The feeding habits of small-bodied fishes mediate the strength of top-down effects on plankton and water quality in shallow subtropical lakes

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.

Prokaryotic and eukaryotic periphyton responses to warming, nutrient enrichment and small omnivorous fish: A shallow lake mesocosms experiment

Global change stressors, including climate warming, eutrophication, and small-sized omnivorous fish, may exert interactive effects on the food webs and functioning of shallow lakes. Periphyton plays a central role in the primary production and nutrient cycling of shallow lakes but constitutes a complex community composed of eukaryotes and prokaryotes that may exhibit different responses to multiple environmental stressors with implications for the projections of the effects of global change on shallow lakes. We analyzed the effects of warming, nutrient enrichment, small omnivorous fish and their interactions on eukaryotic and prokaryotic periphyton structures in shallow lake mesocosms. We performed 16S and 18S rRNA high-throughput sequencing to elucidate the effect of the abovementioned stressors. We found that warming promoted periphytic alpha diversity and network complexity, with multi-tolerant genera becoming dominating (e.g. Spirosomaceae and Azospirillaceae). Contrastingly, nutrient enrichment led to reduced prokaryotic diversity and network complexity and stability, with weak disruption of the eukaryotic structure. Small omnivorous fish were major drivers of changes eukaryotic periphyton, facilitating diversity and network complexity, and increasing prokaryotic and eukaryotic biomarker diversity. Omnivorous fish reduced the grazing pressure on periphyton mainly through selective grazing on zooplankton, contributing to periphytic structural stability and functional diversity, especially the proliferation of prokaryotic biomarkers. Nutrient enrichment counteracted the positive effects of warming on periphyton, while concerted action with omnivorous fish led to high TN and TP concentrations and accelerated the negative development of periphytic alpha diversity and network structure. The co-occurrence of the three environmental pressures ultimately resulted in a disruption of periphytic biodiversity and community structure and weakened connectivity with the environment. Our study provided new insights into the understanding of the response of prokaryotic and eukaryotic community structure and ecological functions of freshwater periphyton to global environmental change.

Formation of Adaptive Trophic Niches of Euryphagous Fish Species in Response to Off-Seasonal Water Level Regulation in Hongze Lake

Off-seasonal water level regulations disrupt the biological traits and phenological rhythms of native fish species, yet their impacts on interspecific trophic interactions remain understudied. This study employed stable isotope analysis to assess the trophic dynamics of three fish species (Parabramis pekinensis, Carassius auratus, and Toxabramis swinhonis) across different water periods in Hongze Lake. The findings revealed that all three species occupied similar mid-level trophic positions, with no significant difference among water periods (p > 0.05). During high-water periods, P. pekinensis and T. swinhonis exploited broader niches, while C. auratus relied on a narrower diet. In contrast, during low-water periods, C. auratus expanded its niche, while P. pekinensis and T. swinhonis reduced their isotopic niche widths. Niche overlap analysis showed minimal trophic overlap among the three species during high-water periods, with increased overlap during low-water periods, except for the highest overlap between C. auratus and T. swinhonis during mid-water periods. This variation in niche overlap aligns with shifts in dietary reliance, as POM was the predominant dietary component for all three species, but its contribution varied significantly across different water periods. These findings indicated that adaptive trophic niche facilitated the coexistence of these fish species, while off-seasonal water level regulation may intensify interspecific competition. These insights are essential for refining water management policies and developing sustainable fishery management strategies of Hongze Lake and other water-level-regulated systems.

