Eutrophic Freshwater Ecosystems Research Articles

Seasonal succession, host associations and biochemical roles of aquatic viruses in a eutrophic lake plagued by cyanobacterial blooms

Viruses are implicated as important biogeochemical mediators and ecological drivers in freshwater ecosystems. However, the dynamics of viruses and host associations over the seasons and blooming periods in eutrophic freshwater ecosystems remain elusive. From the water microbiomes of planktonic biomass from Lake Taihu, a large and eutrophic lake that suffered from yearly Microcystis-dominated harmful algal blooms (HABs) in China, we recovered 41,997 unique viral clusters spanning a wide taxonomic range, including 15,139 Caudovirales clusters targeting bacteria and 1,044 NCLDV clusters targeting eukaryotes. The viral community exhibited distinct seasonal succession driven by microbial communities (mainly Cyanobacteria and Planctomycetes) and environmental factors (mainly nutrients and temperature). Host prediction highlighted a more distinct viral impact on bacteria-driven nitrogen pathways than phosphate cycling through infection. HAB-induced microbial and environmental variations affected viral strategies including lifestyles, host range, and virus-encoded auxiliary metabolic genes (vAMGs) distributions. Viruses infecting Proteobacteria and Actinobacteria showed enhanced lysogenic lifestyle and condensed host ranges during HAB peak in summer, while viruses infecting Bacteroidota selected the opposite strategy. Notably, vAMGs were most abundant before HAB outbreak in spring, compensating for host bacterial metabolism including carbohydrates metabolism, photosynthesis, and phosphate regulation. The elucidated relationship between viruses, host microbes, and bloom-associated environment suggested prominent biochemical roles in the eutrophic freshwater ecosystems.

The Occurrences of Harmful Algal Blooms (HABs) Species and Trophic Status Update in Kedung Ombo Reservoir

Graphical Abstract Highlight Research Water quality conditions in Kedung Ombo Reservoir. Phytoplankton species and their abundance in Kedung Ombo. Potential existence and dominance of HABs species. The updated trophic status is mapped based on the TSI, Secchi depth, chlorophyll-a, and total phosphorus. Abstract Anthropogenic inputs affect the quality of freshwater ecosystems which causes ecological and health problems to aquatic ecosystems. Harmful algal blooms (HABs) associated with cyanotoxins often occur in nutrient-rich or eutrophic freshwater ecosystems. Kedung Ombo Reservoir in Indonesia has been previously classified as eutrophic to hypertrophic. Therefore, this study aimed to identify the occurrences of potential HABs species, measure the bio-physico-chemical water quality parameters, and update the trophic status of Kedung Ombo Reservoir. Sampling was done thrice during the dry season in 2022 from 5 stations. Twenty-two species of phytoplankton were observed in Kedung Ombo Reservoir. Anabaenopsis sp., Aphanizomenon sp., Ceratium sp., Mougeotia sp., Pandorina sp., and Ulothrix sp. were identified as potentially harmful species. Among those, the potentially HABs species, Aphanizomenon sp. was the most abundant (179,344 cells/L) and Cyanophyceae (205,539 cells/L) was the dominant group of phytoplankton. Kedung Ombo Reservoir had a water temperature of 29.49±0.41°C, phosphate of 0.27±0.25 mg/L, and alkaline pH of 7.90±0.39. Kedung Ombo Reservoir also had low transparency coupled with low dissolved oxygen concentration. The occurrences of HABs species were correlated with transparency and dissolved inorganic nutrients, especially phosphate concentrations. Kedung Ombo Reservoir showed eutrophic conditions based on Secchi depth, chlorophyll-a, total phosphorus, and TSI. Based on research findings, control and mitigation efforts are needed to overcome the eutrophication problems which disrupt the balance of the aquatic ecosystem in the Kedung Ombo Reservoir.

