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In-stream water quality reflects the integrated results of hydrological mixing of different water sources and associated biogeochemical transformations. However, quantifying the relative importance of these controls is often challenging, particularly in riparian wetlands due to complex process interactions and marked spatio-temporal heterogeneity in environmental gradients. Here, we established a two-step method to differentiate the dominance of hydrological and biogeochemical controls on water quality in a riparian peatland in northern Germany. First, an isotope-based mixing model was developed for distributed modelling of in-stream water balance over a two-year period. The simulation showed the predominance of groundwater inflows for most of the time period, while lateral inflows and channel leakage became more influential in mid-summer, as stream-groundwater connectivity weakened due to declining groundwater levels. A moderate downstream shift from groundwater to lateral inflow was also observed due to the changing channel network geometries and inflow from field drains. The mixing model was then further applied to predict the in-stream concentrations of nutrients, major ions and trace elements. The predicted concentrations were assumed to be those resulting from hydrological mixing only, while influence of biogeochemical controls were reflected by the prediction deviation from observation. Accordingly, 15 water quality parameters were grouped based on their simulation performances into hydrologically-controlled (Cl

Inland lakes are important ecosystems for the carbon cycle at both regional and global scales. However, a knowledge gap still exists about the correlations between the partial pressure of CO2 (pCO2) and nutrient dynamics in lakes. In this study, we analysed the long-term dynamics of nutrient and pCO2 in Taihu Lake. Strong spatial heterogeneity was observed with highest nutrient concentrations occurring in the River mouth and significant lower concentrations (p

Modelling and predicting nitrate (NO3-N) concentrations at the catchment scale remain challenging as they are controlled by available sources, hydrological connectivity and biogeochemical transformations along the dominant flow paths, which are often spatially heterogenous and highly interacted. To unravel the controlling factors of catchment NO3-N cycling, a grid-based model, mHM-Nitrate, was applied to a 68 km2 mixed land use catchment (Demnitzer Millcreek) near Berlin. Results showed that landscape characteristics dictated the spatial distribution of NO3-N while hydroclimatic variability dominated its temporal dynamics. Restoration of riparian wetlands also mediated the NO3-N concentrations, leading to a modest reduction on NO3-N export (~10% reduction during 2001-2019). Further, the influence of three factors was validated in a spatially distributed sensitivity analysis (SSA) applied on key hydrological and nitrate parameters with a one-year moving window. The SSA results showed that the spatial pattern of parameter sensitivity was determined by NO3-N inputs and hydrological transport capacity, while its temporal dynamics were regulated by annual wetness conditions. Restoration management also contributed to the increase in sensitivity of denitrification parameters. Moreover, SSA identified the influential zones and time periods affecting simulation of NO3-N mobilisation and transport, which provides an evidence base for future model development and optimising of monitoring schemes.

The global increase in the construction of reservoirs has drawn attention given its documented hydrological and biogeochemical impacts on downstream rivers; however, the impact of reservoirs on downstream pCO

Sulfur cycling in freshwater ecosystems has been previously considered minor, and the direct evidence of its impacts on iron and phosphorus cycles in freshwater sediments remains unclear. In this study, mesocosms with amended acetate and various sulfate concentrations (1.5–3.0 mmol L−1) were set up to investigate sulfur cycling and its influences on iron-rich freshwater sediments. Acetate addition induced hypoxia and provided substrates, which stimulated the sulfur cycling with evidence of SO42− decline, ΣS2−, S0 increase and corresponding variations of sulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria. Meanwhile, the growth of iron-reducing bacteria (IRB) was suppressed, and lower Fe(II) release was correspondingly related to larger SRB abundance at higher sulfate level, indicating that microbial iron reduction might be blocked by SRB activities. However, continuous dissolution of Fe(III) oxides and generation of iron sulfides were observed, suggesting that sulfide-mediated chemical iron reduction (SCIR) became the dominant iron-reducing pathway, and Fe(II) was buried as iron sulfides instead of released to water column, which resulted in a transition of iron cycling into unidirectional SCIR. Consequently, continuous dissolution of Fe(III) oxides led to significant increase of PO43− concentration in the water column and sediment pore-water, revealing the phosphorus mobility in sediments derived from the SCIR process. To note, sustained accumulation of iron sulfides was observed even without ΣS2− presence, suggesting that ΣS2− precipitation occurred prior to diffusion. Thus, ΣS2−-missing sulfur cycling seemed “cryptic” in this study. To highlight, the transition of the iron-reducing pathway and resulting PO43− release can be induced even under current sulfate level of Lake Taihu, and elevated sulfate levels could significantly intensify SCIR and phosphorus mineralization. Thus, the stimulated iron deposition and the resulting phosphorus release derived from the sulfur cycling should be paid more attention to in the treatment of eutrophic freshwater ecosystems.

