. Here, we present a comprehensive dataset of unprecedented scale in China’s lakes to address the combined effects of radiation dimming and decreasing water clarity on underwater darkening. Long-term total radiation and sunshine duration showed 5.8% and 7.9% decreases, respectively, after 2000 compared to 1961–1970, resulting in net radiation dimming. An in situ Secchi disk depth (SDD) dataset in 170 lakes showed that the mean SDD significantly decreased from 1.80 ± 2.19 m before 1995 to 1.28 ± 1.82 m after 2005. SDD remote sensing estimations for 641 lakes with areas ≥ 10 km2 showed that SDD markedly decreased from 1.26 ± 0.62 m during 1985–1990 to 1.14 ± 0.66 m during 2005–2010. Radiation dimming and decreasing water clarity jointly caused an approximately 10% decrease in the average available photosynthetically active radiation (PAR) in the euphotic layer. Our results revealed a more important role of decreasing water clarity in underwater darkening than radiation dimming. A meta-analysis of long-term SDD observation data from 61 various waters further elucidated a global extensive underwater darkening. Underwater darkening implies a decrease in water quality for potable water supplies, recession in macrophytes and benthic algae, and decreases in benthic primary production, fishery production, and biodiversity.

The increase of ammonia nitrogen, nitrite, nitrate, urea, and other dissolved nitrogen in prawn pond often causes eutrophication of water body and prawn poisoning, which brings great harm to aquaculture. Microalgae are the main biological factors in prawn pond, which can effectively utilize dissolved nitrogen. The goal of this study was to evaluate the effects of the forms and concentrations of various dissolved nitrogen on the uptake rate (ρ) by Oocystis borgei, and establish the relationship model between ammonia nitrogen uptake rate and key environmental factors using 15N isotope labeling technique. Temperature (A), light intensity (B), salinity (C), pH (D), and algal concentration (E) were the factors used to construct an empirical model. Study results showed that nitrogen concentrations had a significant effect on the uptake rates of ammonium, nitrate, nitrite, and urea (P 0.05), which demonstrated the high fitting degree of simulation equation. This study provides valuable insight into the removal of dissolved nitrogen in a controllable prawn farming mode such as high-elevation ponds by O. borgei.

The South-to-North Canal System (SNCS) is the largest water diversion project in the world, and it transports water from water-sufficient southern and central to water-deficient northeastern China. The oriental river prawn is the most adaptable prawn species in this ecosystem and a dominant member of the benthic community. In this study, in order to obtain more information regarding the ecological functions of the prawn in the benthic food webs of the SNCS ecosystem, the trophic ecology of the prawn from the canal and the upstream reservoir was compared in the two ecosymtems. The prawn stable isotopic compositions of different prawn sizes (large, medium and small), niche spaces and the trophic positions were compared between the two ecosymtems. Results showed that in the main canal, particulate organic matter (POM) composed more than 40% of the food consumed by all three size prawns. Compared to small prawn, the diet of medium and large prawns were proportionally higher in POM and zooplankton. In the upstream reservoir, large prawn fed primarily on benthic microalgae, which composed 72.8% of the prawn diet. The pelagic food webs provided 93.5% food source for the canal prawn, while the benthic food webs afforded 72.4% food source for the reservoir prawn. Thereafter, the ecological functions of the prawn in the two ecosystems were also different due to their varied food diet. Careful considerations were suggested when using biomanipulation in the SNCS ecosystem.

Background Florida’s Everglades is a vast freshwater peatland that has been impacted by the alterations of hydrological pattern and water quality which led to changes in plant species composition and biodiversity. In this study, carbon and nitrogen stable isotopes (δ13C and δ15N) in cattail (Typha domingensis) are evaluated as indicators of environmental changes in the Everglades wetlands along nutrient and hydrological gradients represented by reference sites with total phosphorus (TP) < 10 μg L−1, transition sites with TP ≥ 10 μg L−1 < 20 μg L−1 and impacted sites with TP ≥ 20 μg L−1 which differed in hydrology or habitats (marsh and canal). Results Cattail δ13C values decreased significantly from reference (–24.1‰), transition (–26.8‰) to the impacted sites (–28.2‰). In contrast, δ15N values increased significantly from reference (–5.2‰), transition (2.4‰) to the impacted site (5.9‰). In response to a poor hydrological condition, cattail in marsh area displayed 13C enrichment (–26.0‰) and 15N depletion (0.2‰). By contrast, cattail grown in the canal sites with favorable hydropattern displayed 13C depletion (–27.6‰) and 15N enrichment (5.9‰) from the canal sites with more favorable hydrological condition. Conclusions The different patterns for the changes in δ13C and δ15N in cattail suggested that increased nutrients led to increased stomatal conductance and 13C fractionation during carbon uptake and decreased 15N fractionation with increasing nitrogen demand while poor hydrological condition coupled with low nutrients led to reduced plant growth indicated by higher δ13C and lower δ15N values. Findings from this study suggested that δ13C in emerged macrophytes such as cattail can be used as an indicator for environmental stress while δ15N is a robust indicator for wetland eutrophication.