Survival and Development Strategies of Cyanobacteria through Akinete Formation and Germination in the Life Cycle

Eutrophic freshwater ecosystems are vulnerable to toxin-producing cyanobacteria growth or harmful algal blooms. Cyanobacteria belonging to the Nostocales order form akinetes that are similar to the seeds of vascular plants, which are resting cells surrounded by a thick membrane. They overwinter in sediment and germinate when conditions become favorable, eventually developing into vegetative cells and causing blooms. This review covers the cyanobacterial akinete of the Nostocales order and summarizes the environmental triggers and cellular responses involved in akinete germination and formation based on data from the literature. It also emphasizes the intimate and dynamic relationship that exists between the germination and formation of akinete in the annual life cycle of cyanobacteria. After comparing many published data, it is found that the tolerance ranges for factors affecting both akinete germination and formation do not differ significantly and are broadly consistent with the tolerance ranges for vegetative cell growth. However, the optimal range varies with different species and strains of cyanobacteria. The life cycle of cyanobacteria, as a result of akinete germination and formation, has a seasonal periodicity and spatial connectivity between the water column and the sediment. However, during the summer growing season, intimate coupling between akinete formation and germination can occur in the water column, and this can contribute to high population densities being maintained in the water column. During this time, shallow sediment could also provide suitable conditions for akinete germination, thereby contributing to the establishment of water column populations. The information summarized in this review is expected to help improve our shared understanding of the life cycle of the Nostocales cyanobacteria while also providing insights into the monitoring and management of harmful algal blooms.

Assessing nutrient fate from terrestrial to freshwater systems using a semi-distributed model for the Fuxian Lake Basin, China

The growing and increasingly intensified agricultural sector exerts major pressures on the environment. Specifically, nitrogen (N) and phosphorus (P) runoff can induce eutrophication in freshwater ecosystems. To formulate environmental strategies for controlling eutrophication, decision-makers commonly consider the importance of pollutant contributors before developing sector-specific environmental policies. These types of science-based decisions benefit from nutrient models that quantify nutrient transport and fate. However, due to a lack of fertilizer application data, distributed models are generally not suitable for most rural regions with extensive agriculture, while lumped models cannot properly characterize the spatial variation of nutrient fate in these regions. To assess the nutrient contributions from different emission sources to freshwater, we developed a localized semi-distributed model to simulate total nitrogen (TN) and total phosphorus (TP) in 52 inflow rivers of Fuxian Lake Basin in China. The results show that diffuse sources contributed 82 % TN and 92 % TP loading to the inflow rivers. The highest eutrophication potentials (i.e., loading per area) is from the built environment, which is more than 10 times that of forests, but the contribution of the built environment to total diffuse loading is only the second-highest as it occupies 8.7 % of the surface area. Farmland is the main contributor, generating 49 % of diffuse TN and 57 % TP, respectively. Our results show that promoting a 10 % increase in nutrient use efficiency would reduce 5 % of N and 30 % of P diffuse loadings to the rivers. Through examining the impact of nutrient use efficiency, we emphasize the potential trade-offs between food productivity and environmental effects. This analysis workflow can be applied to other agricultural regions.

Intensification of harmful cyanobacterial blooms in a eutrophic, temperate lake caused by nitrogen, temperature, and CO2

