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With increasing anthropogenic activities, freshwater ecosystems around the world are becoming increasingly affected by various pressures, including eutrophication, overfishing, and irrational stocking, which may have a negative impact on the food web structure. Despite the extensive research and proposed management measures for eutrophic lakes, there are only few analysis on long-term monitoring data regarding fishery resources. Additionally, there is a lack of evaluation and prediction of the effectiveness of current fish management policies. To remedy this, we analyzed long-term monitoring data from Lake Taihu, China, a severely eutrophicated lake with a skewed fish size structure exhibiting dominance of small individuals. We first constructed 14 Ecopath models to investigate how trophic interactions and biomass fluxes changed from 2007 to 2020. Subsequently, the Ecosim model was used to predict how the biomass of fish and the ecosystem network respond to the initiated 10-years fishing ban. Our results demonstrate long-term changes in fish biomass and ecosystem stability. The analyses revealed that 1) the biomass development in different feeding types of fish is controlled by human activities (mainly catches and stocking) and trophic interactions and 2) the rate of decline in ecosystem network stability slows down during the fishing ban. The primary focus of this study was to fill the gap in long-term serial studies of fish monitoring data and ecosystem stability in the lake and, for the first time, to predict the outcome of the fishing ban from an ecosystem perspective using the Ecosim model. Overall, our results emphasize the importance of rational stocking and fishing policies and provide a better understanding of the changes in the ecological dynamics in Lake Taihu of relevance for the management and restoration of the lake.

Phewa Lake, Nepal's second largest lake, has been the subject of many short‐term water quality studies in the past, although the results of such studies have not been consistent. Long‐term monitoring data can distinguish typical seasonal dynamics from atypical occurrences. Accordingly, the objective of the present study was to describe typical seasonal patterns and quantify variability, detect long‐term trends and provide reliable baseline data for future trend detection. The present study provides 17 years of monthly water quality data (1993–2010) and occasional data to 2018 from the 0.5 to 10 m depth at one sampling site. Phewa Lake experiences a subtropical monsoonal climate, typically being stratified from March to September, although heavy rainfalls disrupted stratification. Bottom‐water anoxia, occurring typically between April and June, was associated with increased ammonium concentrations, but not with increased dissolved reactive phosphorus concentrations. The observed nutrient and chlorophyll‐a concentrations were generally typical of an oligo‐ to mesotrophic lake. The water clarity was lowest in the autumn and winter, being associated with high phytoplankton biomass, as well as during the summer, the latter likely attributable to suspended silt in the water. Lake surface waters warmed by 1.1–1.8°C during 1993–2018. Water clarity data suggested slight eutrophication from 1993 to 2018, although no increased nutrient or chlorophyll‐a concentrations was detected up to 2010. The water clarity data gave no evidence of a long‐term increase in the suspended sediment concentrations. All studied water quality variables exhibited high variability within and between years. The results of the present study provide a baseline for the detection of future water quality trends. The data confirm some previous research findings, while at the same time indicating long‐term regular monitoring is needed to accurately characterize Phewa Lake's limnology and to detect long‐term trends. Correct trend detection and proper understanding of the lake limnology are essential for proper management of Phewa Lake and other subtropical lakes. The present study represents one of only a few studies that examine water quality and other seasonality variables in subtropical lakes over multiple years. [ABSTRACT FROM AUTHOR]

Lake Cajititlán is a subtropical and endorheic lake, which is heavily impacted by nutrient pollution. Agricultural runoff and poorly treated wastewater have entered this reservoir at alarming rates during past rainy seasons, causing the cultural eutrophication of this body of water and resulting in several massive fish kill events. In this study, shotgun metagenomic sequencing was used to examine the taxonomic and functional structure of microbial communities in Lake Cajititlán during the rainy season. Several water quality features and their interactions with microbial communities were also assessed to identify the major factors affecting the water quality and biota, specifically fish species. According to current water quality regulations, most of the physicochemical variables analyzed (dissolved oxygen, pH, Secchi disk, NH4 +, NO3 -, blue-green algae, total phosphorus, and chlorophyll-a) were outside of the permissible limits. Planktothrix agardhii and Microcystis aeruginosa were the most abundant phytoplankton species, and the dominant bacterial genera were Pseudomonas, Streptomyces, and Flavobacterium, with Pseudomonas fluorescens, Stenotrophomonas maltophilia, and Aeromonas veronii representing the most abundant bacterial species. All of these microorganisms have been reported to be potentially harmful to fish, and the latter three (P. fluorescens, S. maltophilia, A. veronii) also contain genes associated with pathogenicity in fish mortality (fur, luxS, aer, act, aha, exu, lip, ser). Genetic evidence from the microbial communities analyzed herein reveals that anthropogenic sources of nutrients in the lake altered genes involved in nitrogen, phosphorus, sulfur, and carbon metabolism, mainly at the beginning of the rainy season. These findings suggest that abiotic factors influence the structure of the microbial communities, along with the major biogeochemical cycles of Lake Cajititlán, resulting in temporal variations and an excess of microorganisms that can thrive in high-nutrient and lowoxygen environments. After reviewing the literature, this appears to be the first study that focuses on characterizing the water quality of a subtropical hypereutrophic lake through associations between physicochemical variables and shotgun metagenomic data. In addition, there are few studies that have coupled the metabolism of aquatic ecosystems with nutrient cycles. [ABSTRACT FROM AUTHOR]

