Temporal shifts in the phytoplankton network in a large eutrophic shallow freshwater lake subjected to major environmental changes due to human interventions.

Phytoplankton communities are crucial components of aquatic ecosystems, and since they are highly interactive, they always form complex networks. Yet, our understanding of how interactive phytoplankton networks vary through time under changing environmental conditions is limited. Using a 29-year (339 months) long-term dataset on Lake Taihu, China, we constructed a temporal network comprising monthly sub-networks using "extended Local Similarity Analysis" and assessed how eutrophication, climate change, and restoration efforts influenced the temporal dynamics of network complexity and stability. The network architecture of phytoplankton showed strong dynamic changes with varying environments. Our results revealed cascading effects of eutrophication and climate change on phytoplankton network stability via changes in network complexity. The network stability of phytoplankton increased with average degree, modularity, and nestedness and decreased with connectance. Eutrophication (increasing nitrogen) stabilized the phytoplankton network, mainly by increasing its average degree, while climate change, i.e., warming and decreasing wind speed enhanced its stability by increasing the cohesion of phytoplankton communities directly and by decreasing the connectance of network indirectly. A remarkable shift and a major decrease in the temporal dynamics of phytoplankton network complexity (average degree, nestedness) and stability (robustness, persistence) were detected after 2007 when numerous eutrophication mitigation efforts (not all successful) were implemented, leading to simplified phytoplankton networks and reduced stability. Our findings provide new insights into the organization of phytoplankton networks under eutrophication (or re-oligotrophication) and climate change in subtropical shallow lakes.

Warming alters the network of physiological traits and their contribution to plant abundance.

The impact of global warming on plant abundance has been widely discussed, but it remains unclear how warming affects plant physiological traits, and how these traits contribute to the abundance of aquatic plants. We explored the adjustments in physiological traits of two common aquatic plant species (Potamogeton crispus L. and Elodea canadensis Michx.) and their links to plant abundance in three temperature treatments by determining twelve physiological traits and plant abundance over an 11-month period in outdoor mesocosms. This mesocosms facility has been running uninteruptedly for 16 years, rendering the plants a unique opportunity to adapt to the warming differences. We found that 1) warming reduced the starch storage in winter for P. crispus and in summer for E. canadensis while increased the nitrogenous substances (e.g., TN, FAA, and proline) in winter for P. crispus. 2) For E. canadensis, TC, starch, SC, and sucrose contents were higher in summer than in winter regardless of warming, while TC, SC, and sucrose contents were lower in summer for P. crispus. 3) Warming decreased the association strength between physiological traits and plant abundance for P. crispus but enhanced it for E. canadensis. 4) E. canadensis showed increased interaction strength among physiological traits under warming, indicating increased metabolic exertion in the response to warming, which contributed to the reduction in abundance. Trait interaction strength of P. crispus was reduced under warming, but with less impact on plant abundance compared with E. canadensis. Our study emphasizes that warming alters the network of plant physiological traits and their contribution to abundance and that different strengths of susceptibility to warming of the various plant species may alter the composition of plant communities in freshwater ecosystems.

High resolution chlorophyll- a in-situ fluorescence sensors versus in-vitro chlorophyll- a measurements in mesocosms with contrasting nutrient and temperature treatments.

Harmful algal blooms (HABs) are a significant threat to freshwater ecosystems, and monitoring for changes in biomass is therefore important. Fluorescence in-situ sensors enable rapid and high frequency real-time data collection and have been widely used to determine chlorophyll- a (Chla) concentrations that are used as an indicator of the total algal biomass. However, conversion of fluorescence to equivalent Chla concentrations is often complicated due to biofouling, phytoplankton composition and the type of equipment used. Here, we validated measurements from 24 Chla and 12 phycocyanin (cyanobacteria indicator) fluorescence in-situ sensors (Cyclops-7F, Turner Designs) against spectrophotometrically (in-vitro) determined Chla and tested a data-cleaning procedure for eliminating data errors and impacts of non-photochemical quenching. The test was done across a range of freshwater plankton communities in 24 mesocosms (i.e. experimental tanks) with a 2x3 (high and low nutrient x ambient, IPCC-A2 and IPCC-A2+50% temperature scenarios) factorial design. For most mesocosms (tanks), we found accurate (r 2 ≥ 0.7) calibration of in-situ Chla fluorescence data using simple linear regression. An exception was tanks with high in-situ phycocyanin fluorescence, for which multiple regressions were employed, which increased the explained variance by >16%. Another exception was the low Chla concentration tanks (r 2 < 0.3). Our results also show that the high frequency in-situ fluorescence data recorded the timing of sudden Chla variations, while less frequent in-vitro sampling sometimes missed these or, when recorded, the duration of changes was inaccurately determined. Fluorescence in-situ sensors are particularly useful to detect and quantify sudden phytoplankton biomass variations through high frequency measurements, especially when using appropriate data-cleaning methods and accounting for factors that can impact the fluorescence readings.

