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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.

Stable carbon and nitrogen isotope values (δ13C and δ15N) were used to evaluate trophic niche overlap between two filter-feeding fishes (known together as bigheaded carp) native to China, silver carp (Hypophthalmichthys molitrix) and bighead carp (Hypophthalmichthys nobilis), and three native filter-feeding fish including bigmouth buffalo (Ictiobus cyprinellus), gizzard shad (Dorosoma cepedianum) and paddlefish (Polyodon spathula) in the lower Missouri River, USA, using the Bayesian Stable Isotope in R statistics. Results indicate that except for bigmouth buffalo, all species displayed similar trophic niche size and trophic diversity. Bigmouth buffalo occupied a small trophic niche and had the greatest trophic overlap with silver carp (93.6%) and bighead carp (94.1%) followed by gizzard shad (91.0%). Paddlefish had a trophic niche which relied on some resources different from those used by other species, and therefore had the lowest trophic overlap with bigheaded carp and other two native fish. The trophic overlap by bigheaded carp onto native fish was typically stronger than the reverse effects from native fish. Average niche overlap between silver carp and native species was as high as 71%, greater than niche overlap between bighead carp and native fish (64%). Our findings indicate that bigheaded carps are a potential threat to a diverse and stable native fish community.

The purpose of this study was to assess the water quality and variations in stable carbon and nitrogen isotopes of particulate organic matter (δ 13 C POM and δ 15 N POM ), 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 (SO 4 2− , 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 δ 13 C POM , δ 15 N POM , 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 δ 13 C POM and δ 15 N POM . δ 13 C POM 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 δ 15 N POM (< 0‰) and negative correlation between δ 15 N POM and TN suggested discharge of agricultural waste water (e.g., fertilizers, irrigation tailwater) in Bailanghe, Xinan, and Taihe Reservoir during the fertilization season, while higher values indicated domestic sewage input to waterbodies (e.g., Mishan, Gengjing, Donghai Reservoir). Our results suggested that the aquatic ecosystem in water-receiving area of SNWTP would be potentially affected by the inter-basin water diversion, and thus, ecosystem-based strategies were also presented accordingly.

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 (δ 15 N) in fish are evaluated as indicators of environmental changes in south Florida wetlands. Stable nitrogen isotope (δ 15 N) 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 δ 15 N ratios. The δ 15 N 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 δ 15 N ratios in several fish species. The pattern of changes in δ 15 N ratios along nutrient gradients suggests that increased eutrophication is recorded as the δ 15 N 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 δ 15 N ratios in fish. Results from this analysis demonstrate that δ 15 N ratios in fish integrate biotic responses to eutrophic process over time and could be a robust indicator for early ecological changes. L'eutrophisation d'origine humaine a altéré les processus écologiques des écosystèmes aquatiques. La détection des changements écologiques est une condition préalable à la protection des écosystèmes contre la dégradation. Dans cette étude, les isotopes stables de l'azote (δ 15 N) chez les poissons sont évalués comme indicateurs des changements environnementaux dans les zones humides du sud de la Floride. Les données sur les isotopes stables de l'azote (δ 15 N) de certaines espèces de poissons et sur la qualité de l'eau recueillies dans les Everglades de Floride entre les années 1990 et 2000 ont été utilisées pour évaluer la relation entre les concentrations de phosphore total et les rapports δ 15 N. Les rapports δ 15 N de neuf des dix espèces de poissons sélectionnées augmentent de manière significative lorsque la concentration de phosphore total dans les eaux de surface augmente. On a constaté des relations significatives entre la concentration d'azote total dans les eaux de surface et les rapports δ 15 N chez plusieurs espèces de poissons. Le schéma des changements des rapports δ 15 N le long des gradients de nutriments suggère qu'une eutrophisation accrue est enregistrée dans les rapports δ 15 N chez les poissons. L'absence de charge en eaux usées, la prédominance du ruissellement agricole et le rapport élevé TN:TP suggèrent que le phosphore est le facteur limitant qui détermine la productivité des écosystèmes et les changements des rapports δ 15 N chez les poissons. Les résultats de cette analyse montrent que les rapports δ 15 N chez les poissons intègrent les réponses biotiques au processus d'eutrophisation au fil du temps et pourraient être un indicateur robuste des premiers changements écologiques.

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). Under various salinity and temperature conditions, the relative preference index (RPI) of ammonium was greater than that of nitrite, urea, and nitrate; Ammonium was the preferred nitrogen source for O. borgei. The optimal combination of environmental conditions for ρ(NH4+-N) was temperature of 20 °C, pH of 7.5, light intensity of 81 μmol m−2 s−1, salinity of 15‰, and algal concentration of 5.5 × 108 cell L−1. The model equation wasρ = 0.0017 × (A0.180B0.252C0.748D0.587E) + 0.0512, with the coefficient of determination (R2) of 0.89. No significant difference was observed between the model-predicted values and the measured values (F = 0.789, 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.