Lake Cajititlán is a subtropical and endorheic lake, which is heavily impacted by nutrient pollution. Agricultural runoff and poorly treated wastewater have entered this reservoir at alarming rates during past rainy seasons, causing the cultural eutrophication of this body of water and resulting in several massive fish kill events. In this study, shotgun metagenomic sequencing was used to examine the taxonomic and functional structure of microbial communities in Lake Cajititlán during the rainy season. Several water quality features and their interactions with microbial communities were also assessed to identify the major factors affecting the water quality and biota, specifically fish species. According to current water quality regulations, most of the physicochemical variables analyzed (dissolved oxygen, pH, Secchi disk, NH4+, NO3−, blue-green algae, total phosphorus, and chlorophyll-a) were outside of the permissible limits. Planktothrix agardhii and Microcystis aeruginosa were the most abundant phytoplankton species, and the dominant bacterial genera were Pseudomonas, Streptomyces, and Flavobacterium, with Pseudomonas fluorescens, Stenotrophomonas maltophilia, and Aeromonas veronii representing the most abundant bacterial species. All of these microorganisms have been reported to be potentially harmful to fish, and the latter three (P. fluorescens, S. maltophilia, A. veronii) also contain genes associated with pathogenicity in fish mortality (fur, luxS, aer, act, aha, exu, lip, ser). Genetic evidence from the microbial communities analyzed herein reveals that anthropogenic sources of nutrients in the lake altered genes involved in nitrogen, phosphorus, sulfur, and carbon metabolism, mainly at the beginning of the rainy season. These findings suggest that abiotic factors influence the structure of the microbial communities, along with the major biogeochemical cycles of Lake Cajititlán, resulting in temporal variations and an excess of microorganisms that can thrive in high-nutrient and low-oxygen environments. After reviewing the literature, this appears to be the first study that focuses on characterizing the water quality of a subtropical hypereutrophic lake through associations between physicochemical variables and shotgun metagenomic data. In addition, there are few studies that have coupled the metabolism of aquatic ecosystems with nutrient cycles.

Lakes in subtropical regions are highly susceptible to eutrophication due to the heavy rainfall, which causes significant runoff of pollutants (e.g., nutrients) to reach surface waters, altering the water quality and influencing the microbial communities that regulate the biogeochemical cycles within these ecosystems. Lake Cajititlán is a shallow, subtropical, and endorheic lake in western Mexico. Nutrient pollution from agricultural activity and wastewater discharge have affected the lake’s water quality, leading the reservoir to a hypereutrophic state, resulting in episodes of fish mortality during the rainy season. This study investigated the temporal dynamics of bacterial communities within Lake Cajititlán and their genes associated with the nitrogen, phosphorus, sulfur, and carbon biogeochemical cycles during the rainy season, as well as the influences of physicochemical and environmental variables on such dynamics. Significant temporal variations were observed in the composition of bacterial communities, of which Flavobacterium and Pseudomonas were the dominant genera. The climatological parameters that were most correlated with the bacterial communities and their functional profiles were pH, DO, ORP, turbidity, TN, EC, NH4+, and NO3−. The bacterial communities displayed variations in their functional composition for nitrogen, phosphorus, and sulfur metabolisms during the sampling months. The bacterial communities within the lake are highly susceptible to nutrient loads and low DO levels during the rainy season. Bacterial communities had a higher relative abundance of genes associated with denitrification, nitrogen fixation, assimilatory sulfate reduction, cysteine, SOX system, and all phosphorus metabolic pathways. The results obtained here enrich our understanding of the bidirectional interactions between bacterial communities and major biogeochemical processes in eutrophic subtropical lakes. [ABSTRACT FROM AUTHOR]

Hydrological regulation and land uses are known to alter lake organic carbon stock, while inorganic carbon cycling in karst landscapes may as well be shifted due to its rapid kinetic rate. To date, the synchronous pattern in the variations of sediment organic carbon (TOC) and inorganic carbon (TIC) burial in karst lakes largely remains to be quantified along the gradient of catchment development. Two karst lakes with contrasting nutrient level and water depth from Southwest China, both of which were converted to medium-sized reservoirs in ∼1985, were selected for sediment multi-proxy analyses on the temporal and spatial variation of carbon burial in the last ∼80 years. While hydrological fluctuations were well recorded by the sediment variation of particle size, the gradual decrease in median grain size showed a trend of increasing water depth that is consistent with the damming events for both lakes. Meanwhile, the nutrient variables revealed an acceleration of eutrophication since ∼2000 when a rapid expansion of catchment croplands was documented for both lakes. While diatom assemblage showed a significant response to hydro-climatic fluctuation and nutrients at both lakes, algal production (i.e. chlorophyll and cyanobacterial pigments) significantly increased with eutrophication at both lakes. The TIC content in the mesotrophic Taiping Lake and the TOC content in the eutrophic lake of Sanjiaohai both showed a significant increase with agricultural expansion, however, the contents of TOC in Taiping and of TIC in Sanjiaohai remained relatively stable over time. While a significant increase in the sediment flux with decreasing bulk sediment C:N ratios suggests elevated autochthonous input, the temporal change of TIC and TOC flux showed a significant synchrony at both lakes. Meanwhile, the sediment content and flux of TIC is significantly higher than those of TOC in Taiping Lake with a higher rate of carbon turnover, but lower than those of TOC in Sanjiaohai. Thus, the accumulation of TIC was disproportionally promoted in deep lakes while the burial of TOC is more favorable in eutrophic and shallow lakes. Overall, our sediment evidence highlights that agricultural expansion and hydrological regulation could interact in altering the sediment stock of TIC and TOC, and their synchronous variation in Karst landscapes.

Lakes in subtropical regions are highly susceptible to eutrophication due to the heavy rainfall, which causes significant runoff of pollutants (e.g., nutrients) to reach surface waters, altering the water quality and influencing the microbial communities that regulate the biogeochemical cycles within these ecosystems. Lake Cajititlán is a shallow, subtropical, and endorheic lake in western Mexico. Nutrient pollution from agricultural activity and wastewater discharge have affected the lake’s water quality, leading the reservoir to a hypereutrophic state, resulting in episodes of fish mortality during the rainy season. This study investigated the temporal dynamics of bacterial communities within Lake Cajititlán and their genes associated with the nitrogen, phosphorus, sulfur, and carbon biogeochemical cycles during the rainy season, as well as the influences of physicochemical and environmental variables on such dynamics. Significant temporal variations were observed in the composition of bacterial communities, of which Flavobacterium and Pseudomonas were the dominant genera. The climatological parameters that were most correlated with the bacterial communities and their functional profiles were pH, DO, ORP, turbidity, TN, EC, NH4+, and NO3–. The bacterial communities displayed variations in their functional composition for nitrogen, phosphorus, and sulfur metabolisms during the sampling months. The bacterial communities within the lake are highly susceptible to nutrient loads and low DO levels during the rainy season. Bacterial communities had a higher relative abundance of genes associated with denitrification, nitrogen fixation, assimilatory sulfate reduction, cysteine, SOX system, and all phosphorus metabolic pathways. The results obtained here enrich our understanding of the bidirectional interactions between bacterial communities and major biogeochemical processes in eutrophic subtropical lakes.

