River floodplain ecosystems host one of the highest freshwater molluscan biodiversity on Earth. However, multiple human disturbances, such as loss of hydrological connectivity and deterioration of water quality, are seriously threatening most floodplain lakes throughout the world. Given the high imperilment rate of freshwater molluscs but the scarcity of studies examining the anthropogenic effects on this fauna, we test the response of mollusc assemblages to river-lake disconnection and eutrophication in 30 lakes in the Yangtze River floodplain, China. The species richness of entire Mollusca, Gastropoda and Bivalvia and 6 dominant families were all much lower at disconnected lakes than that in connected lakes, and decreased with increasing water eutrophication. The assemblage structure differed significantly among four lake groups for datasets based on entire Mollusca, Gastropoda and Bivalvia, indicating the serious impacts of hydrological disconnection and eutrophication. Moreover, the connected lakes showed significantly lower values of average taxonomic distinctness (Δ⁺) but higher values of variation in taxonomic distinctness (Λ⁺) than disconnected lakes. Such variations were triggered by the extirpation of congeneric and endemic species (mainly from families Unionidae and Viviparidae), which giving a waring of the loss of mollusc endemism in this region. In general, the present study showed that river-lake disconnection and deterioration of water quality resulted in serious biodiversity declines of both gastropods and bivalves in the Yangtze River floodplain lakes. A systematic approach including restoration of river-lake connectivity and habitats and improvement of water quality should be implemented in the conservation planning in this large river floodplain.

We examined polycyclic aromatic compounds (PACs) and petroleum biomarkers (steranes, hopanes, and terpanes) in radiometrically-dated lake sediment cores from the Athabasca oil sands region (AOSR) and the Peace-Athabasca Delta (PAD) region in Alberta (Canada) to determine whether contributions from petroleum hydrocarbons have changed over time. Two floodplain lakes in the PAD (PAD 30, PAD 31) recorded increased flux of alkylated PACs and increased petrogenic (petroleum-derived) hydrocarbons after ∼1980, coincident with a decline of sediment organic carbon content and a rise of bulk sedimentation rate, likely due to increased Athabasca River flow. A large expansion of upstream oilsands mining, upgrading, and refining may also have contributed to the observed shift to more petrogenic hydrocarbons to sediments since the 1980s. Alkylated PAC flux increased in the floodplain lake analyzed within the AOSR (Saline Lake) since the 1970s–1980s, coincident with a sharp rise in sediment organic carbon content and increased contributions of petrogenic hydrocarbons. These changes identify increased supply of petrogenic PACs occurred as Athabasca River floodwaters waned, and may implicate aerial contributions of petrogenic hydrocarbons from oilsands activity. PACs and petroleum biomarkers (steranes, hopanes, and terpanes) in sediment cores from Saline Lake, PAD 30 and PAD 31 revealed a predominance of petrogenic hydrocarbons in these lakes. In contrast, we recorded minimal petrogenic hydrocarbons in the reference lakes outside the surface minable area of the AOSR and PAD (Mariana Lake and BM11), though we noted slight increases in petrogenic contributions to modern (2010–2016) sediments. We show how a combined analysis of PACs and petroleum biomarkers in sediments is useful to quantify petrogenic contributions to lakes with added confidence and highlight the potential for petroleum biomarkers in lake sediment cores as a novel and effective method to track petroleum hydrocarbons in lake sediment.

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.

This study evaluated three biochars derived from bioenergy by-products — manure-based anaerobic digestate (DIG), distillers’ grains (DIS), and a mixture thereof (75G25S) — as amendments to stabilize Hg in contaminated floodplain soil under long-term saturated (up to 200 d) and cyclic drying and rewetting conditions. Greater total Hg (THg) removal (72 to nearly 100%) and limited MeHg production (<65 ng L⁻¹) were observed in digestate-based biochar-amended systems under initial saturated conditions. Drying and rewetting resulted in limited THg release, increased aqueous MeHg, and decreased solid MeHg in digestate-based biochar-amended systems. Changes in Fe and S chemistry as well as microbial communities during drying and rewetting potentially affected MeHg production. Digestate-based biochars may be more effective as amendments to control Hg release and minimize MeHg production in floodplain soils under long-term saturated and drying and rewetting conditions compared to distillers’ grains biochar.

