Subtropical Eutrophic Lake Research Articles

Historical trajectories of antibiotics resistance genes assessed through sedimentary DNA analysis of a subtropical eutrophic lake

Investigating the occurrence of antibiotic-resistance genes (ARGs) in sedimentary archives provides opportunities for reconstructing the distribution and dissemination of historical (i.e., non-anthropogenic origin) ARGs. Although ARGs in freshwater environments have attracted great attention, historical variations in the diversity and abundance of ARGs over centuries to millennia remain largely unknown. In this study, we investigated the vertical change patterns of bacterial communities, ARGs and mobile genetic elements (MGEs) found in sediments of Lake Chenghai spanning the past 600 years. Within resistome preserved in sediments, 177 ARGs subtypes were found with aminoglycosides and multidrug resistance being the most abundant. The ARG abundance in the upper sediment layers (equivalent to the post-antibiotic era since the 1940s) was lower than those during the pre-antibiotic era, whereas the ARG diversity was higher during the post-antibiotic era, possibly because human-induced lake eutrophication over the recent decades facilitated the spread and proliferation of drug-resistant bacteria. Statistical analysis suggested that MGEs abundance and the bacterial community structure were significantly correlated with the abundance and diversity of ARGs, suggesting that the occurrence and distribution of ARGs may be transferred between different bacteria by MGEs. Our results provide new perspectives on the natural history of ARGs in freshwater environments and are essential for understanding the temporal dynamics and dissemination of ARGs.

Effects of extreme rainfall events on phytoplankton community in a subtropical eutrophic lake: a mesocosm experiment

The impact of global climate change has led to an increase in extreme rainfall events, causing fluctuations in terrigenic inputs that significantly affect aquatic communities. Lake Changhu, the third-largest freshwater lake in Hubei Province, plays a crucial role as a reservoir of aquatic germplasm resources in the middle reaches of the Yangtze River. In recent years, the lake has experienced a series of extreme rainfall events. In response to recent extreme rainfall events, a 42-day mesocosm experiment was conducted to understand the effect of terrigenic inputs on various aspects of phytoplankton in Lake Changhu, such as species composition, abundance, biomass, diversity, community turnover rates, resource use efficiency, and stability. The experiment involved the application of different terrigenic treatments, including a control group (CK, using lake water), low terrigenic input (LT), medium terrigenic input (MT), and high terrigenic input (HT). The results showed a noticeable shift in phytoplankton composition from a co-dominated state of Chlorophyta and Cyanobacteria in the CK treatment to a Cyanobacteria-dominated state in the other terrigenic treatments. Furthermore, the terrigenic inputs increased phytoplankton abundance, community turnover rates, diversity, and resistance. Comparatively, the diversity index of phytoplankton increased by 82.61%, 73.83%, and 70.41% in the LT, MT, and HT treatments, respectively, in contrast to the CK treatment. However, phytoplankton abundance decreased by 6.99%, 15.55%, and 14.76% in the LT, MT, and HT treatments. Additionally, the resource use efficiency decreased by 1.94%, 5.16%, and 14.19% in the LT, MT, and HT treatments, respectively, compared to the CK treatment. These findings provide valuable insights into monitoring and managing the water ecology in Lake Changhu, offering a scientific basis for implementing effective management strategies.

Macroinvertebrate and environmental responses to dredging and submerged macrophytes transplantation

Abstract Eutrophication of freshwater ecosystems is a global problem, but restoration can be difficult due to ongoing problems relating to water pollution, sedimentary nutrient stores and altered aquatic biodiversity. Mitigation of Eutrophication is often conducted alongside transplantation of submerged macrophytes and dredging, but knowledge of ecosystem response to post‐dredging transplantation of submerged macrophytes is limited. We report a long‐term (2008–2018) in situ monitoring study to evaluate the effects of different restoration measures: dredging only (Dredged) and dredged with post‐transplantation of submerged macrophytes (Dredged with macrophytes) conducted in five subtropical eutrophic lakes, Taihu basin. Water and sediment nutrients, bloom‐forming algae Microcystis and macroinvertebrate were monitored every 2 years for each treatment and compared with reference areas (Control) established in unrestored parts of the same lake. Dredging only decreased sediment nutrients (e.g. carbon, phosphorus) significantly; however, this effect diminished about 5 years later. Dredged with macrophytes had a stronger, longer lasting positive effect on water quality than dredged alone. Disturbance caused by dredging (without macrophytes transplantation) decreased the biomass of Microcystis, while transplantation of macrophytes shortly after dredging did not contribute to the decreasing of Microcystis biomass. The biomass of Microcystis in Dredged with macrophytes areas was always similar to Control over the period of our monitoring. A positive effect of submerged macrophytes transplantation post‐dredging was found for macroinvertebrate abundance and diversity: Dredged with macrophytes areas had significantly higher macroinvertebrate biomass and richness than Dredged areas after 9 years' recovery. Macroinvertebrate richness in Dredged with macrophytes areas nearly doubled compared to Control, while Dredged areas were just restored to Control levels. Synthesis and applications: Our study provides an long‐term field monitoring with new findings about the benefits and caution of submerged macrophytes transplantation post‐dredging and the effect of partial restoration, which could inform eutrophic waterbody restoration schemes.