Harmful algal blooms (HABs) may pose a significant threat to freshwater ecosystems and to animal and human health. Therefore, it is important to monitor changes in algal biomass. Traditional methods, while effective, lack the ability to provide rapid, high-frequency, real-time data. In-situ fluorescence sensors, specifically designed to measure chlorophyll- a (total phytoplankton indicator) and phycocyanin (Blue-green algae indicator), offer a promising solution. However, challenges such as biofouling, temporal changes in phytoplankton composition, and equipment variations complicate the conversion of fluorescence data into equivalent chlorophyll- a concentrations. Our study aimed to validate measurements from 24 chlorophyll- a and 12 phycocyanin fluorescence in-situ sensors (Cyclops-7F, Turner Designs). We compared these measurements against spectrophotometrically determined (in-vitro method) chlorophyll- a concentrations. Additionally, we tested a data-cleaning procedure to eliminate errors caused by different sources, such as light. The validation and testing were conducted at Lemming Experimental Mesocosm site (Denmark), in 24 experimental tanks (mesocosms) representing 2 different nutrient levels and 3 temperature scenarios. This study underlines that high-frequency in-situ fluorescence sensors can be useful, only if the user is aware of the possible interacting factors that can influence fluorescence readings (e.g. turbidity, daylight). Therefore, in-situ fluorescence sensors, when properly calibrated and validated, offer a valuable tool for monitoring harmful algal blooms. The high-frequency data provides insights into sudden variations in phytoplankton biomass, demonstrating the potential for improved real-time understanding of freshwater ecosystems.

Species on the move: Impacts of climate change on the spatial range of endemic fishes of the eco-sensitive semi-arid area of the Arabian Peninsula.

Climate change is one of the most significant challenges worldwide in the Anthropocene, and it is predicted to importantly affect biological diversity, especially in freshwaters. Freshwater fishes are facing considerable global threats, particularly in eco-sensitive semi-arid to arid areas such as the Arabian Peninsula, which is considered a highly stressed region in the Middle East. Endemic species are believed to display a narrow range of traits, with rarity reflecting adaptation to specific environmental regimes, and they are thus highly sensitive to environmental disturbances. This study is the first attempt to map the occurrence of endemic freshwater fish species and predict the impact of climate change on their spatial range in the semi-arid area of the Arabian Peninsula using Species Distribution Modeling (SDM). We compared the present and future (2041-2060 and 2061-2080) climate niche for the species under various climatic scenarios. All global circulation models (GCMs) performed well in predicting the species' climatic niche (AUC ranging between 0.72 and 0.92). For certain species (Cyprinion acinaces, Garra buettikeri, Carasobarbus exulatus, Arabibarbus arabicus, and Cyprinion mhalense), variables associated with precipitation were more important than those related to temperature, while for others (Carasobarbus apoensis, G. sahilia, G tibanica, and Aphaniops kruppi), temperature-related variables were most important. Precipitation in the coldest quarter and in the driest quarter was the most sensitive variable for the predictions. The species showed distinct responses to climate change; seven were predicted to lose their climatically suitable habitats (losers) and are thus threatened and highly vulnerable to the effects of climate change, while two species were predicted to expand their range (winners). Regular monitoring of fish in the Arabian Peninsula is recommended to conserve endemic species and their ecosystems.

Composition regulates dissolved organic matter adsorption onto iron (oxy)hydroxides and its competition with phosphate: Implications for organic carbon and phosphorus immobilization in lakes.