Understanding the carbon sources supporting aquatic consumers in large rivers is essential for the protection of ecological integrity and for wildlife management. The relative importance of terrestrial and algal carbon to the aquatic food webs is still under intensive debate. The Yangtze River is the largest river in China and the third longest river in the world. The completion of the Three Gorges Dam (TGD) in 2003 has significantly altered the hydrological regime of the middle Yangtze River, but its immediate impact on carbon sources supporting the river food web is unknown. In this study, potential production sources from riparian and the main river channel, and selected aquatic consumers (invertebrates and fish) at an upstream constricted-channel site (Luoqi), a midstream estuarine site (Huanghua) and a near dam limnetic site (Maoping) of the TGD were collected for stable isotope (δ13C and δ15N) and IsoSource analyses. Model estimates indicated that terrestrial plants were the dominant carbon sources supporting the consumer taxa at the three study sites. Algal production appeared to play a supplemental role in supporting consumer production. The contribution from C4 plants was more important than that of C3 plants at the upstream site while C3 plants were the more important carbon source to the consumers at the two impacted sites (Huanghua and Maoping), particularly at the midstream site. There was no trend of increase in the contribution of autochthonous production from the upstream to the downstream sites as the flow rate decreased dramatically along the main river channel due to the construction of TGD. Our findings, along with recent studies in rivers and lakes, are contradictory to studies that demonstrate the importance of algal carbon in the aquatic food web. Differences in system geomorphology, hydrology, habitat heterogeneity, and land use may account for these contradictory findings reported in various studies.

[This corrects the article DOI: 10.1371/journal.pone.0197584.].

Carbon stable isotopes (δ¹³C) of particulate organic matter (POM) have been used as indicators for energy flow, primary productivity and carbon dioxide concentration in individual lakes. Here, we provide a synthesis of literature data from 32 freshwater lakes around the world to assess the variability of δ¹³CPOM along latitudinal, morphometric and biogeochemical gradients. Seasonal mean δ¹³CPOM, a temporally integrated measure of the δ¹³CPOM, displayed weak relationships with all trophic state indices [total phosphorus (TP), total nitrogen (TN), and chlorophyll a (Chl a)], but decreased significantly with the increase in latitude, presumably in response to the corresponding decrease in water temperature and increase in CO₂ concentration. The seasonal minimum δ¹³CPOM also correlated negatively with latitude while seasonal maximum δ¹³CPOM correlated positively with all trophic state indices, pH, and δ¹³C of dissolved inorganic carbon (DIC). Seasonal amplitude of δ¹³CPOM (the difference between seasonal maximum and minimum values) correlated significantly with pH, TP and Chl a concentrations and displayed small variations in oligotrophic, mesotrophic and low latitude eutrophic lakes, which is attributed to low primary productivity and abundant non-living POM in the low trophic state lakes and relatively stable environmental conditions in the subtropics. Seasonal amplitude of δ¹³CPOM was the greatest in high latitude eutrophic lakes. Greater seasonal changes in solar energy and light regime may be responsible for the large seasonal variability in high latitude productive lakes. This synthesis provides new insights on the factors controlling variations in stable carbon isotopes of POM among lakes on the global scale.

Issue Title: Special Issue: Beijing International Symposium on Biological Invasions Hydrilla verticillata is considered the most problematic aquatic plant in the United States. In south Florida, Hydrilla dominance has also been documented in treatment wetlands. This paper characterizes (1) environmental conditions which favor Hydrilla growth and (2) understand its nutrient removal capability. Despite its occurrence over a wide range of environmental conditions, Hydrilla abundance increased with increasing pH, alkalinity, total P and total N, and decreased with water depth in selected Florida lakes. No relationship was found between color, Secchi depth and Hydrilla abundance. In several Hydrilla-dominated lakes, mean total P concentration (126 μg/l) at inflow was reduced to 106 μg/l at outflow. The maximum inflow total P concentration in a lake with positive nutrient reduction was 148 μg/l. Total P removal efficiency by Hydrilla-dominated lakes and wetlands was comparable to or higher than systems dominated by emergent and other submerged plants. Mean total P settling rates for lakes and a constructed wetland dominated by Hydrilla were estimated at 19 and 34 m/year, respectively, which were higher than or comparable to similar systems dominated by other aquatic plants. Results from this study suggest that reduction of Hydrilla from constructed wetlands will not likely improve nutrient removal performance.[PUBLICATION ABSTRACT]

The purposes of this study were to assess if Lake Apopka (FL, USA) was autotrophic or heterotrophic based on the partial pressure of dissolved carbon dioxide ( p CO 2 ) in the surface water and to evaluate factors that influence the long-term changes in p CO 2 . Monthly average pH, alkalinity and other limnological variables collected between 1987 and 2006 were used to estimate dissolved inorganic carbon (DIC), p CO 2 and CO 2 flux between surface water and atmosphere. Results indicated that average p CO 2 in the surface water was 196 μatm, well below the atmospheric p CO 2 . Direct measurements of DIC concentration on three sampling dates in 2009 also supported p CO 2 undersaturation in Lake Apopka. Supersaturation in CO 2 occurred in this lake in only 13% of the samples from the 20-year record. The surface-water p CO 2 was inversely related to Chl a concentrations. Average annual CO 2 flux was 28.2 g C m −2  year −1 from the atmosphere to the lake water and correlated significantly with Chl a concentration, indicating that biological carbon sequestration led to the low dissolved CO 2 concentration. Low p CO 2 and high invasion rates of atmospheric CO 2 in Lake Apopka indicated persistent autotrophy. High rates of nutrient loading and primary production, a high buffering capacity, a lack of allochthonous loading of organic matter, and the dominance of a planktivorous–benthivorous fish food web have supported long-term net autotrophy in this shallow subtropical eutrophic lake. Our results also showed that lake restoration by the means of nutrient reduction resulted in significantly lower total phosphorus (TP) and Chl a concentrations, and higher p CO 2 .