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Applications of aluminium (Al) salt or lanthanum (La) modified bentonite (LMB) have become popular methodologies for immobilizing phosphorus (P) in eutrophic lakes. The presence of humic substances, has been shown to inhibit this form of treatment due to the complexation with La/Al. However, the effects of other dissolved organic matter (DOM), especially that derived from phytoplankton (the dominant source in eutrophic lakes) are unknown. In this study, the interaction with La/Al of Suwannee River Standard Humic Acid Standard II (SRHA) and algae-derived DOM (ADOM) were investigated and compared. Differed to SRHA which was dominated by polyphenol-like component (76.8%, C1-SRHA), majority in ADOM were protein-like substance, including 41.9% tryptophan-like component (C2-ADOM) and 21.0% tyrosine-like component (C3-ADOM). Two reactions of complexation and coprecipitation were observed between SRHA/ADOM and La/Al. Complexation dominated at low metal inputs less than 10 μM and coprecipitation was the main reaction at higher metal inputs. For ADOM, the tryptophan-like component (C2-ADOM) was the important component to react with metal. The reaction rate for C2-ADOM with La were about two-third of that for C1-SRHA, indicating that the influence of C2-ADOM was significant during the P immobilization by La/Al-based treatment in eutrophic lakes. The P removal data in the presence of ADOM confirmed the significant inhibition of ADOM. In addition, based on the composition of coprecipitates and relatively biodegradable character of tryptophan-like substances (C2-ADOM), the coprecipitation of ADOM was assumed to reduce the stability of precipitated P in eutrophic lakes. The release of P from the potential biodegradation of the coprecipitates and thus the possible decline of the performance of P immobilization by La/Al-based treatments is an important work in the future. Image 1 • Complexation and coprecipitation occurred between SRHA/ADOM and La/Al. • A tryptophan-like component of ADOM complexed strongly with La. • The tryptophan-like component has strong ability to coprecipitate with La and Al. • The behaviors of PARAFAC components explained the inhibitory effect of SRHA/ADOM on P removal. • The stability of coprecipitates of DOM, Al/La and P should be noted. [ABSTRACT FROM AUTHOR]

Due to climate change, tree line advance is occurring in many alpine regions. Within the next 50 to 100 years, alpine lake catchments are expected to develop increased vegetation cover similar to that of sub-alpine lake catchments which currently exist below the tree line. Such changes in vegetation could trigger increased allochthonous DOM inputs to alpine lakes. To understand the fate of allochthonous DOM in alpine lakes impacted by climate change, the photochemical reactivity of DOM in sub-alpine Lake Tiancai (located 200 m below the tree line) was investigated by excitation emission matrix fluorescence combined with parallel factor analysis (EEM-PARAFAC) and UV–Vis spectra analysis. With photo-exposure, a decrease in apparent DOM molecular weight was observed and 32% DOM was photomineralized to CO 2 . Interestingly, the aromaticity of DOM increased after photodegradation, as evidenced by increases in both the specific UV absorbance at 254 nm (SUVA 254 ) and the humification index (HIX). Five EEM-PARAFAC components were identified, including four terrestrially-derived substances (C1, C2, C3 and C4; allochthonous) and one tryptophan-like substance (C5; autochthonous). Generally, allochthonous DOM represented by C2 and C3 exhibited greater photoreactivity than autochthonous DOM represented by C5. C4 was identified as a possible photoproduct with relatively high aromaticity and photorefractive tendencies and contributed to the observed increase in SUVA 254 and HIX. UV light facilitated the photodegradation of DOM and had the greatest effect on the removal of C3. This study provides information on the transformation of EEM-PARAFAC components in a sub-alpine lake, which is important in understanding the fate of increased allochthonous DOM inputs to alpine lakes impacted by climate change. [ABSTRACT FROM AUTHOR]

Streams in the southeastern United States Coastal Plains serve as an essential source of energy and nutrients for important estuarine ecosystems, and dissolved organic matter (DOM) exported from these streams can have profound impacts on the biogeochemical and ecological functions of fluvial networks. Here, we examined hydrological and temperature controls of DOM during low-flow periods from a forested stream located within the Coastal Plain physiographic region of Alabama, USA. We analyzed DOM via combining dissolved organic carbon (DOC) analysis, fluorescence excitation–emission matrix combined with parallel factor analysis (EEM-PARAFAC), and microbial degradation experiments. Four fluorescence components were identified: terrestrial humic-like DOM, microbial humic-like DOM, tyrosine-like DOM, and tryptophan-like DOM. Humic-like DOM accounted for ~70% of total fluorescence, and biodegradation experiments showed that it was less bioreactive than protein-like DOM that accounted for ~30% of total fluorescence. This observation indicates fluorescent DOM (FDOM) was controlled primarily by soil inputs and not substantially influenced by instream production and processing, suggesting that the bulk of FDOM in these streams is transported to downstream environments with limited in situ modification. Linear regression and redundancy analysis models identified that the seasonal variations in DOM were dictated primarily by hydrology and temperature. Overall, high discharge and shallow flow paths led to the enrichment of less-degraded DOM with higher percentages of microbial humic-like and tyrosine-like compounds, whereas high temperatures favored the accumulation of high-aromaticity, high-molecular-weight, terrestrial, humic-like compounds in stream water. The flux of DOC and four fluorescence components was driven primarily by water discharge. Thus, the instantaneous exports of both refractory humic-like DOM and reactive protein-like DOM were higher in wetter seasons (winter and spring). As high temperatures and severe precipitation are projected to become more prominent in the southeastern U.S. due to climate change, our findings have important implications for future changes in the amount, source, and composition of DOM in Coastal Plain streams and the associated impacts on downstream carbon and nutrient supplies and water quality. [ABSTRACT FROM AUTHOR]