In parts of developing countries, the over-exploitation of sands from inland waters has led to serious environmental concerns. However, understanding of the impacts of commercial sand dredging on inland water ecosystem functions remains limited. Herein, we assess the effects of this activity on the functional structure of the macroinvertebrate community and its recovery processes based on a 4-year survey, in the South Dongting Lake in China. Our result showed a simplified macroinvertebrate functional structures within the dredged area, as evidenced by a loss of certain trait categories (e.g., oval and conical body form) and a significant reduction in trait values due to the direct removal of macroinvertebrates and indirect alternations to physical environmental conditions (e.g., water depth and %Medium sand). Moreover, clear increases were observed in certain trait categories (e.g., small body size and swimmer) resulting from the dredging-related disturbance (e.g., increased turbidity) within the adjacent area. Furthermore, one year after the cessation of dredging, a marked recovery in the taxonomic and functional structure of macroinvertebrate assemblages was observed with most lost trait categories returning and an increase in the trait values of eight categories (e.g., body size 1.00–3.00 cm and oval body form) within the dredged and adjacent area. In addition, dispersal processes and sediment composition were the main driver for the structuring of the macroinvertebrate taxonomic and functional assemblages during the dredging stages, whilst water environmental conditions dominated the taxonomic structure and dispersal processes determined the functional structure during the recovery stage. Implications of our results for monitoring and management of this activity in inland waters are discussed.

Plastic, and especially microplastic, contamination of soils has become a novel research field. After the detection of microplastics in soils, spatial distribution and dynamics are still unknown. However, the potential risks associated with plastic particles in soils cannot be sufficiently assessed without knowledge about the spatial distribution of these anthropogenic materials. Based on a spatial research approach, including soil surveys, this study quantified the mesoplastic (MEP, > 5.0 mm) and coarse microplastics (CMP, 2.0–5.0 mm) content of twelve floodplain soils. At four transects in the catchment area of the Lahn river (Germany), soils down to a depth of 2 m were examined for plastic content for the first time. MEP and CMP were detected through visual examination after sample preprocessing and ATR-FTIR analyses. Average MEP and CMP concentrations range between 2.06 kg⁻¹ (±1.55 kg⁻¹) and 1.88 kg⁻¹ (±1.49 kg⁻¹) with maximal values of 5.37 MEP kg⁻¹ to 8.59 CMP kg⁻¹. Plastic particles are heterogeneously distributed in samples. Both plastic size classes occur more frequently in topsoils than in soil layers deeper than 30 cm. The maximal depth of CMP occurrence lies between 75 and 100 cm. Most common CMP polymer type was PE-LD, followed by PP and PA. MEP and CMP particles occur frequently at near channel sides and more often on riparian strips or grassland than on farmland. Vertical distribution of CMP indicates anthropogenic relocation in topsoils and additional deep displacement through natural processes like preferential flow paths or bioturbation. By comparing sedimentation rates of the river with the maximum age of plastic particles, sedimentation as a deposition process of plastic in floodplains becomes probable. From our findings, it can be concluded that an overall widespread but spatial heterogenous contamination occurs in floodplain soils. Additionally, a complex plastic source pattern seems to appear in floodplain areas.

Mining activities have contaminated many riverine floodplains with arsenic (As). When floodplain soils become anoxic under water-saturated conditions, As can be released from the solid phase. Several microbially-driven As solubilization processes and numerous influential factors were recognized in the past. However, the interplay and relative importance of soil properties and the influence of environmental factors such as temperature remain poorly understood, especially considering the (co)variation of soil properties in a floodplain. We conducted anoxic microcosm experiments at 10, 17.5, and 25 °C using 65 representative soils from the mining-impacted Ogosta River floodplain in Bulgaria. To investigate the processes of As solubilization and its quantitative variation we followed the As and Fe redox dynamics in the solid and the dissolved phase and monitored a range of other solution parameters including pH, Eh, dissolved organic C, and dissolved Mn. We related soil properties to dissolved As observed after 20 days of microcosm incubation to identify key soil properties for As solubilization. Our results evidenced reductive dissolution of As-bearing Fe(III)-oxyhydroxides as the main cause for high solubilization. The availability of nutrients, most likely organic C as the source of energy for microorganisms, was found to limit this process. Following the vertical nutrient gradient common in vegetated soil, we observed several hundred μM dissolved As after 1–2 weeks for some topsoils (0–20 cm), while for subsoils (20–40 cm) with comparable total As levels only minor solubilization was observed. While high Mn contents were found to inhibit As solubilization, the opposite applied for higher temperature (Q10 2.3–6.1 for range 10–25 °C). Our results suggest that flooding of nutrient-rich surface layers might be more problematic than water-saturation of nutrient-poor subsoil layers, especially in summer floodings when soil temperature is higher than in winter or spring.