Warmer temperatures can significantly increase the intensity of cyanobacterial harmful algal blooms (CHABs) in eutrophic freshwater ecosystems. However, few studies have examined the effects of CO2 enrichment in tandem with elevated temperature and/or nutrients on cyanobacterial taxa in freshwater ecosystems. Here, we observed changes in the biomass of cyanobacteria, nutrients, pH, and carbonate chemistry over a two-year period in a shallow, eutrophic freshwater lake and performed experiments to examine the effects and co-effects of CO2, temperature, and nutrient enrichment on cyanobacterial and N2-fixing (diazotrophic) communities assessed via high throughput sequencing of the 16S rRNA and nifH genes, respectively. During both years, there were significant CHABs (50–500 μg cyanobacterial chlorophyll-a L−1) and lake CO2 levels were undersaturated (≤300 μatm pCO2). NH4+ significantly increased the net growth rates of cyanobacteria as well as the biomass of the diazotrophic cyanobacterial order Nostocales under elevated and ambient CO2 conditions. In a fall experiment, the N2 fixation rates of Nostocales were significantly higher when populations were enriched with CO2 and P, relative to CO2-enriched populations that were not amended with P. During a summer experiment, N2 fixation rates increased significantly under N and CO2 - enriched conditions relative to N-enriched and ambient CO2 conditions. Nostocales dominated the diazotrophic communities of both experiments, achieving the highest relative abundance under CO2-enriched conditions when N was added in the first experiment and when CO2 and temperature were elevated in the second experiment, when N2 fixation rates also increased significantly. Collectively, this study indicates that N promotes cyanobacterial blooms including those formed by Dolichospermum and that the biomass and N2 fixation rates of diazotrophic cyanobacterial taxa may benefit from enhanced CO2 levels in eutrophic lakes.

Spatial and temporal patterns of phytoplankton community succession and characteristics of realized niches in Lake Taihu, China

Understanding the dynamics and succession of phytoplankton in large lakes can help inform future lake management. The study analyzed phytoplankton community variations in Lake Taihu over a 21-year period, focusing on realized niches and their impact on succession. The study developed a niche periodic table with 32 niches, revealing responses to environmental factors and the optimal number of niches. Results showed that the phytoplankton in Lake Taihu showed significant spatial and temporal heterogeneity, with biomass decreasing as one moved from the northwest to the southeast and expanding towards central lake area, and towards autumn and winter. Different phytoplankton groups in Lake Taihu occupied realized niches shaped by temperature, nitrate, and phosphate. To predict the response of eutrophic freshwater lake ecosystems to human activities and climate change, it is critical to interpret the law of phytoplankton bloom and niche succession.

Increased invasion of submerged macrophytes makes native species more susceptible to eutrophication in freshwater ecosystems

Invasion and eutrophication are considered to pose serious threats to freshwater biodiversity and ecosystem function. However, little is known about the synergistic effects of invasion density and nutrient concentration on native submerged macrophytes. Here, we selected a common invasive species (Elodea nuttallii) and two native plants (Hydrilla verticillata and Potamogeton maackianus) to elucidate the effects of invasion density and eutrophication on native submerged plants. We found that (1) high nutrient concentrations inhibited the growth of both invasive and native species, but E. nuttallii, with a wide ecological niche, was more tolerant to eutrophication than the two native species. (2) High invasion density had a remarkable negative effect on the growth of the two native species under the medium and high nutrient concentrations. (3) Medium and high invasion densities of E. nuttallii made native macrophytes more susceptible to eutrophication. (4) The two native macrophytes had species-specific responses to medium and high invasion densities under medium and high nutrient concentrations. Specifically, a high invasion density of E. nuttallii significantly delayed the growth of H. verticillata rather than P. maackianus. Thus, it is necessary to consider the synergistic effects of invasion with eutrophication when assessing invasion in freshwater ecosystems. And our results implied that invasion with eutrophication was a powerful factor determining the results of interspecific competition among submerged macrophytes, which could change the biodiversity, community structure and functions of freshwater ecosystems.

Microcystin Concentrations and Detection of the mcyA Gene in Water Collected from Agricultural, Urban, and Recreational Areas in a Karst Aquifer in the Yucatan Peninsula of Mexico