Dissolved organic matter (DOM) is a heterogeneous pool of compounds and exhibits diverse adsorption characteristics with or without phosphorous (P) competition. The impacts of these factors on the burial and mobilization of organic carbon and P in aquatic ecosystems remain uncertain. In this study, an algae-derived DOM (ADOM) and a commercially available humic acid (HA) with distinct compositions were assessed for their adsorption behaviors onto iron (oxy)hydroxides (FeOx), both in the absence and presence of phosphate. ADOM contained less aromatics but more protein-like and highly unsaturated structures with oxygen compounds (HUSO) than HA. The adsorption capacity of FeOx was significantly greater for ADOM than for HA. Protein-like and HUSO compounds in ADOM and humic-like compounds and macromolecular aromatics in HA were preferentially adsorbed by FeOx. Moreover, ADOM demonstrated a stronger inhibitory effect on phosphate adsorption than HA. This observation suggests that the substantial release of autochthonous ADOM by algae could elevate internal P loading and pose challenges for the restoration of restore eutrophic lakes. The presence of phosphate suppressed the adsorption of protein-like compounds in ADOM onto FeOx, resulting in an increase in the relative abundance of protein-like compounds and a decrease in the relative abundance of humic-like compounds in post-adsorption ADOM. In contrast, phosphate exhibited no discernible impact on the compositional fractionation of HA. Collectively, our results show the source-composition characters of DOM influence the immobilization of both DOM and P in aquatic ecosystems through adsorption processes. The preferential adsorption of proteinaceous compounds within ADOM and aromatics within HA highlights the potential for the attachment with FeOx to diminish the original source-specific signatures of DOM, thereby contributing to the shared DOM characteristics observed across diverse aquatic environments.

Corrigendum to "Optical measurements of dissolved organic matter as proxies for CODMn and BOD5 in plateau lakes" [Environ. Sci. Ecotech. 19 (2024) 100326].

[This corrects the article DOI: 10.1016/j.ese.2023.100326.].

Response of Functional Traits of Aquatic Plants to Water Depth Changes under Short-Term Eutrophic Clear-Water Conditions: A Mesocosm Study.

Aquatic plants play a key role in the structuring and functioning of shallow lake ecosystems. However, eutrophication often triggers shifts in plant communities and species diversity, especially in the early stages when the water is still clear. Additionally, water depth is an important factor regulating aquatic plant communities. We conducted a 50-day mesocosm study to investigate how water depth (50 cm and 100 cm) affected the functional traits (vertical expansion versus horizontal colonisation) of 20 aquatic plants under eutrophic clear-water conditions. Among the selected species, the submerged plants Hydrocotyle vulgaris and Limnophila indica exhibited higher plant height or biomass in deeper water, while the emergent plants Myriophyllum aquaticum showed the opposite trend. Additionally, Ludwigia peploides subsp. stipulacea exhibited better vertical growth than the remaining species, and the submerged species Vallisneria denseserrulata had better horizontal colonisation. There was a positive correlation between plant height and rhizome length, indicating the absence of a trade-off between vertical growth and horizontal expansion. Our findings suggest an overall resilience of aquatic plants to varying water depths within our study range and highlight the importance of analysing functional traits when selecting appropriate species in freshwater ecosystem restoration, particularly in the face of climate change-induced water depth fluctuations.

Community stability of free-living and particle-attached bacteria in a subtropical reservoir with salinity fluctuations over 3 years.

Changes in salinity have a profound influence on ecological services and functions of inland freshwater ecosystems, as well as on the shaping of microbial communities. Bacterioplankton, generally classified into free-living (FL) and particle-attached (PA) forms, are main components of freshwater ecosystems and play key functional roles for biogeochemical cycling and ecological stability. However, there is limited knowledge about the responses of community stability of both FL and PA bacteria to salinity fluctuations. Here, we systematically explored changes in community stability of both forms of bacteria based on high-frequency sampling in a shallow urban reservoir (Xinglinwan Reservoir) in subtropical China for 3 years. Our results indicated that (1) salinity was the strongest environmental factor determining FL and PA bacterial community compositions - rising salinity increased the compositional stability of both bacterial communities but decreased their α-diversity. (2) The community stability of PA bacteria was significantly higher than that of FL at high salinity level with low salinity variance scenarios, while the opposite was found for FL bacteria, i.e., their stability was higher than PA bacteria at low salinity level with high variance scenarios. (3) Both bacterial traits (e.g., bacterial genome size and interaction strength of rare taxa) and precipitation-induced factors (e.g., changes in salinity and particle) likely contributed collectively to differences in community stability of FL and PA bacteria under different salinity scenarios. Our study provides additional scientific basis for ecological management, protection and restoration of urban reservoirs under changing climatic and environmental conditions.