Purification Efficiency of Two Ecotypes of Wetland Plants on Subtropical Eutrophic Lakes in China

Purification Efficiency of Two Ecotypes of Wetland Plants on Subtropical Eutrophic Lakes in China

Dissolved organic matter and Fe/Mn enhance the combination and transformation of As in Lake Chaohu Basin

The phenomenon of algal blooms resulting from lake eutrophication has the potential to increase the concentration of dissolved organic matter (DOM) and consequently influence the environmental behaviour of arsenic (As). In the subtropical region, the interplay between DOM, Fe/Mn and As becomes complex as Fe/Mn-rich substances from soils and sediments enter eutrophic lakes. The mechanisms by which DOM-Fe/Mn interactions affect the transformation of As species remain uncertain. Therefore, the Chaohu Lake Basin was selected as a representative case study site to investigate the levels of DOM, As, Fe and Mn in the water and to establish their associations. In addition, the interaction mechanism between DOM-Fe/Mn and As was investigated by elucidating the transformation behaviour of DOM-Fe/Mn on As species in a controlled laboratory environment. The results showed that in cases where the coexistence of Fe and Mn concentrations was relatively low (e.g. Fe < ∼0.5 mg/L and Mn < ∼0.6 mg/L), the concentration of As in water would increase proportionally with the simultaneous increase of both Fe and Mn concentrations (As < 5 μg/L). However, when the concentration of either Fe or Mn reached 10 mg/L, the proportion of As complexed by DOM increased significantly, reaching 99.73% and 99.66%, respectively. In the configuration of a metallic bridge, the elements Fe and Mn act as connectors between negatively charged DOM and As, thereby increasing the adsorption capacity of DOM for As. The alcohol and alkene functional groups present on the DOM-Fe/Mn surface show a preference for binding with free species of As in aqueous environments. In addition, the reductive groups on the surface of DOM not only directly convert As(V) to As(III), but also facilitate the reduction of Fe(III) to Fe(II), resulting in the indirect conversion of As(V) to As(III). Thus, this study provides a comprehensive understanding of the transport and transformation processes of arsenic in subtropical eutrophic lakes.

Can a eutrophic lake function as a carbon sink? Case study of a subtropical eutrophic lake in southern USA

Carbon dioxide (CO2) outgassing from inland eutrophic waters is a small but critical component of the global carbon cycle that is currently not well constrained. Eutrophic lake systems have been reported to be supersaturated with dissolved CO2 (pCO2), therefore considered a carbon source to the atmosphere. In this study, we aimed to test two hypotheses: 1) there is a substantial diurnal fluctuation inpCO2 in eutrophic lakes, making them both a source and a sink of carbon; and 2) the diurnal fluctuation is seasonally dependent, with a pronounced variability during the summer. As a case study, we used a hypereutrophic lake in a highly urbanized area in subtropical southern USA. We conducted in-situ measurements on pCO2, photosynthetically active radiation (PAR), and other ambient parameters at 3-hour intervals for two days each in the winter and summer of 2022. The study shows that there was a clear diurnal fluctuation of pCO2 with the peak in the early morning and the lowest in the later afternoon in both seasons. In the winter, the lake water was mainly CO2-unsaturated in reference to the atmospheric CO2 of 420 ppm, ranging from a low of 128 ppm in the mid-afternoon to a high of 557 ppm in the early morning hours. In the summer, the lake water was mostly CO2-oversaturated, withpCO2 increasing drastically from 309 ppm during the early afternoon to 3895 ppm in the early morning hours. In both winter and summer, CO2 outgassing was nearly three times greater at night than in the daytime, increasing from −9.98 to –3.75 mg C m-2h−1 in the winter and from 13.17 to 45.34 mg C m-2h−1 in the summer. Consequently, a clear diurnal trend of CO2 evasion rate (FCO2) was found, showing that the lake often acted as a carbon sink during the daylight hours but as a carbon source at night, strongly affected by PAR availability and colored dissolved organic matter (cDOM). These findings validate our initial hypotheses and suggest that the diurnal and seasonal fluctuations of CO2 evasion in eutrophic lakes should be considered in the global carbon accounting of lake systems to constrain uncertainties. Furthermore, we found that pCO2 measurements at 10:00o’clock in the morning have the least deviation to the diurnal average and, therefore, propose standardizing this time for future studies in field pCO2 measurements in order to make data comparable over seasons, geographical regions, and lake trophic levels.