The eutrophication of freshwater ecosystems allows the proliferation of cyanobacteria that can produce secondary metabolites such as microcystins. The main aim of this study was to explore the occurrence and concentration of microcystin and the mcyA gene in water bodies located in agricultural, urban, and recreational areas in the karst aquifer of the Yucatan peninsula of Mexico (YPM) and to analyze the water quality variables and chlorophyll-a (Chl-a) associated with their presence. Water samples were collected from 14 sites, and microcystin concentrations were quantified using antibody-based ELISA test. Total DNA was isolated from filters and used for PCR amplification of a fragment of the mcyA gene. Amplicons were cloned and sequenced to identify toxin-producing cyanobacteria present in water. Results showed that water bodies had different trophic status based on Carlson’s trophic state index. Dissolved inorganic nitrogen (DIN: NH4+ + NO3− + NO2−) and P-PO43− concentrations were within a range of 0.077–18.305 mg DIN/L and 0.025–2.5 mg P-PO43−/L, respectively, per sampled site. All sampled sites presented microcystin concentrations within a range of ≥0.14 µg/L to ≥5.0 µg/L, from which 21.4% (3/14) exceeded the limit established in water quality standards for water consumption (1 µg/L). The mcyA gene fragment was detected in 28.5% (4/14) of the sites. A total of 23 sequences were obtained from which 87% (20/23) shared >95% nucleotide identity (nt) with the genus Microcystis and 13% (3/23) shared >87% nt identity with uncultured cyanobacteria. No correlation with the presence of the mcyA gene and microcystins was found; however, a positive correlation was detected between microcystin concentrations with pH and Chl-a.

Changes in the morphology and cell ultrastructure of a microalgal community exposed to a commercial glyphosate formulation and a toxigenic cyanobacterium.

Human activities significantly influence the health of aquatic ecosystems because many noxious chemical wastes are discharged into freshwater bodies. Intensive agriculture contributes to the deterioration by providing indirectly fertilizers, pesticides, and other agrochemicals that affect the aquatic biota. Glyphosate is one of the most used herbicides worldwide, and microalgae are particularly sensitive to its formulation, inducing displacement of some green microalgae from the phytoplankton that leads to alterations in the floristic composition, which fosters the abundance of cyanobacteria, some of which can be toxigenic. The combination of chemical stressors such as glyphosate and biological ones, like cyanotoxins and other secondary metabolites of cyanobacteria, could induce a combined effect potentially more noxious to microalgae, affecting not only their growth but also their physiology and morphology. In this study, we evaluated the combined effect of glyphosate (Faena®) and a toxigenic cyanobacterium on the morphology and ultrastructure of microalgae in an experimental phytoplankton community. For this purpose, Microcystis aeruginosa (a cosmopolitan cyanobacterium that forms harmful blooms) and the microalgae Ankistrodesmus falcatus, Chlorella vulgaris, Pseudokirchneriella subcapitata, and Scenedesmus incrassatulus were cultivated, individually and jointly, exposing them to sub-inhibitory concentrations of glyphosate (IC10, IC20, and IC40). Effects were evaluated through scanning electron (SEM) and transmission electron (TEM) microscopy. Exposure to Faena® produced alterations in the external morphology and ultrastructure of microalgae both individually and in combined cultures. SEM evidenced the loss of the typical shape and integrity of the cell wall and an increase in the biovolume. TEM revealed reduction and disorganization of the chloroplast, variation in starch and polyphosphate granules, formation of vesicles and vacuoles, cytoplasm degradation, and cell wall continuity loss. The presence of M. aeruginosa was, for microalgae, an additional stress factor adding to the chemical stress produced by Faena®, increasing the damage in their morphology and ultrastructure. These results alert to the effects that can be caused by glyphosate and the presence of toxigenic bacteria on the algal phytoplankton in contaminated and anthropic and eutrophic freshwater ecosystems.

Macrophyte growth forms and hydrological connectivity affect greenhouse gas concentration in small eutrophic wetlands