Re-oligotrophication and warming stabilize phytoplankton networks.

Phytoplankton taxa are strongly interconnected as a network, which could show temporal dynamics and non-linear responses to changes in drivers at both seasonal and long-term scale. Using a high quality dataset of 20 Danish lakes (1989-2008), we applied extended Local Similarity Analysis to construct temporal network of phytoplankton communities for each lake, obtained sub-network for each sampling month, and then measured indices of network complexity and stability for each sub-network. We assessed how lake re-oligotrophication, climate warming and grazers influenced the temporal dynamics on network complexity and stability of phytoplankton community covering three aspects: seasonal trends, long-term trends and detrended variability. We found strong seasonality for the complexity and stability of phytoplankton network, an increasing trend for the average degree, modularity, nestedness, persistence and robustness, and a decreasing trend for connectance, negative:positive interactions and vulnerability. Our study revealed a cascading effect of lake re-oligotrophication, climate warming and zooplankton grazers on phytoplankton network stability through changes in network complexity characterizing diversity, interactions and topography. Network stability of phytoplankton increased with average degree, modularity, nestedness and decreased with connectance and negative:positive interactions. Oligotrophication and warming stabilized the phytoplankton network (enhanced robustness, persistence and decreased vulnerability) by enhancing its average degree, modularity, nestedness and by reducing its connectance, while zooplankton richness promoted stability of phytoplankton network through increases in average degree and decreases in negative interactions. Our results further indicate that the stabilization effects might lead to more closed, compartmentalized and nested interconnections especially in the deeper lakes, in the warmer seasons and during bloom periods. From a temporal dynamic network view, our findings highlight stabilization of the phytoplankton community as an adaptive response to lake re-oligotrophication, climate warming and grazers.

The effects of different doses of lanthanum-modified bentonite in combination with a submerged macrophyte (Vallisneria denseserrulata) on phosphorus inactivation and macrophyte growth: A mesocosm study.

The combination of chemical phosphorus (P) inactivation and submerged macrophyte transplantation has been widely used in lake restoration as it yields stronger effects than when applying either method alone. However, the dose effect of chemical materials on P inactivation when used in combination with submerged macrophytes and the influences of the chemicals used on the submerged macrophytes growth remain largely unknown. In this study, we investigated P inactivation in both the water column and the sediment, and the responses of submerged macrophytes to Lanthanum modified bentonite (LMB) in an outdoor mesocosm experiment where Vallisneria denseserrulata were transplanted into all mesocosms and LMB was added at four dosage levels, respectively: control (LMB-free), low dosage (570 g m-2), middle dosage (1140 g m-2), and high dosage (2280 g m-2). The results showed that the combination of LMB dosage and V. denseserrulata reduced TP in the water column by 32%-38% compared to V. denseserrulata alone, while no significant difference was observed among the three LMB treatments. Porewater soluble reactive P, two-dimensional diffusive gradient in thin films (DGT)-labile P concentrations, and P transformation in the 0-1 cm sediment layer exhibited similar trends along the LMB dosage gradient. Besides, LMB inhibited plant growth and reduced the uptake of mineral elements (i.e., calcium, manganese, iron, and magnesium) in a dosage-dependent manner with LMB. LMB may reduce plant growth by creating a P deficiency risk for new ramets and by interfering with the uptake of mineral elements. Considering both the dose effect of LMB on P inactivation and negative effect on macrophyte growth, we suggest a "small dosage, frequent application" method for LMB application to be used in lake restoration aiming to recover submerged macrophytes and clear water conditions.

Consistent stoichiometric long-term relationships between nutrients and chlorophyll-a across shallow lakes.