Human-induced eutrophication has altered ecological processes in aquatic ecosystems. Detection of ecological changes is a prerequisite for protecting ecosystems from degradation. In this study, nitrogen stable isotopes (δ15N) in fish are evaluated as indicators of environmental changes in south Florida wetlands. Stable nitrogen isotope (δ15N) data of select fish species and water quality collected from the Florida Everglades between the 1990s and 2000s were used to assess the relationship between total phosphorus concentrations and δ15N ratios. The δ15N ratios in nine of ten select fish species increase significantly as total phosphorus concentration in the surface water increases. There were significant relationships between total nitrogen concentration in the surface water and δ15N ratios in several fish species. The pattern of changes in δ15N ratios along nutrient gradients suggests that increased eutrophication is recorded as the δ15N ratios in fish. The lack of human wastewater loading, the dominance in agricultural runoff and the high TN:TP ratio suggest that phosphorus is the limiting factor driving ecosystem productivity and the changes of δ15N ratios in fish. Results from this analysis demonstrate that δ15N ratios in fish integrate biotic responses to eutrophic process over time and could be a robust indicator for early ecological changes.

Human activities have increased the availability of reactive nitrogen in many freshwater ecosystems, leading to negative impacts on the health and biodiversity of lakes, rivers and wetlands. Yet, understanding the pathways of nutrient cycling in wetlands is limited. We conducted a nitrogen stable isotope (15N) tracer study to assess nitrogenous nutrient transfers within the biota community in a small experimental wetland between August 13, 2016, and September 12, 2016using a. Results showed rapid use of nitrogen by various aquatic plants and subsequent transfer to the consumer community. 15N enrichments were found in all plant samples from the first collection event. The fastest nutrient uptake was found in the simplest form of plants (periphyton and Lemna minor), followed by submerged macrophytes (Ceratophyllum demersum and Utricularia vulgaris). Emergent plants displayed the lowest 15N enrichment, likely due to their inability to directly assimilate nutrients from the water column and their initial large biomass. 15N enrichments in consumers were also pronounced, indicating that the nitrogen added to the wetland surface water was quickly transferred to various trophic levels. Primary consumers were more strongly labeled than secondary consumers; water column feeders were typically more enriched in 15N than benthic consumers. This experiment revealed different uses of nitrogen among the plant and consumer communities, and the results may provide useful information on community secession and ecosystem restoration under anthropogenic influence.

Forest management practices play an important role in soil water conservation. However, the soil water-holding capacity and associated drivers under different management practices remain uncertain, especially when the precipitation varies substantially at the regional scale. Here, we used hydrogen stable isotope to explore the contribution of rainfall to soil water (CRSW) under light, moderate and heavy precipitation in Pinus massoniana plantations with multiple management practices (pure stand, mixed stand, understory removal, light-intensity thinning and high-intensity thinning) in subtropical China. We further used variation partitioning analysis and structural equation modeling to identify the dominant driver affecting CRSW. Our results showed that after light rainfall, the highest CRSW (28.7%) was found in the high-intensity thinning plantation. However, after heavy rainfall, the high-intensity thinning plantation received the lowest CRSW (43.3%), while the mixed stand showed the highest CRSW (67.1%). These results demonstrated that the mixed stand of P. massoniana had a stronger capacity for soil water conservation, whereas high-intensity thinning showed poorer capacity. Furthermore, our results revealed that plant properties (i.e., tree, root and litter biomass) were the dominant controls of the CRSW under light rainfall, while soil properties (i.e., bulk density, total porosity, field capacity) were the primary drivers under moderate and heavy rainfall, indicating that the determinants influencing plantation capacity for intercepting rainfall vary with different levels of precipitation. These results highlight the importance of the level of precipitation in determining the dominant driver of CRSW. More importantly, these results suggest that the mixed stand, rather than high-intensity thinning, was better forest management since the former has a higher capacity for intercepting heavy rainfall.

External inputs and internal cycling are two pathways of nutrients to the aquatic macrophytes in the wetland ecosystem. Internal sources of nutrients include plant die-off and the decomposition of organic matter. In this study, we used a stable nitrogen isotope (15N) enrichment experiment to track the timing and magnitude of cattail (Typha angustifolia) use of coontail (Ceratophyllum demersum) – derived nitrogen (CDN) in a freshwater wetland. Dry coontail powder enriched in 15N (δ15N = 1259 ± 15‰) was buried in the soil close to the roots of cattail plants in field mesocosms at the Yeyahu National Preserve, Beijing, China, from July 22 to September 30, 2014. Different parts (roots, rhizomes, stems, leaves and leaf apexes) of the cattail plants were collected at day 0, 14, 28, 42, 49, 56, 63 and 70 for stable isotope analysis. Results reveal uptake of the nitrogenous nutrient from coontail detritus by cattail four weeks after being exposed to CDN. The maximum uptake rate for excess 15N in roots, leaves, stems, rhizomes and leaf apexes were 1.55 ± 0.84, 0.69 ± 0.11, 0.33 ± 0.08, 0.14 ± 0.11, 0.06 ± 0.02 mg 15N m−2. The root system is the largest pool of nitrogen in cattails, which serves as the source of nitrogen for other plant parts. The uptake rate per unit biomass was highest in the leaf apexes.