In eutrophic freshwater ecosystems, submerged macrophyte communities are replaced by phytoplankton or free-floating plants. In isolated wetlands, vegetation shift occurs over short time scales and leads to water deoxygenation and chemically reduced sediments, conditions that favor the generation, accumulation and degassing of greenhouse gases (GHGs, i.e. CH4, CO2 and N2O) to the atmosphere. However, the relationship between primary producer’s growth forms, hydrological connectivity and GHGs concentration is poorly studied in the literature. A set of 18 freshwater wetlands including isolated and river-connected oxbow lakes, marshes and ponds with different vegetation growth forms was therefore monitored monthly on the annual scale. Potential GHGs diffusive fluxes towards the atmosphere were calculated and compared with direct measurements reported in peer-reviewed papers within a meta-analysis. Our results demonstrate a strong link between the colonization of free-floating plants and the onset of hypoxic conditions and accumulation of dissolved methane. Methane and carbon dioxide concentration peaked in late summer, when floating-leaved and free-floating vegetation covered 100% of the water surface. Carbon dioxide accumulation was particularly evident at hydrological connected wetlands, where nitrate pollution was likely responsible for the concomitant increment of dissolved nitrous oxide. As an increasing number of studies focuses on unravelling environmental drivers of GHGs emission from small lakes and ponds, we encourage to systematically consider the vegetation growth forms and the hydrological connectivity as major drivers of GHGs accumulation and evasion rates.

Evaluating the effectiveness of sediment retention by comparing the spatiotemporal burial of sediment carbon, nitrogen and phosphorus in a plateau lake and its affiliated reservoirs

Evaluating the effectiveness of sediment retention by comparing the spatiotemporal burial of sediment C, N, and P in a plateau lake and its affiliated reservoirs is critical in dealing with global eutrophication in freshwater ecosystems. The objective of the study was to estimate the spatial–temporal of C, N and P accumulation rates (CAR, NAR and PAR) in sediment cores of a plateau Lake Dianchi (DC) and its four affiliated reservoirs (Dashiba-DSB, Xibaisha-XBS, Baoxianghe-BXH, and Shuanlong-SHL). The results indicated that the average sediment accumulation rates (SARs) ranged from 0.040 to 1.187 g cm−2 a−1 in DC and 0.201 to 3.965 g cm−2 a−1 in its affiliated reservoirs. Spatially, all of the values of CAR, NAR and PAR exhibited similar distribution characteristics, along with two high-value regions concentrated in the southern and northern parts of the lake DC. Temporally, these three values of Lake DC and its affiliated reservoirs showed upward trends from bottom to top over the past ∼ 150 years, especially occurring in the 1950s. The change in the C/N ratio indicated that intensive anthropogenic factors promoted the CAR, NAR and PAR in sediments. Based on a developed proportional model, the ratios of sediment retention by each reservoir on TOC, TN and TP were calculated to be 2.15 %, 4.38 % and 9.01 % for DSB, 0.14 %, 0.18 % and 0.14 % for XBS, 1.61 %, 5.77 % and 1.23 % for BXH, and 0.97 %, 2.07 % and 0.46 % for SHL, respectively. Reservoirs located in the upper and middle reaches of the lake catchment play a critical role in sediment retention and prevent a potential risk from water deterioration of the downstream Lake DC. Linkages between reservoir-sources and lake-sinks not only provide valuable information for managing the environmental changes within the catchment, but also contribute to new insight into the regulation of water eutrophication in global lake ecosystem.

Immobilization of Microbes for Biodegradation of Microcystins: A Mini Review.

Harmful cyanobacterial blooms (HCBs) frequently occur in eutrophic freshwater ecosystems worldwide. Microcystins (MCs) are considered to be the most prominent and toxic metabolites during HCBs. MCs may be harmful to human and animal health through drinking water and recreational water. Biodegradation is eco-friendly, cost-effective and one of the most effective methods to remove MCs. Many novel MC-degrading bacteria and their potential for MCs degradation have been documented. However, it is a challenge to apply the free MC-degrading bacterial cells in natural environments due to the long-term operational instability and difficult recycling. Immobilization is the process of restricting the mobility of bacteria using carriers, which has several advantages as biocatalysts compared to free bacterial cells. Biological water treatment systems with microbial immobilization technology can potentially be utilized to treat MC-polluted wastewater. In this review article, various types of supporting materials and methods for microbial immobilization and the application of bacterial immobilization technology for the treatment of MCs-contaminated water are discussed. This article may further broaden the application of microbial immobilization technology to the bioremediation of MC-polluted environments.

