Effects of nutrient reduction and habitat heterogeneity on benthic macroinvertebrate assemblages in a large shallow eutrophic lake

Vigilance against climate change-induced regime shifts for phosphorus restoration in shallow lake ecosystems

The dual pressure of anthropogenic activities and frequent extreme weather events has triggered a transition from macrophyte to algal dominance in shallow lakes. Phosphorus (P) is the key driver of regime shifts that can lead to a decline in the stability and resilience of lake ecosystems. However, the mechanisms underlying such regime shifts, and the effects of state transitions on internal P loading during macrophyte restoration in large shallow eutrophic lakes, remain to be fully elucidated. This study utilised long-term in situ monitoring data, across three distinct lake states (bare ground, macrophyte-dominated stage, and algae-dominated stage) to elucidate the accumulation and release mechanisms of sedimentary P during regime shifts. The findings demonstrated that the rehabilitation of submerged plants efficiently reduced internal P loading (water column P, sediment P fractions, and P flux), while the persistence of algal blooms was driven by the reductive release of Fe-P from sediments and the dissolution of Al-P from suspended particulate matter. High temperature, low dissolved oxygen, and high pH largely modulate the pathways and mechanisms of P supply during regime shifts. The combined pressures of extreme weather (heavy rainfall, strong winds, and extreme heat) and trophic cascades from fish stocking can trigger a shift from macrophytes to algae in shallow lakes. Appropriate management of the structure and biomass of aquatic animal communities (e.g., small-bodied or omnibenthivorous fish) and optimization of the food web structure can effectively improve water quality and maintain ecosystem stability. These findings enrich the theoretical understanding of regime-shift mechanisms from an ecosystem perspective and offer novel insights into P remediation in large shallow eutrophic lakes.

Synergistic effects of environmental factors on benthic diversity: Machine learning analysis

This study examines the water environmental factors of the Cangshan stream and benthic animal communities by using random forest, gradient boosting decision tree, and support vector machine models to analyze the complex response mechanisms of benthic animal diversity and community structure to environmental factors. Feature importance analysis, SHAP values, and 3D response surface analysis are applied to quantitatively assess the non-linear driving effects of environmental factors and their interactions. The findings suggest that total phosphorus and conductivity are central factors influencing benthic animal diversity, with moderate levels fostering community diversity, whereas high levels of total nitrogen and conductivity significantly reduce diversity. Benthic animals exhibit a non-linear response pattern to dissolved oxygen and temperature, with the interaction between dissolved oxygen and temperature highlighting the significant promotion of diversity under low-temperature, high-oxygen conditions, whereas high-temperature, low-oxygen conditions exert evident environmental stress on communities. The results of the multifactor synergistic effect analysis indicate that the moderate synergistic interaction between total phosphorus and conductivity significantly enhances diversity, whereas high total nitrogen levels weaken this positive effect. Model performance comparisons reveal that the RF outperforms the other models in terms of coefficient of determination, mean squared error, and mean absolute error, particularly in capturing complex non-linear relationships and factor interactions. Through machine learning, this study reveals the multidimensional driving mechanisms of environmental factors on benthic animal community characteristics, emphasizing the potential to capture non-linear relationships and multifactor interactions, thereby providing scientific evidence and innovative approaches for stream ecosystem conservation and management.

Impacts of habitat alteration on macroinvertebrates in large shallow lakes: An application of a macroinvertebrate-based multimetric index

Habitat plays a crucial role in shaping the macroinvertebrate community structure in large shallow lakes. In the pursuit of improving the health of freshwater ecosystems, it is imperative to consider their habitat characteristics. To evaluate the impact of habitat variations on lake ecological health, we developed a macroinvertebrate-based multimetric index (MMI) for both the pelagic and littoral zones of Lake Hongze. Additionally, we employed structural equation models to explore the influence of utilization or phytoplankton biomass on ecological health. Historical data served as reference conditions for the pelagic. Seven key attributes were selected for the pelagic MMI, that is, Biological Monitoring Working Party (BMWP), the percentage of Mollusca taxa, the percentage of filter-collector taxa, the percentage of predator taxa, the percentage of gather-collector taxa, and the percentage of sensitive taxa and functional dispersion. The least minimally disturbed conditions and the best attainable conditions were used to develop the littoral. Four key metrics, that is, the percentage of scraper abundance, Mollusca taxa, Biological Pollution Index, and BMWP, were integrated into the littoral MMI. The assessment based on MMI revealed a "poor" health status for the pelagic zone and a "fair" health status for the littoral zone. These findings underscore the high applicability and efficacy of MMIs in assessing and monitoring ecological health in Lake Hongze. Notably, functional feeding groups exhibited heightened sensitivity to disturbance in both zones. Moreover, sediment organic matter strongly influenced the pelagic ecological health, while chlorophyll a and transparency emerged as primary factors influencing the littoral zone, attributable to varying littoral zone utilization. Integr Environ Assess Manag 2024;20:2245-2255. © 2024 SETAC.