The purpose of this study was to assess the water quality and variations in stable carbon and nitrogen isotopes of particulate organic matter (δ13CPOM and δ15NPOM), as well as to evaluate the sources of carbon and nitrogen that contribute to the POM pools in lakes and reservoirs located in the water-receiving area of the Eastern Route of South-to-North Water Transfer Project (SNWTP) in Northern China. During each season from October 2013 to July 2014, samples of POM from 14 lakes and reservoirs in Northern China were collected. The lakes and reservoirs were meso-eutrophic with considerably high brackish ions (SO42−, 173 mg/L; Cl−, 296 mg/L) in Yangtze River lake, and high total nitrogen: total phosphorus ratio (averaged with 772) or dissolved inorganic nitrogen: soluble reactive phosphorus molar ratios (averaged with 1077) in mountainous reservoirs. The δ13CPOM, δ15NPOM, carbon to nitrogen (C/N) ratios showed significant seasonal variation, with ranges of − 32.5 to − 17.4‰, − 3.6 to 13.5‰, and 5.1–13.2, respectively, while they were hard to be distinguished among types of water sources. Principal component analysis (PCA) indicated that brackish ions, nutrients, and their molar ratios were the main factors influencing variations in δ13CPOM and δ15NPOM. δ13CPOM and C/N ratios suggested autochthonous primary production mainly contributed to POM during from April to October, while exogenous organic matter might mainly contribute these carbon pools in January. The low values of δ15NPOM (

Intense storms pose a serious threat to ecosystem functioning and services. However, the effects of typhoons (tropical cyclones) on the biogeochemical processes mediating risk of eutrophication in deep freshwater ecosystems remain unclear. Here, we conducted a three-year study to elucidate linkages between environmental change, stable isotopes and the stoichiometry of particulate organic matter (POM), and nutrient cycling (i.e., carbon, nitrogen and phosphorus) in a subtropical deep reservoir subjected to typhoon events. The typhoons significantly changed the nutrient levels in the deep waters as well as the thermocline position. Increased typhoon-driven organic matter input, algae sinking and heterotrophic decomposition interacted with each other to cause steep and prolonged increases of total nitrogen, ammonium nitrogen and total phosphorus in the bottom waters of the reservoir. Small-sized or pico-sized POM (i.e., 0.2–3 μm) showed a substantial increase in bottom waters, and it exhibited stronger response than large-sized POM (i.e., 3–20, 20–64, 64–200 μm) to the typhoons. Our results also indicated that typhoons boost the nutrient cycling in deep waters mainly through pico-sized POM.

Dajingshan, Fenghuangshan and Meixi reservoirs are located in Zhuhai, a coastal city in southern China, and they function to supply drinking water to Zhuhai and Macau. For effectively supplying waster, they are hydrologically connected and Dajingshan Reservoir first receives the water pumped from the river at Guangchang Pumping Station, and then feeds Fenghuangshan Reservoir, and the two well-connected reservoirs are mesotrophic. Meixi Reservoir is a small and oligotrophic water body and feeds Dajingshan Reservoir only in wet seasons when overflow occurs. Particulate organic matter (POM) was collected from three hydrologically connected water supply reservoirs, and seasonal variations of POM were ascertained from stable carbon and nitrogen isotopes in wet and dry seasons, and the effects of pumping water and reservoir connectivity on POM variations and composition were demonstrated by the relationships of the stable isotope ratios of POM. Seasonality and similarity of stable carbon and nitrogen isotopes of POM varied with hydrodynamics, connectivity and trophic states of the four studied water bodies. The two well-connected reservoirs displayed more similar seasonality for δ13CPOM than those between the river station and the two reservoirs. However, the opposite seasonality appeared for δ15NPOM between the above waters and indicates different processes affecting the stable carbon and nitrogen isotopes of POM. δ13CPOM and δ15NPOM changed little between wet and dry seasons in Meixi Reservoir-a low productive and rain-driven system, suggesting little POM response to environmental changes in that water system. As expected, connectivity enhanced the similarity of the stable isotope ratios of POM between the water bodies.

Anthropogenic activities are substantially changing coastal wetland ecosystems globally. In developing countries such as China, a number of anthropogenic factors associated with rapid population growth and economic development threaten coastal wetlands. In China, notably, coastal wetlands have been increasingly lost to reclamations that are widely adopted to meet the increasing demand for land under rapid economic development. Coastal wetland management requires understanding the patterns of and the mechanisms underlying such human impacts. In this special issue, we synthesize current understanding of environmental changes and human impacts on China’ coastal wetlands, focusing on reclamation. Coastal human activities in China are found to change shoreline evolution and wetland hydrology, to deteriorate soil and water quality, to alter vegetation succession, benthic animal and microbial communities, and fisheries, and to impair ecosystem functioning and services. For some of those impacts, new models and indices are developed or applied. We also outline key research areas that should be further studied for effective management of coastal wetlands and successful wetland restoration. We suggest that developing a multi-objective, multi-scenario, and multi-scale framework of integrated management will be crucial to the success of China’s coastal wetland conservation with increasing human dominance of the nation’s coasts.