Nutrient Remediation Efficiency of the Sedge Plant (Cyperus alopecuroides Rottb.) to Restore Eutrophic Freshwater Ecosystems

The current study investigated the nutrients removal efficiency of the sedge macrophyte Cyperus alopecuroides to treat water eutrophication, besides evaluating the recycling possibility of the harvested material. Samples of sediment, water, and plant tissues were taken seasonally from six polluted and three unpolluted locations for this investigation. The growth properties of C. alopecuroides showed remarkable seasonal differences in plant density and biomass, with the maximum values (7.1 individual/m2 and 889.6 g/m2, respectively) obtained during summer and the minimum (4.1 individual/m2 and 547.2 g/m2, respectively) in winter. In polluted locations, the above-ground tissues had an efficiency to remove more contents of N and P (11.9 and 3.8 g/m2, respectively) than in unpolluted ones (7.1 and 3.4 g/m2, respectively). The high-nutrient standing stock of C. alopecuroides supports its potential use for nutrient removal from eutrophic wetlands. The tissues of C. alopecuroides had the maximum nutrients removal efficiency to remediate great amounts of Na, K, and N in summer, and Ca, P, and Mg in spring. Above- and below-ground parts of C. alopecuroides from unpolluted locations can be considered as a rough forage for beef cattle, dairy cattle, goats, and sheep. The present study indicated the potential of C. alopecuroides in restoring eutrophic freshwater ecosystems, and, thus, it can be used in similar habitats worldwide.

Evaluation of the Potential Release Risk of Internal N and P from Sediments—A Preliminary Study in Two Freshwater Reservoirs in South China

Growing evidence has demonstrated the influence of internal nitrogen (N) and phosphorus (P) on harmful algae blooms in eutrophic freshwater ecosystems. However, the main controlling factors for internal N and P release risks, and whether these factors vary as environmental conditions change, remains poorly understood. We evaluated potential release risks of N and P from sediments in two freshwater reservoirs in Beihai City, southern China, by evaluating apparent nutrient fluxes during simulated static incubation experiments at two temperatures (15 °C and 25 °C). Sediments were analyzed to determine their basic properties as well as N and P fractions. Results showed that the main controlling factors of the apparent fluxes in dissolved total P, soluble reactive P, total N, and ammonium were related to sediment adsorption properties, redox properties, and microbial-mediated properties (e.g., water-extractable P, total inorganic N, redox-sensitive P, total organic carbon, organic P). The primary controlling factors for apparent N and P fluxes were dependent on the form of N and P and changed with temperature. The results suggest that care should be taken when simply using total N and P contents in sediments to evaluate their internal nutrient release risks.

Analysis of oxygen isotopes of inorganic phosphate (δ18Op) in freshwater: A detailed method description for obtaining oxygen isotopes of inorganic phosphate in environmental water samples

The ability to identify the origin of phosphorus and understand processes controlling P cycling is essential for designing effective mitigation and restoration of eutrophic freshwater ecosystems. The oxygen isotope composition of orthophosphate (δ18Op) has significant potential as a tracer for P entering freshwater ecosystems. However, methods of analysis of δ18Op are still in their preliminary stages and have proven challenging to implement for new practitioners. In order to achieve progress in developing the application of δ18Op signatures as a tracing tool, there is a need to eliminate the methodological challenges involved in accurately determining δ18Op. This protocol article describes the various steps needed to concentrate and isolate orthophosphate in freshwater samples into an adequately pure Ag3PO4 analyte, without isotopic alteration during processing. The protocol compiles the disperse experiences from previous studies, combined with our own experience. The twofold aim of the paper is toprovide a baseline for an increasing standardisation of the silver phosphate purification method associated with analysis of the oxygen isotope composition of orthophosphate (δ18Op), and to foster new research in the applicability of δ18Op signatures for P source tracing in catchment science.