Biotic and abiotic properties mediating sediment microbial diversity and function in a river-lake continuum

A river-lake system plays an important role in water management by providing long-term and frequent water diversions. However, hydrological connectivity in the system can have a profound effect on sediment microbial communities through pH, nutrient concentrations, and benthos invertebrates. Consequently, identifying the key environmental factors and their driving mechanisms is vital for microbial adaptation strategies to extreme environments. In this study, we analyzed the significant difference in sediment bacterial and fungal community structures and diversity indices among Dongting Lake and its tributary rivers, which worked as a typical river-connected lake ecosystem. There were significant differences in biotic and abiotic environments in the sediment habitats of Dongting Lake and its tributary rivers. Random forest analysis revealed that pH and Mollusca were found to be the most important abiotic and biotic variables for predicting both bacterial and fungal community structures, respectively. The beta diversity decomposition analyses showed that the bacterial and fungal community compositional dissimilarities among different sections were dominated by species replacement processes, with more than half of the OTUs in each section being unique. Notably, both biotic and abiotic factors affected the number and the relative abundance of these bacterial and fungal unique OTUs, leading to changes in community composition. Mollusca, pH, TP, NO3-N, and NH4-N were negatively related to the relative abundance of Actinobacteria, Acidobacteria, Gemmatimonadetes, Planctomycetes, and Ascomycota, while Annelida and ORP were positively related to the relative abundance of Actinobacteria and Gemmatimonadetes. Additionally, PICRUSt analysis revealed that the functional dissimilarity among lakes and rivers was strengthened in unique species compared to all species in bacterial and fungal communities, and the changes of functional types helped to improve the habitat environment in the main Dongting Lake and promote the process of microbial growth. From our results, the role of macrozoobenthos and physicochemical characteristics in driving the sediment microbial community spatial variations became clear, which contributed to further understanding of the river-lake ecosystem.

Nutrient regeneration patterns for initiating and maintaining algae blooms-a case study of in Lake Taihu

In order to clarify the nitrogen (N) and phosphorus (P) regeneration patterns and internal mechanism for initiating and maintaining algal blooms in Lake Taihu, samples (including surface water and sediment) from 8 sites in Lake Taihu were collected for nine times from May 2010 to April 2011, and analyzed for total and labile organic matter, P fractionation and sorption behaviors, extracellular enzymatic activities (EEA), dehydrogenase activity, the respiratory electron transport system activity, and iron in sediment, EEA, N and P species and chlorophyll a (Chl. a) in surface water, as well as N and P species in interstitial water. In Lake Taihu, although severe blooms occurred in both Meiliang Bay and Zhushan Bay, the nutrient regeneration patterns stimulating the initiation and maintenance of algae blooms in these two bays were different. In Zhushan Bay with low EEA in surface water, abundant N and P flux from sediments, due to the degradation of organic matter and enzymatic hydrolysis in sediment, further stimulated the initiation and maintenance of algae blooms. In Meiliang Bay, in spite of lower nutrient supply from sediment, high EEA in surface water occurred later than the serious blooms, showing that the nutrient regeneration from sediment, not water body, was still the trigger for the start of the bloom, and sediment nutrient release and predominant surface water nutrient regeneration by abundant exoenzymes sustained the algal blooms. In the Western region, algal bloom started in the northern area and further spread in the remaining part of the lake; nutrient regeneration in the surface water sustained the slight bloom. In the East Bays, the decay and decomposition of macrophytes led to anaerobic conditions in sediments and high ammonia in interstitial water, but low iron bound phosphorus resulted in anaerobic release of very few P, thus showed extremely low Chl. a concentration.

Spatial and temporal variation in lake macroinvertebrate communities is decreased by eutrophication

Eutrophication impacts freshwater ecosystems and biodiversity across the world. While temporal monitoring has shown changes in the nutrient inputs in many areas, how spatial and temporal beta diversity change along the eutrophication gradient under a changing context remains unclear. In this regard, analyses based on time series spanning multiple years are particularly scarce. We sampled benthic macroinvertebrates in 32 sites across three lake habitat types (MACROPHYTE, OPEN WATER, PHYTOPLANKTON) along the eutrophication gradient of Lake Taihu in four seasons from 2007 to 2019. Our purpose was to identify the relative contributions of spatial and temporal dissimilarity (i.e., inter-annual dissimilarity and seasonal dissimilarity) to overall benthic biodiversity. We also examined spatio-temporal patterns in community assembly mechanisms and how associated variation in benthic macroinvertebrate communities responded to nutrient indicators. Results showed that eutrophication caused macroinvertebrate community homogenization both along spatial and temporal gradients. Though spatial variability dominated the variation of species richness, abundance and community dissimilarity, seasons within years dissimilarity, inter-annual dissimilarity and seasonal dissimilarity were much more sensitive to eutrophication. Moreover, eutrophication inhibited a strong environmental control in benthic macroinvertebrate community assembly, including a dominant role of deterministic process in the spatial variation of macroinvertebrate communities and transition from stochastic to deterministic process in the temporal assembly of macroinvertebrate communities along the eutrophication gradient. In addition, some sites in PHYTOPLANKTON habitats showed similar spatial dissimilarity and spatial SES as sites in MACROPHYTE habitats, and the decreased spatial dissimilarity of three habitats implying that lake ecosystem recovery projects have achieved their goal at least to a certain degree.