A Water Quality Index (WQI) is a formulation that enables the estimation of the overall quality of a water body based on significant parameters. One example of this is the well-known and widely accepted NSF-WQI, which is frequently used to assess chemical, physical, and microbiologic features of waterbodies in temperate latitudes. In this work, a well-structured method, completely based on multivariate statistical methods and historical data distributions, was used to develop an ecosystem specific water quality index (ES-WQI). Lake Cajititlán, a subtropical Mexican lake located in Tlajomulco de Zúñiga, was selected as a case of study because it is an endorheic shallow lake that shows signs of high levels of eutrophication due to anthropogenic contamination. As a result of the contamination, and its sensibility to changes in the water level, it undergoes important changes in its water features, such as turbidity and intense green color, and experiences massive events of fish mortality. The proposed ES-WQI describes the changes in water quality over the year well and correlates with the capability of the lake to support aquatic life, as the lowest estimated values coincide with the biggest events of massive fish mortality in the lake. Furthermore, the ES-WQI clearly differentiates between typical cyclic behaviors and actual deteriorating trends and is capable of tracking incremental changes all over the range of the possible concentration values of the water quality parameters. [ABSTRACT FROM AUTHOR]

Temporal and spatial variations of organic and inorganic forms of phosphorus in the sediments of a subtropical oligo-mesotrophic shallow lake were investigated in relation to water column parameters, sediment characteristics and benthic fauna. The main results found were: 1) Phosphorus (P) concentration varied seasonally, with organic P and total P increasing in the sediments in warmer periods; 2) Sediment spatial variation, in terms of grain size composition and organic matter content, influenced P concentrations; 3) quantities and qualities of P in the sediments were positively correlated with water parameters, especially DO and NO3, Chl-a, water temperature, pH, and total P. Negative correlations were observed between %OP and DO in sites 1, 3 and 4, and with NO3 in sites 2, 3 and 4; 4) benthic functional feeding groups showed significant relationships with temporal variation in sediment P concentration, including gathering-collectors, shredders, filterers and filtering-collectors. The results suggest a high importance of temperature mediated control of sediment-P release, both directly, through its direct effects on primary production and decomposition rates, and indirectly through its effects on other water and sediment parameters, especially dissolved oxygen concentration.

Temporal and spatial variations of organic and inorganic forms of phosphorus in the sediments of a subtropical oligo-mesotrophic shallow lake were investigated in relation to water column parameters, sediment characteristics and benthic fauna. The main results found were: i) Phosphorus (P) concentration varied seasonally, with organic P and total P increasing in the sediments in warmer periods; i) Sediment spatial variation, in terms of grain size composition and organic matter content, influenced P concentrations; iii) quantities and qualities of P in the sediments were positively correlated with water parameters, especially DO and NO3, Chl-a, water temperature, pH, and total P. Negative correlations were observed between %OP and DO in sites 1, 3 and 4, and with NO3 in sites 2, 3 and 4; iv) benthic functional feeding groups showed significant relationships with temporal variation in sediment P concentration, including gathering-collectors, shredders, filterers and filtering-collectors. The results suggest a high importance of temperature mediated control of sediment-P release, both directly, through its direct effects on primary production and decomposition rates, and indirectly through its effects on other water and sediment parameters, especially dissolved oxygen concentration. [ABSTRACT FROM AUTHOR]

Submerged macrophyte restoration was used to combat with lake eutrophication and improve water quality in a subtropical, shallow lake. A consistent investigation for three years about macrophyte, phytoplankton, zooplankton and water quality has been conducted to explore the effect of macrophyte restoration on improving aquatic ecosystem. The results showed that macrophyte biomass and coverage after the restoration were as high as 821 g/m 2 and 55%, respectively, and the dominant species were Ceratophyllum demersum , Myriophyllum verticillatum and Potamogeton crispus . Moreover, chemical oxygen demand (COD), total nitrogen (TN), total suspended solids (TSS) and nitrate nitrogen (NN) significantly decreased yearly, but the changes in total phosphorus (TP) and ammonia nitrogen (AN) were not significant during the investigation. Phytoplankton density and biomass significantly decreased, and the dominant species changed from blue-green algae to green algae. Zooplankton density and biomass, however, significantly increased yearly, and the small-sized dominant species (such as, Keratella cochlearis , Polyarthra trigla , etc.) were gradually replaced by the large-sized species (such as, Asplanchna priodonala , Alona guttata , etc.) during the restoration. In addition, macrophyte was negatively correlated with nutrients, but positively correlated with zooplankton. Phytoplankton was also positively related with TN. These correlations showed that restored macrophytes could decrease nutrition loading in water body, inhibit phytoplankton growth to decrease the risk of algae blooms, and support more large-sized zooplankton to strengthen the top-down control of phytoplankton. Therefore, our results showed that macrophyte restoration is a very potential and effective method to combat with the eutrophication and develop water quality in the eutrophic subtropical lakes, and deserves more attentions in future subtropical/tropical lake managements. [ABSTRACT FROM AUTHOR]

Hydrogen sulfide (H 2 S) is an important environmental toxin with bi-directional biological effects on organisms. In natural waters, H 2 S complexes with heavy metal ions in an anaerobic environment influence heavy metals' bioavailability and induce phosphorus release and eutrophication in water columns. Traditional detection techniques, such as colorimetric, electrochemical, and chromatographic, cannot simultaneously detect H 2 S and pollution assessment of subtropical lakes. To address these technical defects, we developed small-molecule fluorescent probes to evaluate the pollution level in natural water bodies. This method relies on the combination of the probes' response signals to raw water and the water quality index, thereby enhancing the accuracy and reliability of water quality assessments. Furthermore, this novel material has a large Stokes shift. It can detect complex levels of H 2 S concentrations in natural water bodies by correlating the degree of contamination and fluorescence signals. The development of this visual research tool for detecting environmental H 2 S levels in natural water bodies is expected to have meaningful, practical applications. [Display omitted] • RDM-TPE could emit different fluorescent signals in the subtropical lake. • This fluorescence technology was capable of evaluating pollution level in natural water. • Pollution level was evaluated by the combination of the fluorescence signals and the WQI index. • Traditional fluorescence probes could not achieve pollution assessment in subtropical lakes. [ABSTRACT FROM AUTHOR]