Coastal wetlands are considered to be amongst the most productive ecosystems and can provide invaluable ecological services. However, coastal wetlands are listed amongst the most threatened ecosystems suffering from anthropogenic activities. The loss or degradation of coastal wetlands has drawn a high level of attention to wetland restoration. Improvement of the structure and function of degraded, damaged and destroyed wetlands may be achieved through ecological restoration. Large numbers of restoration projects have been conducted worldwide based on different restoration goals and different methods. It is undoubtedly important to evaluate whether coastal wetland restoration is successful. However, coastal wetland restoration assessment has become challenging because of current disagreement on definitions and concepts of restoration evaluation. We reviewed the methodology of coastal wetland restoration and criteria for success evaluation, and then summarized the issues for current wetland restoration and success evaluation based on literature review. Moreover, we used an estuarine wetland affected by urbanization as a sample to demonstrate how to establish a success indicator system for guiding wetland restoration and evaluating the success of wetland restoration.

Differential distribution of nutrients within an ecosystem can offer insight of ecological and physical processes that are otherwise unclear. This study was conducted to determine if enrichment of phosphorus (P) in tree island soils of the Florida Everglades can be explained by bird guano deposition. Concentrations of total carbon, nitrogen (N), and P, and N stable isotope ratio (δ(15)N) were determined on soil samples from 46 tree islands. Total elemental concentrations and δ(15)N were determined on wading bird guano. Sequential chemical extraction of P pools was also performed on guano. Guano contained between 53.1 and 123.7 g-N kg(-1) and 20.7 and 56.7 g-P kg(-1). Most of the P present in guano was extractable by HCl, which ranged from 82 to 97% of the total P. Total P of tree islands classified as having low or high P soils averaged 0.71 and 40.6 g kg(-1), respectively. Tree island soil with high total P concentration was found to have a similar δ(15)N signature and total P concentration as bird guano. Phosphorus concentrations and δ(15)N were positively correlated in tree island soils (r = 0.83, p0.0001). Potential input of guano with elevated concentrations of N and P, and (15)N enriched N, relative to other sources suggests that guano deposition in tree island soils is a mechanism contributing to this pattern.

Biodiversity has a close relationship with ecosystem functioning. For most biodiversity–ecosystem‐functioning studies, biodiversity has been linked to a single indicator variable of ecosystem functioning. However, there are generally multiple ecosystem processes contributing to ecosystem functioning and they differ in their dependence on biodiversity. Thus, the relationship between biodiversity and ecosystem functioning can be stronger when multiple rather than single ecosystem processes are considered. Using both mass‐balance and stable‐isotope approaches, we explored the effects of plant diversity on nitrogen (N) removal sustained by multiple N‐cycling processes in experimental microcosms simulating constructed wetlands, an ecosystem treating wastewater with high N loading. Four species were used to assemble different plant communities, ranging in richness from one to four species. The removal of N, indicated by low levels of total inorganic N concentration (TIN) present in the effluent, was considered as an integrated measure of ecosystem functioning, combining three constituent N‐cycling processes: plant uptake, denitrification, and substrate adsorption. Our results showed that (1) species richness had a positive effect on N removal, in particular, the four‐species mixture reduced effluent TIN to a lower level than any monoculture; however, polycultures (two‐, three‐, and four‐species mixtures) did not outperform the most efficient monoculture when each of the three constituent N‐cycling processes was considered by itself; (2) species identity had significant impacts on single processes. Communities with the species Coix lacryma‐jobi showed the greatest capacity for N uptake and communities with Phragmites australis had the highest denitrification rates; (3) isotope fractionation in the rhizosphere of Coix lacryma‐jobi was primarily due to microbial denitrification while multistep isotope fractionation was detected for Phragmites australis and Acorus calamus (indicating recycling of N), suggesting that species differed in the way they transformed N; (4) the enhanced N removal at high diversity may be due to mutualistic interactions among species belonging to different functional types. Our findings demonstrated that although plant species richness had negligible effects on individual N‐cycling processes, it enhanced the overall ecosystem functioning (N removal) when these processes were considered collectively. Our study thus contributes to improve the treatment efficiency of constructed wetlands through proper vegetation management.

The goal of this study was to assess the rate of urea nitrogen uptake (ρ) by Oocystis borgei and the relationship between environmental factors and ρ. Light intensity, temperature, pH, salinity, and algal concentration, were used to construct an empirical model. The results showed that light intensity, algal concentration, pH and salinity had significant effects on ρ, and the optimal combination of environmental conditions for ρ was a temperature of 25°C, pH of 7.0, light intensity of 81 µmol m−2 s−1, salinity of 15‰, and algal concentration of 4.5 × 108 cell L−1–5.5 × 108 cell L−1. The model equation was ρ = 2 × 10−5 × (A0.363B0.783C0.045D−0.503E) + 0.0017, with a coefficient of determination (R2) of 0.83. No significant difference in variance was observed between the model-predicted values and the measured values (F = 0.238, p > 0.05), which demonstrated the high fitting degree of the simulation equation. This study provided valuable insight into the reduction of urea nitrogen levels in aquaculture water by O. borgei.

