Modeling microbial impact on macrophyte debris decomposition in macrophyte-dominated eutrophic lakes.

The decomposition of macrophytes plays a crucial role in the nutrient cycles of macrophyte-dominated eutrophication lakes. While research on plant decomposition mechanisms and microbial influences has rapid developed, it is curious that plant decomposition models have remained stagnant at the single-stage model from 50 years ago, without endeavor to consider any important factors. Our research conducted in-situ experiments and identified the optimal metrics for decomposition-related microbes, thereby establishing models for microbial impacts on decomposition rates (k_RDR). Using backward elimination in stepwise regression, we found that the optimal subset of independent variables-specifically Gammaproteobacteria-Q-L, Actinobacteriota-Q-L, and Ascomycota-Q-L-increased the adjusted R-squared (Ra2) to 0.93, providing the best modeling for decomposition rate (p = 0.002). Additionally, k_RDR can be modeled by synergic parameters of ACHB-Q-L, LDB-Q-L, and AB-Q-L for bacteria, and SFQ for fungi, albeit with a slightly lower Ra2 of 0.7-0.9 (p < 0.01). The primary contribution of our research lies in two key aspects. Firstly, we introduced optimal metrics for modeling microbes, opting for debris surface microbes over sediment microbes, and prioritizing absolute abundance over relative abundance. Secondly, our model represents a noteworthy advancement in debris modeling. Alongside elucidating the focus and innovative aspects of our work, we also addressed existing limitations and proposed directions for future research. SYNOPSIS: This study explores optimum metrics for decomposition-related microbes, offering precise microbial models for enhanced lake nutrient cycle simulation.

Phosphorus accumulation during the ice-on season in macrophyte-dominated eutrophic lakes and its implications.

Macrophyte overgrowth in eutrophic lakes can hasten the decline of shallow water bodies, yet the impact of macrophyte deposition on sediment phosphorus (P) accumulation in the ice-on season remains unclear. Comparative analyses of P variations among 13 semi-connected sub-lakes in Wuliangsu Lake in China, a typical MDE lake, considered external flow and macrophyte decomposition as driving forces. Sediment P fractions and water total phosphorus (TP) were analyzed at 35 sampling points across three ice-on season stages, along with macrophyte TP content to assess debris contributions. Our findings reveal that phosphorus accumulation occurs during the ice-on season in the MDE lake, with an average TP content increase of 16 mg/kg. However, we observed a surprisingly small sediment nutrient accumulation ratio (ΔTP/ΔTN=0.006) compared to macrophyte nutrient levels before decomposition. Further analysis of the dominant species, Potamogeton pectinatus, indicates that a significant portion (55%) of macrophyte phosphorus is released before the ice-on season. This highlights the critical importance of timing macrophyte harvesting to precede the phosphorus leaching process, which has implications for lake management and ecosystem restoration efforts. Additionally, our research demonstrates similar transformations among different sediment fractions as previously reported. Macrophyte debris decomposition likely serves as the primary source of Residual P (Res-P) or TP accumulation. In addition, Ca-bound P (Ca-P) generally showed a decrease, which mainly caused by its transformation to Fe/Al-bound P (Fe/Al-P), Exchange-P (Ex-P), and sometimes to Res-P. However, we emphasize the significant impacts of flow dynamics on Ca-P transport and transformations. Its hydrodynamic action increases water dissolved oxygen, which accelerates the transformation of Ca-P to more easily released Fe/Al-P and Ex-P. Furthermore, hydrodynamic transport also leads to upstream Ca-P transport to downstream. This underscores the necessity of considering flow dynamics when estimating phosphorus variations and formulating phosphorus restoration strategies.

Internal phosphorus cycling in macrophyte-dominated eutrophic lakes and its implications.