Temporal variability in some water quality parameters can play an important role in determining the presence and abundance of primary producers, and consequently in the trophic state and other characteristics and uses of lake ecosystems. In this sense, the present study aimed at understanding temporal dynamics of some trophic relevant water quality parameters in different time scales and their correlation and influence in phytoplankton biomass (chlorophyll a) in a shallow subtropical coastal lake. Peri Lake is located in Florianópolis island in Southern Brazil and samples were taken monthly between March 2007 and February 2013. The lake showed low dissolved nutrients concentration, especially phosphorus (P) (median dissolved P: 2.0 µg.l-1) and high chlorophyll a (median: 20.8 µg.l-1) concentration. Total nitrogen (TN) concentration varied broadly, with a median of 672.8 µg L-1, and total P (TP) concentration was low (median: 13.5 µg L-1). A seasonal pattern of variation concerning dissolved and total P and chlorophyll a concentration was observed, associated mainly with temperature and wind speeds, but no clear pattern was observed for nitrogen (N) fractions. Significant differences were observed in different years for some parameters, with higher chlorophyll a and lower N concentration in the last three years sampled. The lake was considered potentially P limited during the majority of the study period and a positive correlation was found between chlorophyll a and total and dissolved P concentration. Phytoplankton biomass (as chlorophyll a) was apparently controlled by water temperature and P availability (TN:TP ratio and dissolved P). Water transparency (as Secchi depth) was strongly and negatively influenced by chlorophyll a concentration. Cylindrospermopsis raciborskii abilities to compete for P and light seem to be important factors determining its success and dominance in this low P coastal ecosystem. The fluctuating P supply, probably associated to sediment resuspension by wind in this shallow waterbody, is an advantageous factor for cyanobacteria and has an important role in chlorophyll a dynamics. Thus, high chlorophyll a concentration in this subtropical lake seems to be related to the P-limited condition, shallowness and low water column transparency, which are probably favouring the dominance of C. raciborskii, especially in higher summer temperatures, and leading to high chlorophyll a concentration even in a low dissolved nutrient environment.

In order to verify the occurrence of invetebrates associated with macro clusters of Limnoperna fortunei (Dunker, 1857) formed on branches of “sarandi” (Chephalanthus glabratus (Spreng.) K. Schum), quantitative samplings (N=28) were conducted for two years (2002 to 2004) at Veludo Beach on Guaíba Lake (municipality of Porto Alegre, RS, Brazil). From the results, the gastropod Heleobia piscium (Orbigny, 1835) was identified as a constant (78.57%) species, while Potamolithus jacuyensis Pilsbry 1899 (35.71%) and the crustaceous Hyalella curvispina Shoemaker 1942 (26%) were indicated as accessory species. The other taxa were accidental (

Seasonal patterns in factors that affect primary producers are an important part of defining the structure and function of aquatic ecosystems. However, defining seasonality is often more difficult in aquatic than in terrestrial ecosystems, particularly in subtropical and tropical environments. In this study, a long-term data set for a shallow subtropical lake (Lake George, Florida, USA) was used to investigate seasonality using a range of physical, chemical and hydrological parameters. K-means cluster analysis of monthly averages among 11 selected environmental factors across 18 years suggested the overall annual pattern consists of three different seasonal clusters: a cold season (January–April), a warm season (May–August) and a flushing season (September–December). High dissolved oxygen and increased Secchi depth are key features of the cold season, while the warm season is characterized by high mean light irradiances, temperature, rainfalls, total nitrogen and phytoplankton biomass (as chlorophyll a level). The flushing season is characterized by high river discharge rates and high levels of dissolved nutrients and colored organic matter. Multiple response permutation procedures indicated that these seasonal cluster arrangements were significantly different than randomly permuted clusters ( A -statistics = 0.3314, significance of delta = 0.0160, based on 1000 permutations). Results from principal component analyses supported the presence of the three seasons in the lake. Linear models explaining chlorophyll a levels using the 3-season system generally indicated better ratios of explained variance compared to the models without seasonal alignments, further indicating that even for sub-tropical systems defining seasons provides a better understanding of phytoplankton dynamics. The approaches used in this study provide statistically-based multivariate tools for the definition of seasonality in aquatic ecosystems. The ability to accurately define seasons is a key step in modeling the structure and dynamics of aquatic ecosystem, which is essential to the development of effective management strategies in a rapidly changing world. [ABSTRACT FROM AUTHOR]

Temporal variability in some water quality parameters can play an important role in determining the presence and abundance of primary producers, and consequently in the trophic state and other characteristics and uses of lake ecosystems. In this sense, the present study aimed at understanding temporal dynamics of some trophic relevant water quality parameters in different time scales and their correlation and influence in phytoplankton biomass (chlorophyll a) in a shallow subtropical coastal lake. Peri Lake is located in Florianópolis island in Southern Brazil and samples were taken monthly between March 2007 and February 2013. The lake showed low dissolved nutrients concentration, especially phosphorus (P) (median dissolved P: 2.0 μg L-1) and high chlorophyll a (median: 20.8 μg L-1) concentration. Total nitrogen (TN) concentration varied broadly, with a median of 672.8 μg L-1, and total P (TP) concentration was low (median: 13.5 μg L-1). A seasonal pattern of variation concerning dissolved and total P and chlorophyll a concentration was observed, associated mainly with temperature and wind speeds, but no clear pattern was observed for nitrogen (N) fractions. Significant differences were observed in different years for some parameters, with higher chlorophyll a and lower N concentration in the last three years sampled. The lake was considered potentially P limited during the majority of the study period and a positive correlation was found between chlorophyll a and total and dissolved P concentration. Phytoplankton biomass (as chlorophyll a) was apparently controlled by water temperature and P availability (TN:TP ratio and dissolved P). Water transparency (as Secchi depth) was strongly and negatively influenced by chlorophyll a concentration. Cylindrospermopsis raciborskii abilities to compete for P and light seem to be important factors determining its success and dominance in this low P coastal ecosystem. The fluctuating P supply, probably associated to sediment resuspension by wind in this shallow waterbody, is an advantageous factor for cyanobacteria and has an important role in chlorophyll a dynamics. Thus, high chlorophyll a concentration in this subtropical lake seems to be related to the P-limited condition, shallowness and low water column transparency, which are probably favouring the dominance of C. raciborskii, especially in higher summer temperatures, and leading to high chlorophyll a concentration even in a low dissolved nutrient environment. [ABSTRACT FROM AUTHOR]