Carbon and nitrogen stable isotopes (δ13C and δ15N) have been used as indicators for environmental changes in aquatic ecosystems. In this study, δ13C and δ15N values of select flora taxa were evaluated as indicators of human-induced eutrophication in an Everglades wetland which has been impaired by the source and quality of inflow and the alteration of hydrological pattern. Results showed that δ13C of cattail (Typha domingensis Pers.) increased with the decrease in total phosphorus (TP) concentration. Limited data for sawgrass (Cladium jamaicenses Crantz) collected at three study sites along the nutrient gradient also show a similar change in δ13C. However, δ13C of periphyton and bladderwort (Utricularia sp.) decreased with the decrease in TP concentration. The elevated δ13C values of emergent vascular plants (cattail and sawgrass) were likely the results of decreases in stomatal conductance, carboxylation and 13C fractionation under nutrient stress. This is supported by corresponding decreases in photosynthetic rate of cattail along the decreasing nutrient gradient. In contrast, δ15N values of all flora taxa increased with the increase in TP concentration. Findings from this study indicate that δ13C of emergent plants is a sensitive indicator for nutrient stress while δ15N of major flora species may serve as a robust indicator for wetland eutrophication.

Wastewater with a high nitrogen (N) loading rate and high ammonium (NH4+-N)/nitrate (NO3−-N) ratio is increasingly becoming a problem in regional environment. An experimental system was established to test the effects of plant diversity on N removal in microcosms of constructed wetlands. The microcosms were treated with simulated wastewater with NH4+-N as the sole chemical form of N, and with mixed NH4+/NO3—N (1:1) as a control, both at a loading rate of 162 g N m−2 yr−1. Results indicated that: (1) high plant species and functional group richness improved N removal efficiency under a high ammonium loading rate, (2) high species richness reduced the difference of N removal between sole NH4+-N and mixed NH4+/NO3−-N treatments, (3) the presence of Coix lacryma-jobi (C4 grass) increased both biomass and N removal, whereas the presence of Aeschynomene indica (legume) increased N removal but decreased biomass of the communities. Overall, the results showed that high plant diversity enhanced N removal efficiency regardless of the N-form ratios and N level in the ecosystem.

Displacement of large areas of native Cladium jamaicense Crantz (sawgrass) by Typha domingensis Pers. (cattail) in the Everglades has occurred during the past several decades, and is widely attributed to phosphorus (P) enrichment. In addition, sulfide toxicity to marsh vegetation, particularly Cladium , has been cited as a possible consequence of increased loading of sulfate to the Everglades (USA) from anthropogenic sources, and a potential contributor to the shift from Cladium to Typha . We initiated a plant growth study at three “low P” sites in the Everglades with differing surface water sulfate (1–48 mg L −1 ) and porewater sulfide (0.09–8.0 mg L −1 ) concentrations. Leaf elongation (LE) rates of Cladium and Typha were monitored, along with surface and porewater concentrations of iron, nutrients, and inorganic sulfur species. During the course of the study, we observed no reduction in Cladium or Typha LE rates at the two sulfate-impacted locations, relative to LE at a sulfate-unimpacted site. Moreover, across all study sites, we observed a positive, instead of a negative, relationship between Cladium LE rates and porewater sulfide concentrations. This suggests overriding effects of other factors such as P availability and supply on plant growth in these low-P environments, which was supported by a positive relationship between dissolved porewater P concentrations and Cladium LE rates, and by the Typha to Cladium LE ratios.

Lake Chenghai, a brackish lake located in southwest China, has changed from oligotrophic to mesotrophic and finally eutrophic conditions since the 1990s. In the late 1990s, planktivorous icefish were introduced into the lake, which dramatically altered the fish population. A paleolimnological evaluation using the cladoceran remains was conducted in order to analyze the effects of increasing nutrient load and fish introduction on the cladoceran community of this lake. Our results showed that the dominant cladocerans were littoral species, with a low abundance of planktonic Bosmina in the sediment. Increasing eutrophication since the late 1990s greatly enhanced the abundance of cladoceran assemblages, especially for the species that prefer eutrophic conditions. Meanwhile, the species which prefer oligotrophic conditions were extirpated. The changes in Daphnia ephippium length suggested that the planktivorous icefish have varying effects on the body size of different species.

Overall mean ± standard error annual mean CO2 flux estimates for the three alternative calculation methods with all pairs were significantly different between WY1999 and 2008 (May 1, 1998–April 30, 2008). Table S2: Model results and statistical evaluation of the models. Global validation statistic estimated using the gvlma R-package. The plot column corresponds to plots represented in Additional file 1: Figure S2. Figure S1: Boxplot of CO2 flux by three estimated associates with the inclusion (daily wind speed or period of record mean wind speed (POR: WY1999–2008) or exclusion of wind speed data to determine surface boundary layer thickness. Letters above each box-and-whisker indicate statistical differences between groups based on Dunn’s multiple comparison test. Figure S2: Cross comparison of different flux estimation methods. Red lines indicate a 1:1 linear line, blue solid line represents fitted linear models and blue dashed line represents the 95% prediction interval for the linear model for each respective comparison. Linear regression model parameters, statistics, and global validation statistics are presented in Additional file 1: Table S2. (DOCX 90 kb)

Scatterplot of monthly mean surface water carbon variables and various water quality parameters corresponding to Spearman’s correlation analysis results (Table 4, main document). Figure S2: Minimum (left), mean (middle), and maximum (right) annual hydroperiod for Water Conservation Area 1, 2, and 3 between water year 1992 to 2015 (May 1, 1991–April 30, 2015). Data Source: Everglades Depth Estimation Network (EDEN); https://sofia.usgs.gov/eden . Figure S3: Boxplot of annual hydroperiod for each water conservation area (WCA) between water year 1992 to 2015 (May 1, 1991–April 30, 2015). (DOCX 2201 kb)