Arsenic contamination of floodplain soils is extensive and additional fresh arsenic inputs to the pedosphere from human activities are ongoing.We investigate the cumulative effects of repetitive soil redox cycles, which occur naturally during flooding and draining, on a calcareous fluvisol, the native microbial community and arsenic mobility following a simulated contamination event.We show through bioreactor experiments, spectroscopic techniques and modelling that repetitive redox cycling can decrease arsenic mobility during reducing conditions by up to 45%. Phylogenetic and functional analyses of the microbial community indicate that iron cycling is a key driver of observed changes to solution chemistry. We discuss probable mechanisms responsible for the arsenic immobilisation observed in-situ. The proposed mechanisms include, decreased heterotrophic iron reduction due to the depletion of labile particulate organic matter (POM), increases to the proportion of co-precipitated vs. aqueous or sorbed arsenic with α-FeOOH/Fe(OH)3 and potential precipitation of amorphous ferric arsenate.

Mercury (Hg) is a persistent environmental contaminant found in many freshwater and marine ecosystems. Historical Hg contamination in rivers can impact the surrounding terrestrial ecosystem, but there is little known about how far downstream this contamination persists. In 2009, we sampled terrestrial forest songbirds at five floodplain sites up to 137 km downstream of an historical source of Hg along the South and South Fork Shenandoah Rivers (Virginia, USA). We found that blood total Hg concentrations remained elevated over the entire sampling area and there was little evidence of decline with distance. While it is well known that Hg is a pervasive and long-lasting aquatic contaminant, it has only been recently recognized that it also biomagnifies effectively in floodplain forest food webs. This study extends the area of concern for terrestrial habitats near contaminated rivers for more than 100 km downstream from a waterborne Hg point source.

Understanding the key drivers of eutrophication in floodplain lakes has long been a challenge. In this study, the Chlorophyll a (Chla) variations and associated relationships with environmental stressors along the temporal hydrological connectivity gradient were investigated using a 11-year dataset in a large floodplain lake (Poyang Lake). A geostatistical method was firstly used to calculate the hydrological connectivity curves for each sampling campaign that was further classified by K-means technique. Linear mixed effect (LME) models were developed through the inclusion of the site as a random effect to identify the limiting factors of Chla variations. The results identified three clear hydrological connectivity variation patterns with remarkable connecting water area changes in Poyang Lake. Furthermore, hydrological connectivity changes exerted a great influence on environmental variables in Poyang Lake, with a decrease in nutrient concentrations as the hydrological connectivity enhanced. The Chla exhibited contrast variations with nutrient variables along the temporal hydrological connectivity gradient and generally depended on WT, DO, EC and TP, for the entire study period. Nevertheless, the relative roles of nutrient and non-nutrient variables in phytoplankton growth varied with different degrees of hydrological connectivity as confirmed by the LME models. In the low hydrological connectivity phase, the Chla dynamics were controlled only by water temperature with sufficient nutrients available. In the high hydrological connectivity phase, the synergistic influences of both nutrient and physical variables jointly limited the Chla dynamics. In addition, a significant increasing trend was observed for Chla variations from 2008 to 2018 in the HHC phase, which could largely be attributed to the elevated nutrient concentrations. This study confirmed the strong influences of hydrological connectivity on the nutrient and non-nutrient limitation of phytoplankton growth in floodplain lakes. The present study could provide new insights on the driving mechanisms underlying phytoplankton growth in floodplain lakes.