Migration and transformation of dissolved carbon during accumulated cyanobacteria decomposition in shallow eutrophic lakes: a simulated microcosm study

Distribution pattern of dissolved organic matter in pore water of sediments from three typical areas of western Lake Taihu and its environmental implications

The migration, transformation, and accumulation of dissolved organic matter (DOM) in pore water of sediment cores play a pivotal role in lacustrine carbon cycling. In order to understand the dynamics of DOM in the sediments of large shallow eutrophic lakes, we examined the vertical profiles of DOM and the benthic fluxes of dissolved organic carbon (DOC) in sediment cores located in algae accumulated, dredged, and central areas of eutrophic Lake Taihu, China. Optical properties showed the significant influence of terrestrial inputs on the DOM components of pore water in the algae accumulated area but an abundant accumulation of autochthonous DOM in the central area. The benthic fluxes of DOC ranging from -458.2 to -139.4 mg·m-2·d-1 in the algae accumulated area displayed an opposite diffusion direction to the other two areas. The flux ranges of 9.5-31.2 mg·m-2·d-1 in the dredged area and 14.6-48.0 mg·m-2·d-1 in the central area were relatively smaller than those in the previously reported lake ecosystems with low trophic levels. Dredging engineering disturbed the pre-dredging distribution patterns of DOM in sediment cores. The deposition, accumulation, and transformation of massive algae scums in eutrophic lakes probably promoted the humification degree of sediments.

Space-for-time substitution leads to carbon emission overestimation in eutrophic lakes

Lacustrine eutrophication is generally considered as an important contributor of carbon emissions to the atmosphere; however, there is still a huge challenge in accuracy estimating carbon emissions from lakes. To test the effect of widely used space-for-time substitution on lake carbon emissions, this study monitored different processes of carbon emissions, including the carbon production potential, dissolved carbon concentrations, and carbon release fluxes in eight lakes along the trophic gradients on a spatial scale and the typical eutrophic Lake Taihu for one year on a temporal scale. Eutrophication promoted carbon production potential, dissolved carbon concentrations, and carbon release fluxes, especially for CH₄. Trophic lake index (TLI) showed positive correlations with the CH₄ production potential, dissolved CH₄ concentrations, and CH₄ release fluxes, and also positive correlations with the CO₂ production potential, dissolved CO₂ concentrations, and CO₂ release fluxes. The space-for-time substitution led to an overestimation for the influence of eutrophication on carbon emissions, especially the further intensification of lake eutrophication. On the spatial scale, the average CH₄ production potential, dissolved CH₄ concentrations and CH₄ release fluxes in eutrophic lakes were 268.6, 0.96 μmol/L, and 587.6 μmol m^-2·h^-1, respectively, while they were 215.8, 0.79 μmol/L, and 548.6 μmol m^-2·h^-1 on the temporal scale. Obviously, CH₄ and CO₂ emissions on the spatial scale were significantly higher than those on the temporal scale in eutrophic lakes. The primary influencing factors were the seasonal changes in the physicochemical environments of lake water, including dissolved oxygen (DO) and temperature. The CH₄ and CO₂ release fluxes showed negative correlations with DO, while temperature displayed positive correlations, respectively. These results suggest that the effects of DO and temperature on lake carbon emissions should be considered, which may be ignored during the accurate assessment of lake carbon budget via space-for-time substitution in eutrophic lakes.

High-resolution characteristics and mechanisms of endogenous phosphorus migration and transformation impacted by algal blooms decomposition

Extremely high phosphorus (P) concentrations can be found in eutrophic freshwater sediments during algal blooms (ABs). However, few investigations have revealed the mechanism of labile P production in anoxic sediments following ABs decomposition. This limits our understanding of P cycling and mitigation of ABs in aquatic ecosystems. To identify such a mechanism, we conducted a microcosm experiment to identify how ABs decomposition enhances endogenous P release, using the combined techniques of diffusive gradients in thin films, high-resolution dialysis, and 16S rRNA amplicon sequencing. We show the concentrations of labile iron, manganese, sulfide, and P can be well predicted by quality and quantity of algal biomass. The relative abundance of iron reduction bacteria positively correlated with the decrease of pH induced by ABs decomposition, suggesting that this decomposition facilitates microbial iron and manganese reduction. In addition, the reductive dissolution of iron and manganese oxides leads to the labile P release, resulting in higher concentrations of labile P in those sediments affected by ABs compared with those not affected. The P fluxes in the algae-dominated regions exhibited higher values in the algae group than in the control group, with gains of 14.07-100.04%. Furthermore, endogenous P release is strongly controlled by Mn when the Fe(II):Mn(II) ratio is low (below 0.47), and by both Fe and Mn when the Fe(II):Mn(II) ratio is high (above 0.63). Our results quantify the endogenous P diffusion fluxes across the sediment-water interface can be attributed to ABs decomposition, and are therefore useful for further understanding of P cycling in freshwater.