Five isonitrogenous and isocaloric diets were fed to juvenile cobia, to assess the relative contribution of different proteins (fish meal, soybean meal, corn gluten and beer yeast) to the growth of cobia. The dietary effects on nitrogen and carbon turnover and on the isotopic diet-consumer discrimination factors (Δ15N and Δ13C) were also assessed. Growth results showed that the final body weight, growth rate, feed conversion ratio and protein efficiency ratio of cobia fed diets with alternative protein were significantly lower (P

The abundant riparian wetlands supported by the complex river network in Pearl River Delta (PRD) are under an intensely anthropogenic disturbance that threatens the local ecological environment. This paper examined the benthic invertebrate diversity and biomass within the Pearl River through a field study. A fieldwork was conducted to assess a hydrological connectivity gradient among the main channel of the Pearl River. The responses of macroinvertebrate metrics to the connectivity gradient and the restoration history in the river channel were examined. The Shannon diversity index ranged from 0.75 to 2.08, which was the highest among most connectivity river channels. Statistical analyses (PCA and CCA) revealed that the benthic abundance in high hydrological connectivity riverine habitats is clearly distinct from the low connectivity habitats. Results demonstrated that the diversity and density of macrobenthos are determined by hydrological connectivity and time span of the restoration (river channels). Therefore, conservation and restoration efforts should focus on the macroinvertebrate response to the hydrological connectivity of the river channels in PRD.

Quantifying and predicting the food web consequences of anthropogenic changes is difficult using traditional methods (based on gut content analysis) because natural food webs are variable and complex. Here, stable and radioactive carbon isotopes are used, in conjunction with nitrogen isotopes and mercury (Hg) concentration data, to document the effects of land-use change on food webs and Hg bioaccumulation in the Everglades - a subtropical wetland ecosystem in the US. Isotopic signatures of largemouth bass and sunfish in reference (relatively pristine) wetlands indicate reliance on the food supply of modern primary production within the wetland. In contrast, both fish in areas impacted by agricultural runoff had radiocarbon ages as old as 540 years B.P., and larger isotopic variability than counterparts in reference wetlands, reflecting differences in the food web between impacted and reference wetlands. Consistent with this difference, particulate and dissolved organic matter in impacted areas had old radiocarbon ages (>600 years B.P.), indicating that old carbon derived from historic peat deposits in the Everglades Agricultural Area was passed along the food chain to consumers. Significant radiocarbon deficiencies in largemouth bass and sunfish, relative to mosquitofish, in impacted areas most likely indicate a reduced dependence on small fish. Furthermore, largemouth bass and sunfish from impacted areas had much lower Hg contents than those from reference wetlands. Taken together, these data suggest a shift toward lower trophic levels and a possible reduction in mercury methylation in impacted wetlands. Our study provides clear evidence that hydrological modification and land-use change in the Everglades have changed the system from one driven primarily by in-situ productivity to one that is partially dependent on allochthonous carbon input from peat soils in the agricultural area and altered the Hg biogeochemical cycle in the wetlands. The results have implications for the restoration and management of wetland ecosystems.

Environmental controls on cladoceran community structure in lake ecosystems are complex and may involve many environmental parameters including trophic state and fish populations. In Lake Erhai, a plateau lake located in southwest China, it was hypothesized that a combination of lake eutrophication and planktivorous fish introduction would increase the abundance of cladoceran, while also decreasing cladoceran size. To test this hypothesis, we examined temporal changes in cladoceran microfossils in the sediments of Lake Erhai over the past century. The influence of changing macrophyte coverage within the littoral region of the lake was also considered. Results demonstrated that cladoceran abundance (measured as flux of cladoceran fossils in the sediments) increased markedly accompanying eutrophication of the lake. In addition, there was a shift in the dominant cladoceran species from those species that prefer oligotrophic conditions to those that prefer mesotrophic and eutrophic conditions. A reduction in the ephippium length of Daphnia spp. was observed and attributed to the introduction of the planktivorous fish Neosalanx taihuensis. Our findings indicated that eutrophication and fish introduction were the main controls affecting cladoceran community structure during the recent decades, and predation by planktivorous fish had an important impact on Daphnia body size.

Tree islands are habitat for wading birds and a characteristic landscape feature in the Everglades. A total of 93 surface soil and 3 soil core samples were collected from 7 degraded/ghost and 34 live tree islands. The mean Hg concentration in surface soils of ghost tree islands was low and similar to marsh soil. For live tree islands, Hg concentrations in the surface head region were considerably greater than those in mid and tail region, and marsh soils. Hg concentrations in bird guano (286 μg kg −1 ) were significantly higher than those in mammal droppings (105 μg kg −1 ) and plant leaves (53 μg kg −1 ). In addition, Hg concentrations and δ 15 N values displayed positive correlation in soils influenced by guano. During 1998–2010, estimated annual Hg deposition by guano was 148 μg m −2 yr −1 and ∼8 times the atmospheric deposition.