Aquatic ecosystems are threatened by eutrophication from nutrient pollution. In lakes, eutrophication causes a plethora of deleterious effects, such as harmful algal blooms, fish kills and increased methane emissions. However, lake-specific responses to nutrient changes are highly variable, complicating eutrophication management. These lake-specific responses could result from short-term stochastic drivers overshadowing lake-independent, long-term relationships between phytoplankton and nutrients. Here, we show that strong stoichiometric long-term relationships exist between nutrients and chlorophyll a (Chla) for 5-year simple moving averages (SMA, median R² = 0.87) along a gradient of total nitrogen to total phosphorus (TN:TP) ratios. These stoichiometric relationships are consistent across 159 shallow lakes (defined as average depth < 6 m) from a cross-continental, open-access database. We calculate 5-year SMA residuals to assess short-term variability and find substantial short-term Chla variation which is weakly related to nutrient concentrations (median R² = 0.12). With shallow lakes representing 89% of the world's lakes, the identified stoichiometric long-term relationships can globally improve quantitative nutrient management in both lakes and their catchments through a nutrient-ratio-based strategy.

Quality matters: Response of bacteria and ciliates to different allochthonous dissolved organic matter sources as a pulsed disturbance in shallow lakes.

Shallow lake ecosystems are particularly prone to disturbances such as pulsed dissolved organic matter (allochthonous-DOM; hereafter allo-DOM) loadings from catchments. However, the effects of allo-DOM with contrasting quality (in addition to quantity) on the planktonic communities of microbial loop are poorly understood. To determine the impact of different qualities of pulsed allo-DOM disturbance on the coupling between bacteria and ciliates, we conducted a mesocosm experiment with two different allo-DOM sources added to mesocosms in a single-pulse disturbance event: Alder tree leaf extract, a more labile (L) source and HuminFeed® (HF), a more recalcitrant source. Allo-DOM sources were used as separate treatments and in combination (HFL) relative to the control without allo-DOM additions (C). Our results indicate that the quality of allo-DOM was a major regulator of planktonic microbial community biomass and/or composition through which both bottom-up and top-down forces were involved. Bacteria biomass showed significant nonlinear responses in L and HFL with initial increases followed by decreases to pre-pulse conditions. Ciliate biomass was significantly higher in L compared to all other treatments. In terms of composition, bacterivore ciliate abundance was significantly higher in both L and HFL treatments, mainly driven by the bacterial biomass increase in the same treatments. GAMM models showed negative interaction between metazoan zooplankton biomass and ciliates, but only in the L treatment, indicating top-down control on ciliates. Ecosystem stability analyses revealed overperformance, high resilience and full recovery of bacteria in the HFL and L treatments, while ciliates showed significant shift in compositional stability in HFL and L with incomplete taxonomic recovery. Our study highlights the importance of allo-DOM quality shaping the response within the microbial loop not only through triggering different scenarios in biomass, but also the community composition, stability, and species interactions (top-down and bottom-up) in bacteria and plankton.

Influence of hydrological features on CO2 and CH4 concentrations in the surface water of lakes, Southwest China: A seasonal and mixing regime analysis.

Due to the large spatiotemporal variability in the processes controlling carbon emissions from lakes, estimates of global lake carbon emission remain uncertain. Identifying the most reliable predictors of CO2 and CH4 concentrations across different hydrological features can enhance the accuracy of carbon emission estimates locally and globally. Here, we used data from 71 lakes in Southwest China varying in surface area (0.01‒702.4 km2), mean depth (< 1‒89.6 m), and climate to analyze differences in CO2 and CH4 concentrations and their driving mechanisms between the dry and rainy seasons and between different mixing regimes. The results showed that the average concentrations of CO2 and CH4 in the rainy season were 23.9 ± 18.8 µmol L-1 and 2.5 ± 4.9 µmol L-1, respectively, which were significantly higher than in the dry season (10.5 ± 10.3 µmol L-1 and 1.8 ± 4.2 µmol L-1, respectively). The average concentrations of CO2 and CH4 at the vertically mixed sites were 24.1 ± 21.8 µmol L-1 and 2.6 ± 5.4 µmol L-1, being higher than those at the stratified sites (14.8 ± 13.4 µmol L-1 and 1.7 ± 3.5 µmol L-1, respectively). Moreover, the environmental factors were divided into four categories, i.e., system productivity (represented by the contents of total nitrogen, total phosphorus, chlorophyll a and dissolved organic matter), physicochemical factors (water temperature, Secchi disk depth, dissolved oxygen and pH value), lake morphology (lake area, water depth and drainage ratio), and geoclimatic factors (altitude, wind speed, precipitation and land-use intensity). In addition to the regression and variance partitioning analyses between the concentrations of CO2 and CH4 and environmental factors, the cascading effects of environmental factors on CO2 and CH4 concentrations were further elucidated under four distinct hydrological scenarios, indicating the different driving mechanisms between the scenarios. Lake morphology and geoclimatic factors were the main direct drivers of the carbon concentrations during the rainy season, while they indirectly affected the carbon concentrations via influencing physicochemical factors and further system productivity during the dry season; although lake morphology and geoclimatic factors directly contributed to the carbon concentrations at the vertically mixed and stratified sites, the direct effect of system productivity was only observed at the stratified sites. Our results emphasize that, when estimating carbon emissions from lakes at broad spatial scales, it is essential to consider the influence of precipitation-related seasons and lake mixing regimes.