Human activities can alter dissolved organic matter (DOM) in lakes through both direct (i.e., exporting DOM of anthropogenic sources) and indirect effects (i.e., enhancing the autochthonous production of DOM via nutrient loading). Distinguishing between the direct and indirect effects is important to better understand human impacts on aquatic systems, but it remains highly challenging due to the interdependence of associated environmental variables. Here, we demonstrated that disentangling the direct and indirect effects can be achieved through combining large-scale environmental monitoring with the Partial Least Squares Path Modeling (PLS-PM). We presented DOM data from 61 lakes within the floodplain of the Yangtze River (Lakes-YR), China, a region that has been subjected to intense anthropogenic disturbances. We analyzed the amount and composition of DOM through dissolved organic carbon (DOC), chromophoric DOM (CDOM), and fluorescent DOM (FDOM). Four fluorescence components were identified, including one tyrosine-like component, one tryptophan-like component, and two humic-like components. Most of the lakes were dominated by freshly produced DOM with small molecular weights and low humification. Results from the PLS-PM models showed that the autochthonous production was more important than anthropogenic inputs in mediating DOC and CDOM. In contrast, FDOM parameters in lakes were more sensitive to the direct, anthropogenic sources, including treated domestic, industrial wastewater, and the effluents of aquaculture. These sources can be identified by elevated FDOM content per DOC (FDOM: DOC ratio) relative to autochthonous DOM, suggesting the potential of using FDOM as a tracer to identify and monitor the contribution of anthropogenic organic matter to inland waters. Unlabelled Image • Human activity can alter DOM in lakes directly and indirectly. • Direct and indirect effects are disentangled by path analysis modeling. • DOC and CDOM are more sensitive to nutrient loading and autochthonous DOM production. • FDOM parameters are more sensitive to direct DOM inputs from anthropogenic sources. • FDOM: DOC ratio may serve as a tracer for anthropogenic sourced DOM. [ABSTRACT FROM AUTHOR]

The biogeochemical cycles of phosphorus (P) and organic matter (OM) are significantly influenced by iron (Fe) through forming OM-Fe-P associations. The sources and compositions of organic matter in the two typical states (algae-dominated and macrophyte-dominated) of shallow lakes are different, which could modulate the formation of OM-Fe-P associations and influence the internal loadings of P in lakes. In this study, OM and P bound with Fe were extracted from the sediments of algae-dominated zone (A-zone) and macrophyte-dominated zone (M-zone) in Lake Taihu via the citrate-bicarbonate-dithionite (CBD) reduction method, and were analyzed to elucidate the differences in OM-Fe-P associations between the two zones. The results showed that OM-Fe-P associations in the sediments of M-zone had higher a molar C/Fe ratio but a lower molar P/Fe ratio. Four components identified by excitation-emission matrix fluorescence coupled with parallel factor analysis (EEM-PARAFAC) in OM-Fe-P associations in the two zones were all humic-like substances and the relative abundance of the corresponding PARAFAC components had no significant difference between the two zones. However, the total fluorescence intensity of the humic-like components was higher in M-zone than those in A-zone. This could be attributed to the higher aromaticity of OM produced from macrophyte than that from algae, as evidenced by the difference in the easily-desorbed OM between two zones. In M-zone, high aromatic compounds which are preferentially associated with Fe, could be more produced from macrophyte plants than from algae and resulted in the higher C/Fe molar ratio. The higher C/Fe molar ratio (2.1 ± 0.9) of OM-Fe-P associations in M-zone reflected that more OM originated from macrophyte could be involved in the coprecipitation with Fe (III) and OM-Fe-P associations in M-zone were probably more resistant to the microbial and chemical reduction. Our results from the real situation confirmed the mechanism related to the role of OM composition in the reduction of OM-Fe-P associations to explain the lower internal loading of P in M-zone than that in A-zone. [ABSTRACT FROM AUTHOR]