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.

Lead concentrations in hard antlers of adult European roebucks (Capreolus capreolus) were analyzed to assess lead exposure of roe deer roaming the floodplain of the Innerste River, a river system contaminated due to historical metal ore mining, processing, and smelting in its upper reaches. Antler lead concentrations of roebucks culled in the period 1939–2018 within or close to the Innerste floodplain ranged between <0.17 mg Pb/kg (limit of detection) and 51.5 mg Pb/kg (air-dry weight). Median lead concentration in antlers of roebucks culled within the floodplain was 11.1 mg Pb/kg, compared to 2.3 mg Pb/kg in antlers of bucks culled in the floodplain vicinity (P < 0.01). Sampling year had no significant effect on antler lead concentrations (P = 0.748). Lead isotope ratios of antlers from the Innerste downstream area (²⁰⁶Pb/²⁰⁷Pb: 1.179–1.181; ²⁰⁸Pb/²⁰⁶Pb: 2.083–2.085) fell within the range of those reported for hydrothermal vein deposits from the upper catchment area of the Innerste River in the Harz Mountains. Our study demonstrates the long-lasting impact of the historical metal ore mining, processing, and smelting in the Harz Mountains on lead pollution in floodplains of rivers draining this area and the lead exposure of wild herbivores inhabiting the floodplains. Furthermore, it highlights the suitability of roe deer antlers for monitoring environmental lead levels and the usefulness of lead isotope signatures in antlers for source apportionment of lead pollution.

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.

Reservoirs typically have elevated fish mercury (Hg) levels compared to natural lakes and rivers. A unique feature of reservoirs is water-level management which can result in sediment exposure to the air. The objective of this study is to identify how reservoir water-level fluctuations impact Hg cycling, particularly the formation of the more toxic and bioaccumulative methylmercury (MeHg). Total-Hg (THg), MeHg, stable isotope methylation rates and several ancillary parameters were measured in reservoir sediments (including some in porewater and overlying water) that are seasonally and permanently inundated. The results showed that sediment and porewater MeHg concentrations were over 3-times higher in areas experiencing water-level fluctuations compared to permanently inundated sediments. Analysis of the data suggest that the enhanced breakdown of organic matter in sediments experiencing water-level fluctuations has a two-fold effect on stimulating Hg methylation: 1) it increases the partitioning of inorganic Hg from the solid phase into the porewater phase (lower log Kd values) where it is more bioavailable for methylation; and 2) it increases dissolved organic carbon (DOC) in the porewater which can stimulate the microbial community that can methylate Hg. Sulfate concentrations and cycling were enhanced in the seasonally inundated sediments and may have also contributed to increased MeHg production. Overall, our results suggest that reservoir management actions can have an impact on the sediment-porewater characteristics that affect MeHg production. Such findings are also relevant to natural water systems that experience wetting and drying cycles, such as floodplains and ombrotrophic wetlands.

Arsenic and other trace element concentrations were determined for tube-well water collected in the Lao PDR provinces of Attapeu, Bolikhamxai, Champasak, Savannakhet, Saravane, and Vientiane. Water samples, especially from floodplain areas of central and southern Laos, were significantly contaminated not only with As, but with B, Ba, Mn, U, and Fe as well. Total As concentrations ranged from <0.5μgL⁻¹ to 278μgL⁻¹, with over half exceeding the WHO guideline of 10μgL⁻¹. 46% of samples, notably, were dominated by As(III). Samples from Vientiane, further north, were all acceptable except on pH, which was below drinking water limits. A principal component analysis found associations between general water characteristics, As, and other trace elements. Causes of elevated As concentrations in Lao tube wells were considered similar to those in other Mekong River countries, particularly Cambodia and Vietnam, where young alluvial aquifers give rise to reducing conditions.