A bioturbator, a holobiont, and a vector: The multifaceted role of Chironomus plumosus in shaping N‐cycling

Abstract Tube‐dwelling chironomid larvae are among the few taxa that can withstand and thrive in the organic‐rich sediments typical of eutrophic freshwater ecosystems. They can have multiple effects on microbial nitrogen (N) cycling in burrow environments, but such effects cease when chironomid larvae undergo metamorphosis into flying adults and leave the sediment. Here we investigated the ecological role of Chironomus plumosus by exploring the effect of its different life stages (as larva and adult midge) on microbial N transformations in a shallow freshwater lagoon by means of combined biogeochemical and molecular approaches. Results suggest that sediment bioturbation by chironomid larvae produce contrasting effects on nitrate ()‐reduction processes. Denitrification was the dominant pathway of reduction (>90%), primarily fuelled by from bottom water. In addition to pumping ‐rich bottom water within the burrows, chironomid larvae host microbiota capable of reduction. However, the contribution of larval microbiota is lower than that of microbes inhabiting the burrow walls. Interestingly, dinitrogen fixation co‐occurred with reduction processes, indicating versatility of the larvae's microbial community. Assuming all larvae (averaging 1,800 ind./m2) leave the sediment following metamorphosis into flying adults, we estimated a displacement of 47,787 µmol of organic N/m2 from the sediment to the atmosphere during adult emergence. This amount of particulate organic N is similar to the entire N removal stimulated by larvae denitrification over a period of 20 days. Finally, the detection of N‐cycling marker genes in flying adults suggests that these insects retain N‐cycling microbes during metamorphosis and migration to the aerial and terrestrial ecosystems. This study provides evidence that chironomids have a multifaceted role in shaping the N cycle of aquatic ecosystems.

Identifying the Mechanisms behind the Positive Feedback Loop between Nitrogen Cycling and Algal Blooms in a Shallow Eutrophic Lake

Algal blooms have increased in frequency, intensity, and duration in response to nitrogen (N) cycling in freshwater ecosystems. We conducted a high-resolution sedimentary study of N transformation and its associated microbial activity in Lake Taihu to assess the accumulation rates of the different N fractions in response to algal blooms, aiming to understand the mechanisms of N cycling in lacustrine environments. Downcore nitrification and denitrification processes were measured simultaneously in situ via diffusive gradients in thin-films technique, peeper, and microelectrode devices in a region of intensified algal blooms of shallow lake. The decomposition of different biomasses of algal blooms did not change the main controlling factor on different N fractions in profundal sediment. However, the decomposition of different algal biomasses led to significant differences in the nitrification and denitrification processes at the sediment–water interface (SWI). Low algal biomasses facilitated the classic process of N cycling, with the balanced interaction between nitrification and denitrification. However, the extreme hypoxia under high algal biomasses significantly limited nitrification at the SWI, which in turn, restricted denitrification due to the lack of available substrates. Our high-resolution results combined with estimates of apparent diffusion fluxes of the different N fractions inferred that the lack of substrates for denitrification was the main factor influencing the positive feedback loop between N and eutrophication in freshwater ecosystems. Moreover, this positive feedback can become irreversible without technological intervention.

Dynamic sulfur-iron cycle promoted phosphorus mobilization in sediments driven by the algae decomposition.

Direct evidence of the algae bloom in eutrophic freshwater lakes on sulfur cycle and the subsequent iron oxide reduction and the iron oxides-bound phosphate (Fe-P) release in sediments is lacking. In this study, microcosms experiment was carried out to investigate the dynamic variations of S, Fe and P species in the water column and sediments as well as the sulfate reducing bacteria (SRB) abundance variation in the sediments during algae decomposition. The sulfate reduction was stimulated by the algae decomposition, which resulted in dramatic sulfate decline, sulfide increase and SRB growth. In addition, large amounts of acid volatile sulfide (AVS), pyrite sulfur (Pyrite-S) and elemental sulfur (S0) accumulated in the sediment. In particular, the contents of sedimentary Fe(II) and Pyrite-S in surface sediments continuously accumulated until the end of the experiment. Moreover, the terminal Fe-P content reduced by 35.4% compared with the initial concentration at high algae density group. These results suggested the irreversible reduction of iron oxides and revealed iron chemical reduction mediated by sulfide during algae decomposition. In addition, the connection of sulfur-iron cycle and the significant promotion of Fe-P mobilization in sediments was established, which should be paid more attention in the eutrophic freshwater ecosystems.