To assess the effectiveness of aquaculture cessation for improving water quality, we analysed the response of the phytoplankton community to the cessation of aquaculture in Lake Dianshan, Shanghai, China. Quantitative analyses of phytoplankton and nutrient content were undertaken monthly from 2004 through 2008. After the cessation of aquaculture, no significant changes in nutrient concentration were detected. Nevertheless, the phytoplankton community changed greatly after the cessation. Firstly, the phytoplankton species richness as well as the Shannon-Wiener diversity index increased. Secondly, the chlorophylla concentration and the phytoplankton density declined sharply. This decrease was caused primarily by a decrease in Cyanophyta. Additionally, the Chlorophyta, Bacillariophyta and Cryptophyta increased, leading to a decrease in the relative biomass of Cyanophyta. Thirdly, the dominant species shifted from the toxic cyanophyte Microcystis sp. to less harmful Cyanophyta such as Merismopedia sp. and Chroococcus sp., along with Chlorophyta, Bacillariophyta and Cryptophyta. The phytoplankton community composition improved, indicating that the overall water quality was improved after the cessation. However, the changes in the phytoplankton community were more likely to be associated with the physical and biotic changes caused by the removal of aquaculture than with the variation in nutrient levels. Our study indicated that phytoplankton monitoring, especially in summer, is essential for assessing the water quality because an overall improvement in water quality may also be achieved without significant changes in nutrient concentrations. The rebounding of Cyanophyta in 2008 highlighted the importance of controlling nutrient loading by inflow for a substantial improvement in water quality. These results are relevant for managing subtropical eutrophic lakes. [ABSTRACT FROM AUTHOR]

Excess loading of phosphorus (P) and nitrogen (N) triggers a shift in the trophic structure of shallow lakes from a clear-water, macrophyte-dominated state to an algal-dominated turbid state. However, the role of N in the shift is debated, and experimental evidence is, with a few exceptions, based on short-term studies (days to a few months)., We studied the effect of N loading on macrophytes (dominated by Potamogeton lucens and Cabomba caroliniana), periphyton, filamentous algae and phytoplankton in mesocosms over 10 months (starting in October) in subtropical China (Wuhan). There were three N treatments: controls ( CN) without nitrogen addition (mean TN = 1.9 mg L−1), low nitrogen ( LN) addition (mean TN = 3.5 mg L−1) and high nitrogen ( HN) addition (mean TN = 5.5 mg L−1). Total phosphorus ( TP) concentration in the water column remained moderate (0.05-0.07 mg L−1) during the experiment in all treatments., Macrophyte abundance declined in the LN and HN treatments in the first 6 months, but not in controls, followed by a partial recovery in the LN treatments. They disappeared completely in the HN treatments the following summer. Periphyton (biofilm on plastic) and phytoplankton biomass remained unaffected during the first 6 months but increased over the summer by two or three times, compared with controls, in low and high nitrogen treatments, respectively. By contrast, the abundance of filamentous algae increased over winter but declined during the summer with no obvious relationship to the N treatments. There was no difference in the TN or nitrate concentrations or soluble protein, soluble sugar and Chl-a content of P. lucens leaves and stems with increasing N load., Macrophyte populations are partially resilient to abrupt increases in N loading at moderate TP concentrations, but, after prolonged exposure, a complete collapse occurs. Our results further indicate that macrophyte loss is exacerbated by shading by filamentous algae during the winter, and by phytoplankton and periphyton in the summer, while there was no indication of direct N toxicity. [ABSTRACT FROM AUTHOR]

To understand the dynamics of planktonic prokaryotes in a subtropical lake and its relationship with carbon, we conducted water sampling through four 48 h periods in Peri Lake for one year. Planktonic prokaryotes were characterized by the abundance and biomass of heterotrophic bacteria and of cyanobacteria (coccoid and filamentous cells). During all experiments, we measured wind speed, water temperature (WT), pH, dissolved oxygen (DO), precipitation, dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and carbon dioxide (CO2). DOC was higher in the summer experiment (average = 465 μM - WT = 27°C) and lower in the winter experiment (average = 235 μM – WT = 17°C), with no significant variability throughout the daily cycles. CO2 concentrations presented a different pattern, with a minimum in the warm waters of the summer period (8.31 μM) and a maximum in the spring (37.13 μM). Daily trends were observed for pH, DO, WT, and CO2. At an annual scale, both biological and physical-chemical controls were important regulators of CO2. Heterotrophic bacteria abundance and biomass were higher in the winter experiment (5.60 x 109 cells L-1 and 20.83 μmol C L-1) and lower in the summer (1.87 x 109 cells L-1 and 3.95 μmol C L-1). Filamentous cyanobacteria (0.23 x 108 – 0.68 x 108 filaments L-1) produced up to 167.16 μmol C L-1 as biomass (during the warmer period), whereas coccoid cyanobacteria contributed only 0.38 μmol C L-1. Precipitation, temperature, and the biomass of heterotrophic bacteria were the main regulators of CO2 concentrations. Temperature had a negative effect on the concentration of CO2, which may be indirectly attributed to high heterotroph activity in the autumn and winter periods. DOC was positively correlated with the abundance of total cyanobacteria and negatively with heterotrophic bacteria. Thus, planktonic prokaryotes have played an important role in the dynamics of both dissolved inorganic and organic carbon in the lake.