The acid volatile sulphide (AVS) and simultaneously extracted metals (ΣSEM) method is increasingly used for risk assessment of toxic metals. In this study, we assessed spatial and temporal variations of AVS and ΣSEM in river sediments and floodplain soils, addressing influence of flow regime and flooding. Slow-flowing sites contained high organic matter and clay content, leading to anoxic conditions, and subsequent AVS formation and binding of metals. Seasonality affected these processes through temperature and oxygen concentration, leading to increased levels of AVS in summer at slow-flowing sites (max. 37 μmol g-1). In contrast, fast-flowing sites hardly contained AVS, so that seasonality had no influence on these sites. Floodplain soils showed an opposite AVS seasonality because of preferential inundation and concomitant AVS formation in winter (max. 3-30 μmol g-1). We conclude that in dynamic river systems, flow velocity is the key to understanding variability of AVS and ΣSEM. Flow velocity is the key to understanding variability of AVS and ΣSEM in river sediment.

Assessments of antimony (Sb) and arsenic (As) contamination in sediments are reported on a wide range of different particle size fractions, including <63 μm, < 180 μm and <2 mm. Guidelines vary between jurisdictions which limits comparative assessment between contamination events and complicates ecotoxicity assessment, and almost no information exists on Sb size distribution in contaminated sediments. This study quantified and compared the size distribution of Sb and As in 11 sediments (and 2 floodplain soils) collected along 320 km of waterway contaminated by historic mining activity. Sediment particle size distribution was the primary determinant of total metalloid load in size fractions across the varying substrates of the waterway. Minerals and sorption complexes influenced metalloid particle distribution but relative importance depended on location. Arsenic concentrations were greatest in the fine <63 μm fraction across all the different river environments (7.3–189 mg kg⁻¹, or 1–26% of total sample As), attributed to fine-grained primary arsenopyrite and/or sorption of As(V) to fine solid-phases. The Sb particle size concentrations were greatest in mid-size fractions (205–903 mg kg⁻¹) in the upper catchment and up to 100 km downstream to the mid-catchment as a result of remnant Sb minerals. Antimony concentrations in the lower catchment were greatest in the <63 μm fraction (8.8–12.1 mg kg⁻¹), reflecting the increasing importance of sorption for Sb particle associations. This work demonstrates the importance of particle size analysed for assessment of sediment quality, and provides support for analysis of at least the <250 μm fraction for Sb and As when comparing pollutant distribution in events impacted by primary contamination. Analysis of the <63 μm fraction, however, provides good representation in well-dispersed contaminated sediments.

Juvenile Chinook salmon (Oncorhynchus tshawytscha) of the Sacramento River system encounter many anthropogenically-induced stressors while rearing and migrating to the Pacific Ocean. Located in a prominent agricultural region, the watershed serves as a source of notable contaminants including pesticides. Salmon rearing in riverine and floodplain areas are potentially exposed to these compounds via dietary exposure, which can vary based on selected food webs. Previous studies have suggested that juvenile Chinook salmon rearing in riverine and floodplain environments of the Sacramento River watershed are characterized by different dietary preferences, with potential for contrasting pesticide exposure between habitats. To examine the potential for pesticide exposure, juvenile Chinook salmon and known dietary items were collected in the mainstem Sacramento River and an adjacent floodplain, the Yolo Bypass, in 2019 and 2020, and analyzed for 33 pesticides, including degradates and isomers. Organochlorine pesticides including the DDX group (p,p’-DDT, p,p’-DDD and p,p’-DDE) were prevalent in all examined biota. There was a significantly greater number of total pesticide detections across all classes in zooplankton compared to macroinvertebrates, coupled with higher bifenthrin concentrations in zooplankton across regions and years, which may indicate different exposure potential depending on fish dietary preferences. Detection frequencies and concentrations of organochlorines were higher in prey items during flooding than in drought conditions, suggesting resuspension of legacy compounds. Significantly higher concentrations of organochlorines were recorded in floodplain rearing fish compared to the Sacramento River. These findings suggest that within these habitats, juvenile Chinook salmon feeding primarily on zooplankton within the water column may be exposed to a greater range of pesticides than those feeding on benthic macroinvertebrates, and that the benefits of floodplain rearing may come at a cost of increased organochlorine exposure.

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.

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.