Habitat-specific regulation of bacterial community dynamics during phytoplankton bloom succession in a subtropical eutrophic lake

Phytoplankton blooms, an important indicator of severe eutrophication, are a globally significant consequence of anthropogenic activities and climate change on freshwater lakes. Shifts in microbial communities during phytoplankton blooms have been extensively investigated, yet we have a limited understanding of how distinct assembly processes underlying the temporal dynamics of freshwater bacterial communities within different habitats respond to the succession of phytoplankton blooms. To address this knowledge gap, we collected both water and sediment samples in a subtropical eutrophic lake over a complete period of phytoplankton blooms to assess the dynamics of bacterial communities and the temporal shifts in assembly processes. Our results showed that phytoplankton blooms strongly altered the diversity, composition, and coexistence patterns of both planktonic and sediment bacterial communities (PBC and SBC), but the successional patterns differed between PBC and SBC. PBC were less temporally stable under bloom-induce disturbances, with higher variations in temporal dynamics and greater sensitivity to environmental fluctuations. Furthermore, the temporal assembly patterns of bacterial communities in both habitats were mainly driven by homogeneous selection and ecological drift. In the PBC, the role of selection decreased over time, while ecological drift became increasingly important. Conversely, in the SBC, the relative impact of selection and ecological drift on community assemblages fluctuated less over time, with selection remaining the dominant process throughout the bloom.

Response of zooplankton to nutrient reduction and enhanced fish predation in a shallow eutrophic lake.

As a key link between top-down regulators and bottom-up factors, zooplankton responds sensitively to environmental variations and provides information on the ecological state of freshwater systems. Although the response of zooplankton to anthropogenic pressures and fluctuating natural conditions, such as nutrient loading and climate change, has been extensively examined, findings have varied markedly. The mechanistic basis for the correlation between environmental variability and the zooplankton community is still debated, particularly for subtropical eutrophic lakes. We used two methods to analyze physicochemical and selected biological variables derived from long-term monitoring of Lake Taihu, a subtropical shallow lake in China. We first applied random forest regression to examine how changes in zooplankton were related to a set of environmental variables on interannual time scales. Then we used the results to guide the construction of a conceptual model for piecewise structural equation modeling (pSEM) to quantify more precisely the zooplankton-environment relationship. Zooplanktivorous fish and nutrient concentrations were the most important predictors of long-term trends in zooplankton in RF regression. Intensification of planktivorous fish predation led to a lower zooplankton biomass and smaller individuals through the removal of larger crustaceans. Moreover, suppression of zooplankton can in part be explained by increases in inedible algae, triggered by a combination of reduced nutrient concentrations and weakened grazer control. These results were also confirmed in the pSEM, which further indicated that top-down regulators might be more important than bottom-up factors for the zooplankton community in Lake Taihu. Our results suggest that stocking of filter-feeding fish in the lake did not meet the expectation that they would control algae, but that the use of biomanipulation measures considering both water quality and fishery management seems promising. This study offers insights into how indicator metrics of zooplankton can improve our understanding of the associations between plankton communities and ecosystem alterations.

Cover Image

The cover image is based on the Original Article A trophic cascade triggers blooms of Asterionella formosa in subtropical eutrophic Lake Taihu, China, by Xia Liu et al. https://doi.org/10.1111/fwb.13986.