Improvement of water quality through coordinated multi-trophic level biomanipulations: Application to a subtropical emergency water supply lake

Artificial emergency water source lakes have been built in most cities in the middle and lower reaches of the Yangtze River, China, to ensure water safety for residents. However, these new ecosystems are prone to algal blooms or other degraded water quality conditions. A newly built water supply lake in the lower reaches of the Yangtze River was selected as a model system to test whether the coordinated manipulation of fish and submerged macrophyte communities could enhance ecosystem function and quality. The coordinated manipulations spanned a five-year period, aiming to enhance both top-down and bottom-up control of phytoplankton. As a result of these manipulations, the catch per unit effort of small-bodied zooplanktivorous fishes decreased by >95 % from year two and remained low. The coverage and biomass of submerged macrophytes increased year by year. Water transparency increased from 1.07 to 3.33 m. Total phosphorus and total nitrogen showed a decreasing trend (not significant though). The annual mean biomass of Cyanophyta, Chlorophyta and Bacillariophyta decreased from 2.99 to 0.03 mg/L, 3.90 to 0.16 mg/L, and 3.50 to 0.3 mg/L, respectively. The biomass of phytoplankton in different groups decreased in all four seasons. The annual mean biomass of Cladocera and Copepoda remained low. The biomass of Cladocera and Copepoda decreased in summer, fall, and winter. The Ecosystem Health Index - increased from 15.9 to 32.0. The pros and cons of the various top-down and bottom-up control measures employed are discussed. This research presents a valuable case study on the enhancement of ecosystem structure and function in newly constructed emergency water supply lakes and offers insights into the restoration of other subtropical shallow lakes.

Identification of 2, 4-di-tert-butylphenol from Microcystis lysate after bloom control and its potential risks to aquatic ecosystems

With the increasing concern of cyanobacterial blooms, numerous techniques have been developed to mitigate these environmental nuisances. During bloom control, the allelopathic effects of compounds released from cyanobacterial cells are considered as secondary hazards. In this study, the findings indicated that Microcystis lysate inhibited algal proliferation and disrupted the development of zebrafish embryos. Then, allelochemicals in Microcystis lysate were identified using gas chromatography-mass spectrometry, with 2, 4-di-tert-butylphenol (2, 4-DTBP) being the only identified phenol, which was selected for further study. The results showed that 2,4-DTBP caused oxidative damages, disrupted metabolic activity, and suppressed photosynthetic activity, consequently impeding the growth of Microcystis aeruginosa (M. aeruginosa) and Chlorella pyrenoidosa (C. pyrenoidosa). Moreover, it enhanced the interspecies competitive advantages of M. aeruginosa by increasing phosphate uptake rate. Furthermore, at a concentration of 2 mg L-1, 2, 4-DTBP negatively affected the development of zebrafish embryos, manifesting in mortality, malformation, and hatching delay. Therefore, the investigation identified 2, 4-DTBP as a potential allelochemical within Microcystis lysate. Although the effective concentration for freshwater algae and zebrafish embryos was higher than that found in Microcystis lysate, it highlighted the need for careful monitoring of aquatic ecosystem health during cyanobacterial bloom mitigation.

Phosphorus cycle in shallow lakes affected by crucian carp (Carassius auratus): Effects of fish density and size

Crucian carp (Carassius auratus) is an omni-benthivorous fish common in many shallow lakes in China. The presence of crucian carp can contribute to the nutrient cycles in lakes and thus affect water quality. In this work, a two-by-two factorial mesocosm experiment was performed with crucian carp of different sizes and densities, to investigate their effects on the cycle of phosphorus (P). Results showed that nutrients in particulate form increased in overlying water due to crucian carp disturbance, especially for treatments with higher fish densities and larger individuals. Smaller individuals at high density have a greater ability to promote P release from sediment, due to a stronger combined effects of physical disturbance and excretion. Accumulation of feces led to sediment anaerobiosis and the reductive dissolution of iron oxide-hydroxide, which were the main factors affecting the desorption of P. Our results quantify the endogenous P diffusion fluxes across the sediment-water interface attributed to different densities and sizes of crucian carp disturbance, and suggest controlling crucian carp at low density and small size to minimize their impact on sediment P flux.