Seasonal variation in effects of urea and phosphorus on phytoplankton abundance and community composition in a hypereutrophic hardwater lake

Abstract Urea accounts for half of global agricultural fertiliser applications, yet little is known of its role in eutrophication of freshwater ecosystems, nor how it interacts with phosphorus (P) in regulating phytoplankton composition, especially during spring and autumn. To identify when and how urea and P inputs interact across the ice‐free period, we conducted seven monthly fertilisation experiments in 3,240‐L mesocosms from ice‐off to ice‐formation in a hypereutrophic lake. In addition, we ran bioassays with ammonium (NH4+) to compare the effects of urea with those of NH4+, the immediate product of chemical decomposition of urea. Analysis of water‐column chlorophyll a and biomarker pigments by high‐performance liquid chromatography revealed that addition of inorganic P alone (100 µg P L–1 week–1) had no significant impact on either algal abundance or community composition in hypereutrophic Wascana Lake. Instead, fertilisation with urea (4 mg N L−1 week–1) alone, or in concert with P, significantly (p < 0.05) increased algal abundance in spring and much of summer, but not prior to ice formation in October. In particular, urea amendment enhanced abundance of cryptophytes, chlorophytes, and non‐diazotrophic cyanobacteria during April and May, while fertilisation in summer and early autumn (September) increased only chlorophytes and non‐diazotrophic cyanobacteria. Comparison of urea mesocosms with NH4+ bioassays demonstrated that urea lacked the inherent toxicity of NH4+ in cool waters, but that both compounds stimulated production during summer experiments. This study showed that urea pollution can degrade water quality in P‐rich lakes across a variety of seasonal conditions, including spring, and underscores the importance of quantifying the timing and form of N inputs when managing P‐rich freshwaters.

Experimental evidence on the effects of temperature and salinity in morphological traits of the Microcystis aeruginosa complex

Microcystis aeruginosa complex (MAC) encompasses noxious colonial bloom forming cyanobacteria. MAC representatives bloom in eutrophic freshwater and brackish ecosystems with stagnant water, were temperature and salinity are the main variables modulating their distribution, biomass and toxicity. Cell abundance and biovolume of MAC colonies define regulatory standards for public health. These variables depend upon colony size that in turn changes with environmental conditions. Here, we conducted two series of experiments to evaluate the response of MAC colonies morphological traits (length, volume, mucilage and number of cells) to temperature and salinity. In two series of experiments in the laboratory, we exposed natural MAC communities to three different temperatures (10, 21 and 30 °C) and four salinity levels (0, 5, 10 and 25 ppt) typically found in estuaries. We found that average colony length, volume and mucilage thickness did not change with temperature, but the cell-free space inside the colonies was smaller at the highest evaluated temperature (30 °C). Salinity fostered an increase in colony length, volume and mucilage thickness, while cell-free space diminished, resulting in higher cell density. The number of cells per colony was significantly related to colony size (length and volume) and both, temperature and salinity, affected the parameters of the relationships. Based on present results we propose statistical models to predict cell number per colony based on length and volume and accounting for the effect of salinity and temperature on these traits. This is applicable to ecological studies and to the monitoring of estuarine aquatic environments, by means of a fast and more accurate estimation of cell numbers to define MAC toxic populations early warning systems. A protocol is suggested for its application while the analysis of the interaction of temperature and salinity, as well as the variability in natural environments are objectives for future researches.