Lakes are regarded as one of the important sources of atmospheric CH4. However, the role of cyanobacteria blooms (CBBs) play in the CH4 production in eutrophic lakes is not fully clear. In this study, the spatial distribution characteristics of CH4 concentrations in surface water and sediment columns were investigated in Zhushan Bay of Taihu lake, China. Results showed that CH4 concentrations in CBBs accumulated zones were much higher than that in the open lake areas, with the highest values of 3.79 μmol·L−1 and 2261.88 μmol·L−1 in surface water and sediment columns, respectively. CH4 concentrations were strongly influenced by various factors. In surface water, the occurrence of CBBs greatly contributed to CH4 productions, as evidenced by the well-predicting for CH4 concentrations using Chl-a and NH4+ concentrations. In the sediments, the Ignition Loss and C:N ratio values were two indicators of CH4 contents, suggesting that the methanogenesis processes were influenced by not only the quantities, but also the qualities of organic matter. The labile substrates produced during the CBBs decomposition processes promoted the CH4 production and migration from sediments to the water column, resulting in the coherence in CH4 concentrations between the sediments and the surface water. The high-resolution determinations of CH4 concentrations in surface water and sediments clarified that the CBBs were a neglected facilitator of CH4 productions, which should be considered in the future estimation of CH4 emissions in eutrophic lakes.

Sulfur reduction in freshwater ecosystems has previously been considered as negligible because of often very low sulfate concentrations and generally low sulfate reducing capacity in freshwater sediments. In this study, seasonal variations on three types of sediments from central lake, dredged and algae accumulated areas in a eutrophic lake in China, Lake Taihu, were investigated. The high temperature in summer and the accumulation of algae are conducive to the reduction processes in freshwater lake sediments. Iron reduction was observed as the major anaerobic process in all types of sediments, while sulfate reduction was weak in central and dredged lake areas. However, strong sulfate reduction with increase of sulfate reducing bacteria and sulfides generation (119.5 ± 0.2 μmol L−1) was found in surface sediments in algae accumulated areas. Based on the results of Fe reduction rate and the quantity of Fe reducing bacteria, extensive sulfate reduction in algae accumulated sediments inhibited the microbial Fe reduction, and the ΣS2−-mediated chemical Fe reduction (SCIR) dominated instead. Iron was principally stored in the sediments as Fe sulfide compounds, which weakened the rebinding of phosphorus and stimulated phosphorus mobilization. Therefore, attention should be paid to the alteration of Fe cycling and phosphorus mobility caused by the SCIR in algae accumulated sediments and the consequent effects on the eutrophication of freshwater lakes.

Inland lakes are important ecosystems for the carbon cycle at both regional and global scales. However, a knowledge gap still exists about the correlations between the partial pressure of CO

Climate change and riparian management are major drivers of hydrological change, with important implications for nutrient fluxes in lowland streams. However, the coupling of hydrological and nutrient dynamics is complex due to time-variant flow paths, interacting biogeochemical processes and variation in land use, fertilization levels and management measures. Here, we assessed the long-term (30 year) changes in climate, discharge, groundwater levels and stream water quality in a mixed land use catchment (Demnitzer Mill Creek catchment, DMC) in northern Germany. The catchment was subject to wetland restoration and subsequent beaver (Castor fiber) recolonization over the last 15 years. Climatic variability is the primary driver of the hydrological regime, with discharge and groundwater variations closely coupled to increases in annual air temperature and fluctuations of annual rainfall. Over the past 8 years, stream flows have been greatly reduced during a prolonged drought period. Meanwhile, beaver dams have also moderated flow regimes and facilitated groundwater recharge. In terms of water quality regimes, the concentration-discharge (C-Q) relationship for nitrate (NO3-N) and soluble reactive phosphate (SRP) showed positive and negative chemodynamic behavior respectively, whilst dissolved organic carbon (DOC) exhibited chemostatic behavior. These differences were dictated by the intrinsic properties of each solute in terms of contrasting catchment storage and immobilization processes. Moreover, the changing hydrological conditions regulated such patterns and strengthened the C-Q relationships for nutrients during the drought. As an integrated result of drought and wetland restoration, nutrient concentrations and fluxes in stream water have been reduced in recent years. Headwater catchments like DMC, where much of the hydrochemical signature of larger, downstream river networks are generated, are critical observatories for integrating understanding of the linkages between water quality and land management in an era of climate change.

Lakes are regarded as one of the important sources of atmospheric CH

Climate warming and eutrophication are regarded as two important contributors to the occurrence of cyanobacteria blooms in aquatic ecosystems. However, the feedback of cyanobacteria blooms to climate warming and eutrophication is not fully clear. In this study, a microcosm system was established to simulate the decomposition processes of cyanobacteria blooms. It was observed that a large amount of nitrogen and phosphorus was released into the overlying water, and the concentrations of nitrogen and phosphorus were increased with the amount of added cyanobacteria bloom biomass addition. Subsequently, these released nutrients became available for primary production and intensified the eutrophic state of freshwater lakes. During the decomposition of cyanobacteria blooms, the microenvironment acquired low DO, low pH, and reductive conditions. Together with abundant organic matter in the water column and sediment, a large amount of CH 4 and CO 2 produced through organic matter mineralization, in which CH 4 was the dominant fraction, occupied 50%–92% in mass of emitted carbon. Furthermore, a certain amount of N 2 O, probably underestimated, was produced with a strong greenhouse effect, even though its magnitude was small. These observations clarify that the feedbacks among cyanobacteria blooms formation and climate warming as well as the eutrophication of freshwater lakes are not unidirectional, but bidirectional. Given that climate warming enhanced the occurrence of cyanobacteria blooms, it was proposed that there are two vicious loops between cyanobacteria blooms, lake eutrophication and climate warming, which should be considered in the future management of aquatic ecosystems.