Angiogenesis plays an essential role in many physiological and pathological processes. Quercetin, a plant pigment and traditional Chinese medicinal herb, is an important flavonoid that has anti-cancer activity. However, the function of quercetin in blood vessel development in vivo and in vitro is still unclear. In this study, we investigated the anti-angiogenic activity of quercetin in zebrafish embryos and in human umbilical vein endothelial cells (HUVECs). Our results showed that quercetin disrupted the formation of intersegmental vessels, the dorsal aorta and the posterior cardinal vein in transgenic zebrafish embryos. In HUVECs, quercetin inhibited cell viability, the expression of vascular endothelial growth factor receptor 2 and tube formation in a dose-dependent manner. In inhibiting angiogenesis, quercetin was found to be involved in suppressing the extracellular signal-regulated kinase signaling pathway in vivo and in vitro. This study has shown that quercetin has potent anti-angiogenic activity and may be a candidate anti-cancer agent for future research.

Spatially variable areas, or hotspots, of elevated mercury (Hg) concentrations in soil, water, and wildlife occur throughout the Everglades wetland ecosystem. This study investigates the stoichiometric controls of Hg relative to soil, water, and biotic components. Surface water, porewater, soil, periphyton, and Gambusia spp. (mosquitofish) were collected from hotspots and non-spot stations and analyzed for various parameters, including total mercury (THg), organic carbon (OC), total carbon (TC), total phosphorus (TP), and total nitrogen (TN) between late 2010 and early 2013. Soil nutrient ratios were significantly different between hotspot and non-hotspot stations, indicating a difference in trophic status and position along the decay continuum or differences in limiting nutrients. Overall, soil total Hg concentrations were negatively correlated with soil TC/TN, while soil TC/TP and soil TN/TP molar ratios and soil THg were negatively correlated at hotspot stations. Meanwhile, mosquitofish THg was negatively correlated with soil TC/TN molar ratio and positively correlated with soil TC/TP and TN/TP molar ratios, suggesting trophic truncation. Soil, surface water, and porewater THg, TC, and OC interactions resulted in significant differences between hotspot and non-hotspot stations and between molar ratios of C, N, and P. Periphyton-surface water THg/OC homeostasis and soil nutrient ratios significantly explained mosquitofish THg concentrations, further indicating a trophic influence on mosquitofish THg and potential hotspot dynamics. Several factors and processes including bottom-up trophic interaction and vegetation influence on Hg accumulation dynamics and food-chain length explain the development and persistence of Hg hotspot formation within the Everglades system.

Variations of total mercury (THg) and methylmercury (MeHg) concentrations at the inflow and outflow of a constructed treatment wetland, Stormwater Treatment Area 2 (STA-2) in south Florida were investigated from 2000 to 2011. The linkages between inflow water quality parameters and outflow THg and MeHg concentrations were assessed. THg and MeHg concentrations at the outflow in 2001 (2.88 and 0.95 ng L −1 ) and 2002 (2.33 and 0.87 ng L −1 ) were significantly greater than those (1.14 and 0.14 ng L −1 , 1.16 and 0.12 ng L −1 , respectively) at the inflow during the same time period. This was due to dryout and rewetting which may lead to Hg release from soil and promote Hg methylation. The average concentrations (0.75 and 0.11 ng L −1 ) of THg and MeHg at the outflow from 2005 to 2011 (except 2007) were lower than those (1.00 and 0.13 ng L −1 ) at the inflow, indicating that net THg and MeHg retention occurred in STA-2 during those years, which ranged from 0.35% to 49% for THg and from 3.5% to 37% for MeHg, respectively. Stepwise regression identified several inflow water quality parameters which explained 68% and 58% of the variances in the outflow THg and MeHg concentrations from 2000 to 2011. Certain inflow quality parameters were strong predictors of the variations in outflow THg and MeHg concentrations. These parameters included sulfate, chloride, dissolved organic carbon (DOC), dissolved oxygen (DO), pH, and temperature for outflow THg; and sulfate, chloride, DOC, and DO for outflow MeHg. Inflow sulfate and DO concentrations separately exhibited significant correlations with outflow THg and MeHg concentrations only prior to 2004. The results indicate that STA-2 was a source of THg and MeHg during early years of operation and a sink for THg and MeHg when the wetland passed the initial operation period and especially as water management improved to prevent dryout from occurring. Findings from this study also underscore the important role of inflow water quality to the variations of the outflow THg and MeHg concentrations.

We investigated the mechanisms of Hg sorption onto biochars produced from Brazilian pepper (BP; Schinus terebinthifolius) at 300, 450, and 600 °C using different analytical techniques. The Hg sorption capacity of BP300, BP450, and BP600 was 24.2, 18.8, and 15.1 mg g(-1) based on Langmuir isotherm. FTIR data suggested the participation of phenolic hydroxyl and carboxylic groups in Hg sorption by biochars. XPS analysis showed that 23-31% and 77-69% of sorbed Hg was associated with carboxylic and phenolic hydroxyl groups in biochars BP300-450, whereas 91% of sorbed Hg was associated with a graphite-like domain on an aromatic structure in BP600 biochar, which were consistent with flow calorimetry data. Based on flow calorimetry, sorption of K and Ca onto biochar was exchangeable with the molar heat of sorption of 3.1 kJ mol(-1). By comparison, Hg sorption was via complexation with functional groups as it was not exchangeable by K or Ca with molar heat of sorption of -19.7, -18.3, and -25.4 kJ mol(-1) for BP300, BP450, and BP600. Our research suggested that Hg was irreversibly sorbed via complexation with phenolic hydroxyl and carboxylic groups in low temperature biochars (BP300 and BP450) and graphite-like structure in high temperature biochar (BP600).