• Using biomanipulation to restore a eutrophic highland lake in subtropical China was tested. • Fish removal led to increased abundance of cladoceran (Daphnia spp., especially Bosmina spp.). • Enhanced top-down control was found leading to lower phytoplankton and cyanobacterial biomass. • Fish removal may be a feasible additional restoration tool to nutrient loading reduction such lakes. Removal of planktivorous fish is used extensively in northern temperate lakes to reduce phytoplankton abundance via enhanced zooplankton grazing. However, whether this method would work also in large subtropical highland lakes to alleviate cyanobacterial blooms is unknown. We conducted a one-year pilot in situ experiment where we removed a substantial biomass of fish in a fenced-in area, followed by a 3-year whole-lake experiment where the dominant fish species (Japanese smelt) was removed in Lake Erhai in southwest China. The fencing experiments showed that between July and November, when the biomass of the removed stock reached 4 g/m2, the zooplankton biomass inside the fence increased significantly compared to a control fence. In the full-lake experiment, we found that sustained removal of Japanese smelt led to an increase in the biomass of cladocerans (Daphnia spp. but especially of Bosmina spp.) and a significant decrease in the biomass of Cyanobacteria and Chlorophyta. Additionally, a marked increase in the ratio of zooplankton to phytoplankton biomass, as well as an increase in the body size of cladocerans, emphasising the importance of enhanced top-down control for mitigating cyanobacterial blooms following extensive fish removal. Our results reveal that removal of small fish (here Japanese smelt) can lead to a reduction of the phytoplankton and cyanobacteria biomass through a trophic cascade in highland deep subtropical lakes. Thus fish removal may be a feasible additional restoration tool to external nutrient loading reduction in such lakes. [Display omitted] [ABSTRACT FROM AUTHOR]

Enclosure lake aquaculture causes lake eutrophication and emits CH 4 to the atmosphere. So far, little is known about the rate of CH 4 emission from lake aquaculture and about how ecological restoration (or aquaculture abandonment) affects the emission. In this study, the eddy covariance (EC) technique was deployed to quantify the CH 4 flux in an enclosure lake aquaculture farm and to investigate the flux response to ecological restoration. The lake site under aquaculture farming emitted 36.0 g C-CH 4 m−2 yr−1 to the atmosphere, an amount that is comparable to the global mean value of semi-intensive aquaculture systems. The annual CH 4 emission decreased from the pre-restoration level by 34% and 37% to 23.7 and 22.8 g C-CH 4 m−2 yr−1 in the first and second year of ecological restoration, respectively, but was still much higher than that at a reference lake site not impacted by aquaculture farming (6.12 g C-CH 4 m−2 yr−1). The high emission values after aquaculture abandonment suggest that aquafeed input in the decades of farming may have caused accumulation of a large amount of organic carbon in the sediment that continues to fuel CH 4 production and that it may take a long time for the system to recover to a natural state. After aquaculture abandonment, floating-leaved plants expanded rapidly within the EC flux footprint, resulting in a high net ecosystem productivity (NEP) in the growing season. These plants appeared to be able to transport CH 4 to the atmosphere through aerenchyma tissues and stomata. • The enclosure aquaculture farm was a large source of CH 4. • The annual CH 4 emission decreased by 36% in year 1 and 2 after aquaculture abandonment. • The CH 4 flux response to sediment temperature was asymmetric throughout the year. • Floating-leaved plants appeared to be able to transport CH 4 to the atmosphere. [ABSTRACT FROM AUTHOR]

While numerous reports exist on the results of lake restoration initiatives in temperate regions, only a few exist from subtropical lakes. We present results of the lake restoration of shallow, subtropical Lake Wuli, China, conducted between 1999 and 2010. After restoration, annual average concentrations of total nitrogen, total phosphorus (TP), and chlorophyll a and the chemical oxygen demand declined significantly, though summer TP remained high. Suspended solids increased significantly over the years, whereas transparency decreased, though not significantly so. The contribution of cryptophytes to total phytoplankton biomass decreased, while the proportion of cyanobacteria, especially potentially N-fixing species, increased. Rotifers were superseded by crustaceans as the dominant taxon of the zooplankton community. Enhanced abundance of Daphnia spp., appearance of Leptodora kindti, and increased biomass ratios of zooplankton to phytoplankton, calanoids to cyclopoids, and nauplii to copepods in the post-restoration period indicate reduced fish predation and stronger top-down control of phytoplankton. However, the increase in non-algal turbidity, probably caused by the higher biomass of benthivorous fish, apparently prevented the re-establishment of submerged macrophyte communities. We conclude that removal of fish, particularly benthivorous species, will further improve water quality in this and other subtropical shallow lakes. [ABSTRACT FROM AUTHOR]

An enclosure experiment was conducted in July-September 2001 in subtropical eutrophic Lake Donghu (China) to test a hypothesis that a moderate cyanobacterial biomass would have a positive effect on small-sized cladocerans. Eight enclosures (12.5 m) were arranged with different nutrient concentrations using the lake water, tap water, and sediment from Lake Donghu. Microcystis blooms appeared in enclosures with higher nutrient concentrations and the average fresh weight biomass of Microcystis spp. ranged from 4.6 to 30.4 mg l during the bloom period. Three cladocerans ( Moina micrura, Diaphanosoma brachyurum, and Ceriodaphnia cornuta) and two cyclopoids ( Mesocyclops dissimilis and Thermocyclops taihokuensis) dominated the crustacean plankton community during the experimental period. The C. cornuta biomass constituted the greatest percentage (55.9-90.0%) of cladoceran biomass in the Microcystis bloom treatments. When the Microcystis biomass increased, the average biomass of C. cornuta increased and the biomass of M. micrura and D. brachyurum decreased, whereas the cyclopoid biomass did not change significantly. The total biomass of cladoceran and crustacean plankton were significantly positively correlated with the Microcystis biomass. Our results indicate that a moderate biomass of Microcystis spp. can favor crustacean plankton to some extent and, furthermore, may impact food web structures in a eutrophic lake. [ABSTRACT FROM AUTHOR]

Vertical mixing affects the vertical transport of biochemical materials and modifies the characteristics of basin-scale internal waves in subtropical, subalpine Yang Yuan Lake (YYL), in the north central region of Taiwan. Vertical mixing in YYL is generally caused by typhoons (strong winds and heavy rainfalls) during the spring, summer and early fall, or by cooling of the water column during late fall and winter. Vertical mixing caused by typhoons significantly affects internal thermal dynamics and biochemical processes, with basin-scale internal waves enhanced before complete mixing occurs, and temporal variations of water temperature dramatically changing after typhoon events. This study quantitatively determines when the complete mixing (defined as homogenous temperature in a water column) occurs on the basis of the balance between kinetic energy and potential energy and also shows what meteorological conditions contribute to the vertical mixing. Data on water column thermal profiles and meteorological variables were collected by a wireless, instrumented buoy in the deepest location of YYL and from a nearby meteorological station, from spring 2004 to summer 2006. This study also investigated how a physical process (internal waves) is affected by the mixing associated with typhoons. Signal processing and a two-layer model of the water column are used to understand the characteristics of basin-scale internal waves. [ABSTRACT FROM AUTHOR]