Terrestrial dissolved organic matter inputs accompanied by dissolved oxygen depletion and declining pH exacerbate CO2 emissions from a major Chinese reservoir.

Terrestrial inputs and subsequent degradation of dissolved organic matter (DOM) in lake ecosystems can result in rapid depletion of dissolved oxygen (DO). Inputs of terrestrial DOM including organic acids can also lead to decreases in pH. However, to date, few studies have investigated the linkages between terrestrial DOM inputs, DO and pH levels in the water column, and carbon dioxide (CO2) emissions from lake ecosystems. Based on monthly field sampling campaigns across 100 sites in Lake Qiandao, a major man-made drinking water reservoir in China, from May 2020 to April 2021, we estimated an annual CO2 efflux (FCO2) of 37.2 ± 29.0 gC m-2 yr-1, corresponding to 0.02 ± 0.02 TgC yr-1 from this lake. FCO2 increased significantly with decreasing DO, chlorophyll-a (Chl-a) and δ2H-H2O, while FCO2 increased with increasing specific UV absorbance (SUVA254) and a terrestrial humic-like component (C2). We found that DO concentration and pH declined with increasing terrestrial DOM inputs, i.e. increased SUVA254 and terrestrial humic-like C2 levels. Vertical profile sampling revealed that the partial pressure of CO2 (pCO2) increased with increasing terrestrial DOM fluorescence (FDOM), while DO, pH, and δ13C-CO2 declined with increasing terrestrial FDOM. These results highlight the importance of terrestrial DOM inputs in altering physico-chemical environments and fueling CO2 emissions from this lake and potentially other aquatic ecosystems.

Climate-associated variation in the drivers of benthic macroinvertebrate species-area relationships across shallow freshwater lakes.

The island species-area relationship (ISAR) describes how species richness increases with increasing area of a given island or island-like habitat, such as freshwater lakes. While the ISAR is one of the most common phenomena observed in ecology, there is variation in both the form of the relationship and its underlying mechanisms. We compiled a global data set of benthic macroinvertebrates from 524 shallow freshwater lakes, ranging from 1 to 293,300 ha in area. We used individual-based rarefaction to determine the degree to which ISAR was influenced by mechanisms other than passive sampling (larger islands passively sample more individuals from the regional pool and, therefore, have more species than smaller islands), which would bias results away from expected relationships between rarefied species richness (and other measures that capture relative abundances) and lake area. We also examined how climate may alter the shape of the ISARs. We found that both rarefied species richness (the number of species standardized by area or number of individuals) and a measure of evenness emphasizing common species exhibit shallow slopes in relationships with lake area, suggesting that the expected ISARs in these lakes most likely result from passive sampling. While there was considerable variation among ISARs across the investigated lakes, we found an overall positive rarefied ISAR for lakes in warm (i.e. tropical/subtropical) regions (n = 195), and in contrast, an overall negative rarefied ISAR in cool (i.e. north temperate) lakes (n = 329). This suggested that mechanisms beyond passive sampling (e.g. colonization-extinction dynamics and/or heterogeneity) were more likely to operate in warm lakes. One possible reason for this difference is that the area-dependent intensity of fish predation, which can lead to flatter ISARs, is weaker in warmer relative to cooler lakes. Our study illustrates the importance of understanding both the pattern and potential processes underlying the ISARs of freshwater lakes in different climatic regions. Furthermore, it provides a baseline for understanding how further changes to the ecosystem (i.e. in lake area or climate) might influence biodiversity patterns.