Silver carp ( Hypophthalmichthys molitrix ) and bighead carp ( Aristichthys nobilis ) are important cultivated fish and their dietary sources have been subject to many studies. In the present work, stable isotope analysis (δ 13 C and δ 15 N) was used to assess the relative contributions of natural and artificial diets to the growth and dietary overlap in these fish. In an indoor growth experiment, the δ 13 C and δ 15 N values of muscle tissue of silver carp and bighead carp fed a fish meal as the sole protein source revealed identical isotope enrichment. In an outdoor growth experiment conducted in fishponds, the relative contribution of particulate organic matter (POM) and a commercial composite diet (CD) to the growth of both carps was estimated using a two-source isotope mixing model. Results revealed that bighead carp used more POM than silver carp for their growth. Estimates of dietary overlap between these fish suggest that silver carp and bighead carp partially shared the same diets for growth. This finding is consistent with the results of several recent studies, demonstrating that the trophic niche of these planktivores was not segregated, as was previously suggested.

The transport of dissolved inorganic nitrogen in wetlands was investigated in the Yeyahu National Preserve of Beijing, China, from July to December 2011. Stable isotope of nitrogen ( 15 N) as NH 4 + was added to experimental mesocosms vegetated by seven common wetland plants. The 15 N abundance in soil and various plant parts were measured at prescribed time intervals during the 49-day experimental period. Rapid 15 N enrichments were found in soil and aquatic plants during the first sampling event at day 3 and typically increased linearly during the experimental period. Highest 15 N enrichment was found in submerged plants, followed by emergent plants and soil. No differences in δ 15 N values were found among leaves and stems of all submerged plants ( Potamogeton pusillus , Potamogeton perfoliatus , Najas marina , Ceratophyllum demersum ), suggesting that these plant structures are equally effective in nitrogen assimilation. The roots of P. perfoliatus displayed the lowest enrichment compared to the stems and leaves. Such results indicate that submerged plant roots may have a relatively weak role in nitrogen assimilation. The two emergent plants ( Phragmites australis and Typha orientalis ) showed no differences in δ 15 N values among all plant parts, suggesting rapid translocation of nitrogen from roots to other portions. This experiment demonstrates that wetland soil and plant stems and leaves are major sinks for dissolved nitrogen from the surface water and provides valuable insight into nitrogen retention and transport pathways in various wetland plants.

a b s t r a c t Plant species richness and hydraulic retention time (HRT) are two important variables in mediating pollu- tant removals of constructed wetlands, but their collective effects have not been clear. Pollutant removals in vertical flow mesocosms were investigated using both plant species richness and HRT as fixed factors. Both BOD5 and COD removals were enhanced by the longer HRT and the significant species richness × HRT interaction, as shown by two-way ANOVA. Differently, the removals of the NH4-N, NO3-N and phosphate were improved by the species richness, HRT, and the species richness × HRT interaction. The present findings highlight the importance of the interaction between the plant species richness and the HRT in improving the pollutant removals in vertical flow constructed wetlands.

Damming is one of the dramatic impacts to river food webs due to habitat fragmentation and changes in the hydrological regimes. The world’s largest dam, the Three Gorges Dam (TGD), has been constructed in the middle of Yangtze River since 2003. The objective of this study was to understand the effects of altered hydrological regime on the trophic structure of fish food web along the hydrological gradient in the TGD. Fish samples were collected from an upstream, a midstream and a downstream site of the Yangtze River upstream of the TGD in May and September of 2004. Muscle tissue of each fish species was determined for stable isotopic compositions (δ13C and δ15N) which were used to calculate isotope-based Bayesian community-wide trophic metrics. Stable isotope trophic niche analysis reveals reduced utilization of basal resource and trophic niche space at the midstream and downstream sites. By contrast, community trophic diversity and species redundancy were higher at the downstream than the upstream and midstream, likely as the result of reduced the spectrum of basal resources. These findings suggested that the negative impacts on the overall trophic niche space from reduced flow, increased water depth and stagnancy occurred only one year after the completion of the TGD. Altered hydrological regimes posed multiple impacts to the aquatic food web. Reduced flow and increased water depth within TGD has posed initial effects to the fish community trophic structure. Further collections ana analysis of ecological data to compare the fish trophic structure revealed in this study are needed to assess the long-term impacts of TGD on fish community structure and resources utilization.

A paleolimnological evaluation was made in order to analyze the effects of increasing nutrient load, macrophyte degradation and fish introductions on the cladoceran community of a large, shallow plateau lake in southwestern China. The trophic state of Lake Dianchi has increased rapidly during recent decades, its macrophyte vegetation has suffered severe degradation, and fish introductions in the late 1950s and early 1980s have had a marked effect on the structure of the fish community. Our results show an increase in abundance of cladoceran species with a preference for eutrophic conditions over the last few decades, while species preferring oligotrophic conditions have decreased or disappeared. These changes correspond to the eutrophication in Lake Dianchi. The loss of the cladocerans Kurzia latissima and Disparalonarostrata is likely to be a reflection of the degradation of the macrophyte community. An increase in Daphnia body size indicated by the ephippia length since the early 1990s is associated with the decline of planktivorous species.