Increasing sulfate concentration and sedimentary decaying cyanobacteria co-affect organic carbon mineralization in eutrophic lake sediments

Sulfate (SO₄^2-) concentrations in eutrophic lakes are continuously increasing; however, the effect of increasing SO₄^2- concentrations on organic carbon mineralization, especially the greenhouse gas emissions of sediments, remains unclear. Here, we constructed a series of microcosms with initial SO₄^2- concentrations of 0, 30, 60, 90, 120, 150, and 180 mg/L to study the effects of increased SO₄^2- concentrations, coupled with cyanobacterial blooms, on organic carbon mineralization in Lake Taihu. Cyanobacterial blooms promoted sulfate reduction and released a large amount of inorganic carbon. The SO₄^2- concentrations in cyanobacteria treatments significantly decreased and eventually reached close to 0. As the initial SO₄^2- concentration increased, the sulfate reduction rates significantly increased, with maximum values of 9.39, 9.44, 28.02, 30.89, 39.68, and 54.28 mg/L∙d for 30, 60, 90, 120, 150, and 180 mg/L SO₄^2-, respectively. The total organic carbon content in sediments (51.16-52.70 g/kg) decreased with the initial SO₄^2- concentration (R² = 0.97), and the total inorganic carbon content in overlying water (159.97-182.73 mg/L) showed the opposite pattern (R² = 0.91). The initial SO₄^2- concentration was positively correlated with carbon dioxide (CO₂) emissions (R² = 0.68) and negatively correlated with methane (CH₄) emissions (R² = 0.96). The highest CO₂ concentration and lowest CH₄ concentration in the 180 mg/L SO₄^2- treatment were 1688.78 and 1903 μmol/L, respectively. These biogeochemical processes were related to competition for organic carbon sources between sulfate reduction bacteria (SRB) and methane production archaea (MPA) in sediments. The abundance of SRB was positively correlated with the initial SO₄^2- concentration and ranged from 6.65 × 10⁷ to 2.98 × 10⁸ copies/g; the abundance of MPA showed the opposite pattern and ranged from 1.99 × 10⁸ to 3.35 × 10⁸copies/g. These findings enhance our understanding of the effect of increasing SO₄^2- concentrations on organic carbon mineralization and could enhance the accuracy of assessments of greenhouse gas emissions in eutrophic lakes.

A latest review on the application of microcosm model in environmental research

Microcosms are used experimentally to simulate ecosystems. This technology has received increasing attention and is widely used for environmental research. This review briefly introduces the origin and development of microcosm theory, summarizes classification and applications of microcosms across decades, and describes the advantages and limitations of microcosm technology in comparison with other methods. Finally, trends in the development of microcosm models are discussed.

Carbon and phosphorus transformation during the deposition of particulate matter in the large deep reservoir

As the running time of reservoirs is increasing, a large number of reservoirs are becoming eutrophicated. Organic phosphorus (OP) is a key factor in eutrophication. However, the mechanism and extent to which organic matter degradation affects P recycling in water column of large deep reservoirs are unclear, especially for the newly-built ones. In this study, different forms of carbon (C) and P in the water column of Hongjiadu Reservoir were investigated. The contents of particulate organic carbon (POC) and particulate organic phosphorus (POP) both decreased with depth in summer, indicating that organic matter was degraded during the deposition of particulates. In contrast, the contents of POC and POP varied slightly with depth in winter. This difference may result from the double thermal stratification and the corresponding double oxygen stratification in summer. The POC/POP ratios were lower in the epilimnion and increased with depth, suggesting that P was preferentially regenerated relative to C during organic matter degradation. The contents of particulate inorganic phosphorus (PIP) and POP were significantly negatively correlated, indicating that POP transformed into PIP in deeper water. The double thermoclines and oxyclines in Hongjiadu Reservoir lead to very low dissolved oxygen (DO) concentrations in the hypolimnion, which should receive sufficient attention. If water becomes hypoxic, enhanced P release during organic matter degradation will promote phytoplankton growth, leading to higher phytoplankton biomass and more severe DO depletion. Thus, a positive feedback loop may form among hypoxia, enhanced P release, higher primary productivity, and more severe hypoxia, accelerating P recycling in large deep reservoirs. Once if eutrophication occurs in these reservoirs, it will be very difficult to restore the water ecosystem. Thus, it is particularly important to prevent the occurrence of eutrophication and the formation of positive feedback loop as early as possible. This highlights the importance of both reducing external loading and improving DO level in large deep reservoirs.