Native molluscs alleviate water quality impacts of invasive crayfish.

Freshwaters are considered to be the most vulnerable ecosystems facing biological invasions, and the red swamp crayfish (Procambarus clarkii) is one of the most widespread aquatic invasive species in the world. P. clarkii has negative impacts on water quality in the lakes that it invades by, for instance, increasing their turbidity and nutrient concentrations and reducing macrophyte biomass. However, native taxa such as snails and mussels could potentially help to maintain a clear-water status in lakes by grazing on periphyton or by phytoplankton filtration. To examine the potential negative effects of P. clarkii on the clear-water state in lakes dominated by the macrophyte Vallisneria denseserrulata and the potential for native species to buffer these effects, we tested the crayfish impact in the absence and presence of the snail Bellamya aeruginosa and the mussel Sinanodonta woodiana at different biomasses. In the presence of crayfish, total suspended solids, total phosphorus, and chlorophyll a concentrations significantly increased compared to the control treatments without crayfish. However, when crayfish coexisted with snails or mussels, these three environmental variables all decreased in concentration compared to the crayfish-only treatment. Low (500 g/m2) and high (1500 g/m2) snail or mussel biomass had similar buffering effects. Macrophyte biomass in the crayfish and high mussel biomass treatment was 43 % higher than in the crayfish-only treatment. Native molluscs therefore alleviated the negative effects of crayfish on lake water quality and promoted native macrophyte growth. We conclude that a thriving native mollusc community may help in maintaining the clear-water state in lakes following crayfish invasion.

Terrestrial dissolved organic matter inputs drive the temporal dynamics of riverine bacterial ecological networks and assembly processes.

Rivers receive, transport, and are reactors of terrestrial dissolved organic matter (DOM) and are highly influenced by changes in hydrological conditions and anthropogenic disturbances, but the effect of DOM composition on the dynamics of the bacterial community in rivers is poorly understood. We conducted a seasonal field sampling campaign at two eutrophic river mouth sites to examine how DOM composition influences the temporal dynamics of bacterial community networks, assembly processes, and DOM-bacteria associations. DOM composition and seasonal factors explained 34.7% of the variation in bacterial community composition, and 14.4% was explained purely by DOM composition where specific UV absorbance (SUVA254) as an indicator of aromaticity was the most important predictor. Significant correlations were observed between SUVA254 and the topological features of subnetworks of interspecies and DOM-bacteria associations, indicating that high DOM aromaticity results in more complex and connected networks of bacteria. The bipartite networks between bacterial taxa and DOM molecular formulae (identified by ultrahigh-resolution mass spectrometry) further revealed less specialized bacterial processing of DOM molecular formulae under the conditions of high water level and DOM aromaticity in summer than in winter. A shift in community assembly processes from stronger homogeneous selection in summer to higher stochasticity in winter correlated with changes in DOM composition, and more aromatic DOM was associated with greater similarity in bacterial community composition. Our results highlight the importance of DOM aromaticity as a predictor of the temporal dynamics of riverine bacterial community networks and assembly.

Optical measurements of dissolved organic matter as proxies for CODMn and BOD5 in plateau lakes.

The presence of organic matter in lakes profoundly impacts drinking water supplies, yet treatment processes involving coagulants and disinfectants can yield carcinogenic disinfection by-products. Traditional assessments of organic matter, such as chemical oxygen demand (CODMn) and biochemical oxygen demand (BOD5), are often time-consuming. Alternatively, optical measurements of dissolved organic matter (DOM) offer a rapid and reliable means of obtaining organic matter composition data. Here we employed DOM optical measurements in conjunction with parallel factor analysis to scrutinize CODMn and BOD5 variability. Validation was performed using an independent dataset encompassing six lakes on the Yungui Plateau from 2014 to 2016 (n = 256). Leveraging multiple linear regressions (MLRs) applied to DOM absorbance at 254 nm (a254) and fluorescence components C1-C5, we successfully traced CODMn and BOD5 variations across the entire plateau (68 lakes, n = 271, R2 > 0.8, P < 0.0001). Notably, DOM optical indices yielded superior estimates (higher R2) of CODMn and BOD5 during the rainy season compared to the dry season and demonstrated increased accuracy (R2 > 0.9) in mesotrophic lakes compared to oligotrophic and eutrophic lakes. This study underscores the utility of MLR-based DOM indices for inferring CODMn and BOD5 variability in plateau lakes and highlights the potential of integrating in situ and remote sensing platforms for water pollution early warning.