The autotoxicity of dissolved organic matter from Microcystis may be a contributor to the decline of its bloom

Microcystis blooms frequently occur in freshwaters worldwide, causing detrimental impacts on the ecosystems and human health. Studying the mechanisms behind the decline of Microcystis blooms can aid in mitigating their harmful effects. However, there is currently a dearth of research in this area. In this study, we examined the dissolved organic matter (DOM) collected during the growth of axenic Microcystis. Axenic Microcystis produces toxic DOM during its growth, which accumulate in the medium. Further, the DOM obtained from decline phase of Microcystis blooms exhibited a strong inhibitory effect on the growth of fresh Microcystis cultures. It is postulated that DOM may contribute to decline of Microcystis blooms. Meanwhile, potential autotoxic components in Microcystis DOM were analyzed by integrating column chromatography fractionation, identification and Microcystis growth inhibition assay. The results showed that the autotoxic components consisted mainly of small molecule hydrophobic base. Moreover, we evaluated the influence of key environmental factors such as nutrient and temperature on the autotoxic DOM in Microcystis cultures. Nutrient deficiency and low temperature may promote the accumulation of autotoxic substances. The study provided a new perspective on the decay process of cyanobacterial bloom, and offered new ideas for the development of natural algicidal agents.

Application of Synchronous Evaluation-Diagnosis Model with Quantitative Stressor-Response Analysis (SED-QSR) to Urban Lake Ecological Status: A Proposed Multiple-Level System

Ecological integrity assessment and degradation diagnosis are used globally to evaluate the health of water bodies and pinpoint critical stressors. However, current studies mainly focus on separate evaluation or diagnosis, leading to an inadequate exploration of the relationship between stressors and responses. Here, based on multiple data sets in an urban lake system, a synchronous evaluation-diagnosis model with quantitative stressor-response analysis was advanced, aiming to improve the accuracy of evaluation and diagnosis. The weights for key physicochemical stressors were quantitatively determined in the sequence of NDAVIadj > CODMn > TP > NH4+-N by the combination of generalized additive model and structural equation modeling, clarifying the most significant effects of aquatic vegetation on the degradation of fish assemblages. Then, sensitive biological metrics were screened by considering the distinct contributions of four key stressors to alleviate the possible deviation caused by common methods. Finally, ecological integrity was evaluated by summing the key physicochemical stressors and sensitive biological metrics according to the model-deduced weights instead of empirical weights. Our system's diagnosis and evaluation results achieved an accuracy of over 80% when predicting anthropogenic stress and biological status, which highlights the great potential of our multiple-level system for ecosystem management.

Legacy and novel brominated flame retardants in a lab-constructed freshwater ecosystem: Distribution, bioaccumulation, and trophic transfer

The extensive utilization of both legacy and novel brominated flame retardants (BFRs) leads to high environmental concentrations, which would be bioaccumulated by organisms and further transferred through the food webs, causing potential risks to humans. In this study, five BFRs, that showed high detection frequencies and concentrations in sediments from an e-waste dismantling site in Southern China, namely 2,3,4,5,6-pentabromotoluene (PBT), hexabromobenzene (HBB), 1,2-bis(2,4,6-tribromophenoxy) ethane (BTBPE), decabromodiphenyl ethane (DBDPE), and decabromodiphenyl ether (BDE209), were selected as target pollutants in the lab-constructed aquatic food web as part of a micro-ecosystem, to investigate their distribution, bioaccumulation, and trophic transfer patterns. The significant correlations between different samples in the food web indicated that the dietary uptake appeared to influence the levels of BFRs in organisms. Significant negative correlations were observed between the trophic level of organisms and the lipid-normalized concentrations of BTBPE and DBDPE, indicating the occurrence of trophic dilution after 5-month exposure. However, the average values of bioaccumulation factors (BAFs) were from 2.49 to 5.17 L/kg, underscoring the importance of continued concern for environmental risks of BFRs. The organisms occupying higher trophic levels with greater bioaccumulation capacities may play a pivotal role in determining the trophic magnification potentials of BFRs. This research provides a helpful reference for studying the impacts of feeding habits on bioaccumulation and biomagnification, as well as for identifying the fate of BFRs in aquatic environment.