Abstract: In many aquatic ecosystems, increased nutrient loading has caused eutrophication, which is reflected in the trophic structure of the ecosystem. In Lake Mangueira, a large shallow subtropical lake in Brazil, nutrient loading has also increased, but it is still unclear what the effects of this increase will be and how this relates to climate change. To evaluate the effects of increased nutrient loadings in such large lake one would need to integrate hydrological and ecological processes into one model, an approach that has rarely been used before. Here, we apply different versions of a complex 3D ecological model, called IPH-TRIM3D-PCLake, which describes the integrated hydrodynamic, water-quality, and biological processes in the lake. First, the nutrient loadings from the watershed were estimated using a separate hydrological water quality model of the watershed based on field data. Second, we calibrated the 3D ecological model for a 6-year monitoring period in the lake using a simplified non-spatial version of the model. Finally, the calibrated ecological model was applied to evaluate the spatial explicit effects of different scenarios of land use, water pumping for irrigation, and climate change. On short term (1.5 year), the system seemed to be rather resilient, probably because of the lake size related to its high inertia. Our simulations indicated warming can increase water transparency in Lake Mangueira which may be related to two factors: (a) the current meso-oligotrophic state of the lake which may easily lead to nutrient limitation; and (b) submerged macrophytes grow during the whole season. The combined effect of climate change and increased nutrient loading, less strong than increased nutrient loading alone. The model can only be used for qualitative predictions of the effect of management scenarios, such as maintenance of water levels in the dry season, and water-pumping rules for irrigation in order to maintain the ecosystem structure and functions in the future under additional stress caused by increased use or climate change. [Copyright &y& Elsevier]

Coastal lagoons are ecologically and economically important environments but a relative low number of studies were carried out in subtropical and permanently closed coastal lagoons. The present study aimed at assessing the temporal and spatial dynamic of trophic relevant water quality parameters in the small, deep and freshwater Peri coastal lagoon, South Brazil. During the 19 sampled months (March/2008-September/2009) spatial homogeneity (horizontal and vertical) was registered in all seasons for all variables, a condition related to the strong wind influence and low human occupation in the lagoon watershed. Seasonal variations of the water quality could be observed and they can be explained mainly by variation on temperature, wind forces and direction and rainfall, characteristic from the subtropical weather. Comparing this study with two others conducted in Peri lagoon in 1996 and 1998, no critical differences that evidence alteration in the water quality were found, but climate differences may have influenced in some small variations observed. The use of four trophic state indices indicated that indices designed for temperate lakes are inappropriate for the subtropical Peri lagoon. The lagoon was classified as oligotrophic for nutrients concentrations and meso-eutrophic for transparency and chlorophyll-a, which can be explained by the high densities and monodominance of the cyanobacterium Cylindrospermopsis raciborskii and the high recycling rates observed in warmer water bodies, when compared to the temperate ones. [ABSTRACT FROM AUTHOR]

The purposes of this study were to assess if Lake Apopka (FL, USA) was autotrophic or heterotrophic based on the partial pressure of dissolved carbon dioxide ( pCO) in the surface water and to evaluate factors that influence the long-term changes in pCO. Monthly average pH, alkalinity and other limnological variables collected between 1987 and 2006 were used to estimate dissolved inorganic carbon (DIC), pCO and CO flux between surface water and atmosphere. Results indicated that average pCO in the surface water was 196 μatm, well below the atmospheric pCO. Direct measurements of DIC concentration on three sampling dates in 2009 also supported pCO undersaturation in Lake Apopka. Supersaturation in CO occurred in this lake in only 13% of the samples from the 20-year record. The surface-water pCO was inversely related to Chl a concentrations. Average annual CO flux was 28.2 g C m year from the atmosphere to the lake water and correlated significantly with Chl a concentration, indicating that biological carbon sequestration led to the low dissolved CO concentration. Low pCO and high invasion rates of atmospheric CO in Lake Apopka indicated persistent autotrophy. High rates of nutrient loading and primary production, a high buffering capacity, a lack of allochthonous loading of organic matter, and the dominance of a planktivorous-benthivorous fish food web have supported long-term net autotrophy in this shallow subtropical eutrophic lake. Our results also showed that lake restoration by the means of nutrient reduction resulted in significantly lower total phosphorus (TP) and Chl a concentrations, and higher pCO. [ABSTRACT FROM AUTHOR]

1. We used high-frequency in situ dissolved oxygen measurements to investigate the seasonal variability and factors regulating metabolism in a subtropical alpine lake in Taiwan between May 2004 and October 2005, specifically exploring how the typhoon season (from June or July to October) affects lake metabolism. 2. Gross primary production (GPP) and ecosystem respiration ( R) both peaked in early summer and mid-autumn but dropped during the typhoon season and winter. Yuan-Yang Lake is a net heterotrophic ecosystem (annual mean net ecosystem production −39.6 μmole O2 m−3). 3. Compared to the summer peaks, seasonal averages of GPP and R decreased by approximately 50% and 25%, respectively, during the typhoon season. Ecosystem respiration was more resistant to external disturbances than GPP and showed strong daily variation during typhoon seasons. 4. Changes in the quality and quantity of dissolved organic carbon controlled the temporal dynamics and metabolic regulation. External disturbances (typhoons) caused increased allochthony, increasing DOC and water colour and influencing lake metabolism. 5. Seasonal winter mixing and typhoon-induced water mixing in summer and autumn play a key role in determining the extent to which the lake is a seasonal carbon sink or source to the atmosphere. [ABSTRACT FROM AUTHOR]