Weakened casual feedback loops following intensive restoration efforts and climate changes in a large shallow freshwater lake.

Understanding how phytoplankton interacts with local and regional drivers as well as their feedbacks is a great challenge, and quantitative analyses of the regulating role of human activities and climate changes on these feedback loops are also limited. By using monthly monitoring dataset (2000-2017) from Lake Taihu and empirical dynamic modelling to construct causal networks, we quantified the strengths of causal feedbacks among phytoplankton, local environments, zooplankton, meteorology as well as global climate oscillation. Prevalent bidirectional causal linkages between phytoplankton biomass (chlorophyll a) and the tested drivers were found, providing holistic and quantitative evidence of the ubiquitous feedback loops. Phytoplankton biomass exhibited the highest feedbacks with total inorganic nitrogen and ammonia and the lowest with nitrate. The feedbacks between phytoplankton biomass and environmental factors from 2000 to 2017 could be classified into two groups: the local environments (e.g., nutrients, pH, transparency, zooplankton biomass)-driven enhancement loops promoting the response of the phytoplankton biomass, and the climate (e.g., wind speed)-driven regulatory loops suppressing it. The two counterbalanced groups modified the emergent macroecological patterns. Our findings revealed that the causal feedback networks loosened significantly after 2007 following nutrient loading reduction and unsuccessful biomanipulation restoration attempts by stocking carp. The strength of enhancement loops underwent marked decreases leading to reduced phytoplankton responses to the tested drivers, while the climate (decreasing wind speed, warming winter)-driven regulatory loops increased- like a tug-of-war. To counteract the self-amplifying feedback loops, the present eutrophication mitigation efforts, especially nutrient reduction, should be continued, and introduction of alternative measures to indirectly regulate the critical components (e.g., pH, Secchi depth, zooplankton biomass) of the loops would be beneficial.

Food quality upgrade of carbon from submerged macrophytes by flagellates via a heterotrophic pathway can stimulate growth of Daphnia magna.

Submerged macrophytes play crucial roles in maintaining the stability of clear-water states in shallow lakes. Recent stable isotope studies have shown that crustacean zooplankton can utilize submerged macrophyte carbon, but macrophytes alone cannot support the growth and reproduction of such grazers, being deficient in highly unsaturated fatty acids (HUFA). We hypothesized that flagellates feeding on macrophytes can synthesize HUFA and thereby support crustacean zooplankton. To test this hypothesis, we conducted a feeding experiment in which Daphnia magna were provided with a diet of submerged macrophyte Hydrilla verticillata detritus which had been degraded by lake microbes. The chlorophyte Scenedesmus bijuga and undegraded macrophyte detritus were used as controls for comparison of Daphnia's performance. Using biochemical analysis, we examined how the degradation process affected the food quality of the macrophyte. Flagellates were subsequently isolated from the degraded macrophyte and cultured heterotrophically to detect their HUFA synthesis. The 5-day degraded H. verticillata showed significantly higher HUFA concentrations than undegraded macrophyte detritus. They supported better Daphnia performance than undegraded macrophyte, being comparable with S. bijuga. Two isolated flagellates (SL-1 and SL-2), identified as Ochromonas sp. and Poterioochromonas sp., were found to contain HUFA when cultured heterotrophically without dietary sources of fatty acids, suggesting their HUFA synthesis ability. Our results demonstrate that submerged macrophytes may thus indirectly support crustacean zooplankton via flagellate mediation. As crustacean zooplanktons are of key importance for water quality in the grazer control of phytoplankton, this microbial facilitation may contribute to the maintenance of macrophyte clear-water conditions in shallow lakes.