Macrophyte-dominated eutrophication (MDE) generally exhibits different characteristics from phytoplankton-dominated eutrophication (PDE). However, the significance of P cycling on eco-environmental management of MDE lakes is still not fully recognized. In this study, P-cycling mechanism was studied in a typical MDE lake (Wuliangsu Lake, China) based on a Before-After-Control-Impact design, taking advantage of the contrasting states between its 13 sub-lakes (with/without macrophytes and with/without external water flow). Our study demonstrated that P cycling predominantly occurs through "macrophyte ↔ sediment" in the MDE lakes, rather than "water ↔ sediment" as in PDE lakes; the biodynamics of the macrophytes acts as a primary driving force for this self-enforced P cycling. Our findings challenge the present lake eutrophication management strategies, which primarily limited to the water nutrient content, and demonstrate that successful MDE lake restoration should focus on stressors caused by the sustainable "macrophyte-sediment" P cycling. Macrophyte harvesting immediately before withering is recommended as the most sustainable technique in environment management for periodically frozen shallow MDE lakes. By this technique, sediment P can be gradually pumped up by the overgrown macrophytes each year until the advent of an alternative stable state (low sediment P, small biomass, and higher diversity), thereby forming a positive feedback loop "macrophyte harvesting → less sediment P → less macrophyte → higher diversity." Also, the catastrophic shift from MDE to PDE is no longer a concern. Furthermore, the macrophyte debris will not pose a problem as long as the macrophytes are removed during the harvest.

Evaluating Different Methods for Determining the Velocity-Dip Position over the Entire Cross Section and at the Centerline of a Rectangular Open Channel.

The velocity profile of an open channel is an important research topic in the context of open channel hydraulics; in particular, the velocity-dip position has drawn the attention of hydraulic scientists. In this study, analytical expressions for the velocity-dip position over the entire cross section and at the centerline of a rectangular open channel are derived by adopting probability methods based on the Tsallis and general index entropy theories. Two kinds of derived entropy-based expressions have the same mathematical form as a function of the lateral distance from the sidewall of the channel or of the aspect ratio of the channel. Furthermore, for the velocity-dip position over the entire cross section of the rectangular open channel, the derived expressions are compared with each other, as well as with two existing deterministic models and the existing Shannon entropy-based expression, using fifteen experimental datasets from the literature. An error analysis shows that the model of Yang et al. and the Tsallis entropy-based expression predict the lateral distribution of the velocity-dip position better than the other proposed models. For the velocity-dip position at the centerline of the rectangular open channel, six existing conventional models, the derived Tsallis and general index entropy-based expressions, and the existing Shannon entropy-based models are tested against twenty-one experimental datasets from the literature. The results show that the model of Kundu and the Shannon entropy-based expression have superior prediction accuracy with respect to experimental data compared with other models. With the exception of these models, the Tsallis entropy-based expression has the highest correlation coefficient value and the lowest root mean square error value for experimental data among the other models. This study indicates that the Tsallis entropy could be a good addition to existing deterministic models for predicting the lateral distribution of the velocity-dip position of rectangular open channel flow. This work also shows the potential of entropy-based expressions, the Shannon entropy and the Tsallis entropy in particular, to predict the velocity-dip position at the centerline of both narrow and wide rectangular open channels.

Nitrogen variations during the ice-on season in the eutrophic lakes.

Nitrogen accumulation in sediments, and the subsequent migration and transformations between sediment and the overlying water, plays an important role in the lake nitrogen cycle. However, knowledge of these processes are largely confined to ice-free seasons. Recent research under ice has mainly focused on the water eco-environmental effects during winter. Sediment N accumulation during the ice-on season and its associated eco-environmental impacts have never been systematically investigated. To address these knowledge gaps, we chose Wuliangsu Lake in China as a case study site, taking advantage of the spatial disparity between the 13 semi-separated sub-lakes. Based on samples of 35 sampling sites collected before, in the middle, and at the end of ice-on season separately, we performed a quantitative analysis of under-ice lake N accumulation and water-sediment N exchange by analyzing N fraction variations. Hierarchical Cluster Analysis and Relevance Analysis were used to help elucidate the main causes and implications of under-ice N variation. Our results clearly show that existing studies have underestimated the impact of under-ice N accumulation on the lake ecology throughout year: 1) Sediment N accumulated 2-3 times more than that before winter; 2) residual nitrogen (Res-N) contributed to the majority of the accumulated sediment N and was mainly induced by the debris of macrophytes; 3) total available nitrogen (TAN) was the most easily exchanged fractions between sediment and water, and it mainly affected the water environment during winter; 4) the Res-N accumulation during the ice-on season may have a strong impact on the eco-environment in the subsequent seasons. Our research is valuable for understanding the mechanism of internal nutrient cycle and controlling the internal nitrogen pollution, especially in shallow seasonally-frozen lakes that have long suffered from macrophyte-phytoplankton co-dominated eutrophication.