1. Structural complexity may stabilise predator–prey interactions and affect the outcome of trophic cascades by providing prey refuges. In deep lakes, vulnerable zooplankton move vertically to avoid fish predation. In contrast, submerged plants often provide a diel refuge against fish predation for large-bodied zooplankton in shallow temperate lakes, with consequences for the whole ecosystem. 2. To test the extent to which macrophytes serve as refuges for zooplankton in temperate and subtropical lakes, we introduced artificial plant beds into the littoral area of five pairs of shallow lakes in Uruguay (30°–35°S) and Denmark (55°–57°N). We used plants of different architecture (submerged and free-floating) along a gradient of turbidity over which the lakes were paired. 3. We found remarkable differences in the structure (taxon-richness at the genus level, composition and density) of the zooplankton communities in the littoral area between climate zones. Richer communities of larger-bodied taxa (frequently including Daphnia spp.) occurred in the temperate lakes, whereas small-bodied taxa characterised the subtropical lakes. More genera and a higher density of benthic/plant-associated cladocerans also occurred in the temperate lakes. The density of all crustaceans, except calanoid copepods, was significantly higher in the temperate lakes ( c. 5.5-fold higher). 4. Fish and shrimps (genus Palaemonetes) seemed to exert a stronger predation pressure on zooplankton in the plant beds in the subtropical lakes, while the pelagic invertebrate Chaoborus sp. was slightly more abundant than in the temperate lakes. In contrast, plant-associated predatory macroinvertebrates were eight times more abundant in the temperate than in the subtropical lakes. 5. The artificial submerged plants hosted significantly more cladocerans than the free-floating plants, which were particularly avoided in the subtropical lakes. Patterns indicating diel horizontal migration were frequently observed for both overall zooplankton density and individual taxa in the temperate, but not the subtropical, lakes. In contrast, patterns of diel vertical migration prevailed for both the overall zooplankton and for most individual taxa in the subtropics, irrespective of water turbidity. 6. Higher fish predation probably shapes the general structure and dynamics of cladoceran communities in the subtropical lakes. Our results support the hypothesis that horizontal migration is less prevalent in the subtropics than in temperate lakes, and that no predator-avoidance behaviour effectively counteracts predation pressure in the subtropics. Positive effects of aquatic plants on water transparency, via their acting as a refuge for zooplankton, may be generally weak or rare in warm lakes. [ABSTRACT FROM AUTHOR]

The macrozoobenthic community of Lake Naukuchiyatal, which has gradually turned eutrophic during the past 30 years, was investigated for the period January, 2000 to December, 2001. Among the 21 species collected during the study, Diptera were numerically most significant whereas in terms of biomass the gastropods dominated the community. Numerically, the most important taxa were: Chironomus plumosus, Glossiphonia weberi, Mesostoma sp., Tubifex tubifex and Dugesia sp. Gravimetrically, the most dominant species included Lymnaea acuminata, Chironomus plumosus, G. weberi, Barbronia weberi and Gyraulus convexiusculus. The community abundance was characterized by a single peak, which coincided with the phytoplankton peak. Both the community abundance and biomass revealed that the macrobenthic fauna was unaffected by eutrophication. [ABSTRACT FROM AUTHOR]

Abstract: Up to now, there have been few studies in the annual fluxes of greenhouse gases in lakes of subtropical regions. The fluxes of methane (CH4) and carbon dioxide (CO2) across air–water interface were measured in a shallow, hypereutrophic, subtropical Lake Donghu (China) over a year cycle, using a static chamber technique. During the year, Lake Donghu emitted CH4 and CO2; the average flux of CH4 and CO2 was 23.3±18.6 and 332.3±160.1mgm−2 d−1, respectively. The fluxes of CH4 and CO2 showed strong seasonal dynamics: CH4 emission rate was highest in summer, remaining low in other seasons, whereas CO2 was adsorbed from the atmosphere in spring and summer, but exhibited a large emission in winter. Annual carbon (C) budget across air–water interface in Lake Donghu was estimated to be 7.52±4.07×108 g. CH4 emission was correlated positively with net primary production (NPP) and temperature, whereas CO2 flux correlated negatively with NPP and temperature; however, there were no significant relationships between the fluxes of CH4 and CO2 and dissolved organic carbon, a significant difference from boreal lakes, indicating that phytoplankton rather than allochthonous matter regulated C dynamics across air–water interface of subtropical lake enriched nutrient content. [Copyright &y& Elsevier]

Two separate studies measured the biomass of benthic charophytes in Lake Okeechobee, a large subtropical lake located in south-central Florida, USA. In one study, five stations in the southern region of the lake were sampled monthly or bimonthly over a 6 year period (1994–1999). A distinct seasonal pattern was observed, with maximum biomass occurring in summer and early fall. Charophyte abundance was positively related to irradiance and negatively related to water depth. A second study took place in 1999–2000, before and after a managed lake drawdown in April, 2000. This whole-ecosystem experiment was designed to lower lake stage by ∼12.5% (0.6 m). We compared charophyte biomass at 42 stations throughout the lake prior to and after this managed recession and evaluated the influence of various environmental variables on growth. This analysis indicated that the recession resulted in increased transparency/bottom irradiance and greater charophyte biomass at nearshore stations. The data are consistent with prior studies, both in Lake Okeechobee and other lakes around the world, which indicate charophyte growth and abundance are strongly influenced by irradiance. We hypothesize that a positive feedback loop resulted in Lake Okeechobee when water levels were lowered, whereby shallower water allowed greater light penetration to the bottom, resulting in increased growth of submerged charophytes. Chara, in turn, probably stabilized sediments, leading to greater transparency and a further improvement of environmental conditions for plant recruitment and growth. [Copyright &y& Elsevier]

The contribution of metazoan zooplankton to internal phosphorus (P) cycling was investigated in Meiliang and Wuli Bays of Lake Taihu, a eutrophic, shallow lake in China by determining biomass and biomass specific P release rates. The zooplankton community of eutrophic Meiliang Bay was dominated by cladocerans, particularly Daphnia spp. and copepods, while dominant groups in hypereutrophic Wuli Bay included rotifers, copepods, and cladocerans. Release rates of PO43--P ranged from 0.20 to 0.57 mg·g (dry weight)-1·h-1 in Meiliang Bay and 0.20 to 0.76 mg·g-1·h-1 in Wuli Bay. In most cases, P release rates were higher in Wuli Bay than Meiliang Bay. Phosphorus fluxes from zooplankton excretion varied from 5.34 to 57.41 mg·m-2·d-1 in Meiliang Bay and 8.20 to 70.02 mg·m-2·d-1 in Wuli Bay. Since P released by zooplankton in this study was in a form available to phytoplankton, zooplankton may represent a significant source of P contributing to high phytoplankton biomass in Lake Taihu.