Seasonal variations alter the effect of an invasive plant on the decomposition of a native plant in a subtropical eutrophic lake, China

Seasonal variations alter the effect of an invasive plant on the decomposition of a native plant in a subtropical eutrophic lake, China

The influences of nitrogen and phosphorus availability on periphytic algae in eutrophic experimental mesocosm

ABSTRACT To investigate the influence of nitrogen (N) and phosphorus (P) availability on the periphytic algal biomass and assemblage composition (by pigment analysis) in the eutrophic lake, natural littoral intact sediment cores in a subtropical eutrophic lake (Lake Taihu) were collected, and subjected to three levels of water N and P loading (N-P [mg. L−1]: 1.5, 0.075; 2.5, 0.125; 3.5, 0.175) in the tanks, eighteen artificial substances (glass slides) were fixed at three water depths (1, 17, and 33 cm above the surface sediment) for development of periphytic algae in each tank, respectively. The results showed that the biomass of periphytic algae on artificial substance was significantly enhanced with increasing N and P levels in water column and water depth (p < 0.05), while sediment-associated periphytic algae did not significantly increase with increasing N and P levels in water column. The development of sediment-associated periphytic algae changed the phosphorus form, calcium-bound phosphorus and inorganic phosphorus in the sediments decreased slightly, and the total phosphorus and organic phosphorus in the sediments increased. Our results suggest that N and P availability is conducive to the development of periphytic algae in subtropical eutrophic shallow lakes in spring.

Reducing nutrient increases diatom biomass in a subtropical eutrophic lake, China–Do the ammonium concentration and nitrate to ammonium ratio play a role?

Response of aquatic organisms to eutrophication have been well reported, while less studies are available for the recovery of eutrophic lakes following a reduction in the external loading, especially for systems where nitrogen is reduced but the phosphorus concentration is maintained high due to internal loading. Diatoms are nitrate (NO3-N) opportunists but can also use ammonium (NH4-N). They may, therefore, be more sensitive to nitrogen reduction than other algae that typically prefer NH4-N. We document the variations of nutrients and diatoms in subtropical, eutrophic Lake Taihu over 28 yr during which a reduction of the external loading resulted from lake management. According to the results of change point analysis, data on environmental variables were divided into two periods (P1: 1992–2006; P2: 2007–2019) with two different seasons (WS: Winter-Spring; SA: Summer-Autumn), respectively. Compared with P1-WS, the concentration of NH4-N decreased significantly whereas NO3-N showed no significant change in P2-WS. In contrast, NH4-N concentrations were low and showed no significant changes in P1-SA and P2-SA and NO3-N decreased significantly in the latter period. Accordingly, NO3-N: NH4-N mass ratios in P1-SA and P2-WS were all significantly higher than those in P2-SA and P1-WS, respectively. The biomass of WS diatom increased significantly and the timing of the peak biomass shifted from P1-SA to P2-WS since 2007. The SEM analysis showed that NO3-N was retained as a statistically significant predictor for diatom biomass in P1-SA and significant effects of windspeed, zooplankton and NH4-N on diatom biomass in P2-WS. Windspeed and zooplankton have further changed the biomass of diatoms in the case of declining inorganic nitrogen. We conclude that the magnitude of vernal suppression or stimulation of diatom assemblages has increased, concomitant with the variations of NH4-N and NO3-N: NH4-N mass ratios. Diatoms response to NH4-N or NO3-N is apparently changing in response to water temperature in this eutrophic shallow lake. Thus, parallel reductions in external nitrogen loading, along with variations in dominant inorganic nitrogen, will stimulate the growth of diatom and therefore increase the total biomass of phytoplankton in still high internal phosphorus loading, which is should be regarded as a good sign of restoration measures.

Ecosystem stability and water quality improvement in a eutrophic shallow lake via long-term integrated biomanipulation in Southeast China

Biomanipulation is one of the most promising measures in ecological restoration and used to mitigate eutrophication in water bodies. Current biomanipulation methods, such as traditional biomanipulation (based on large-size zooplankton) and non-traditional biomanipulation (based on filter-feeding fish), are often carried out independently and focus on algae control. Integrated biomanipulation, including traditional and non-traditional biomanipulation, was implemented in Longhu Lake, a drinking water source, to successfully control cyanobacterial blooms and improve water quality for 20 years. In order to study the mechanism of algae control and the ecosystem stability, we analysed the effects of integrated biomanipulation and single-biomanipulation on the phytoplankton community and the ecosystem via model simulation. The results revealed that compared with the other single-biomanipulation scenarios of ‘filter-feeding fish biomanipulation’, ‘omnivorous fish biomanipulation’, ‘piscivore biomanipulation’, and ‘without biomanipulation’, integrated biomanipulation combined the advantages of traditional and non-traditional biomanipulation. In this case, cyanobacteria biomass (4.5 ton/km2) was kept at a low level, although the biomass of small phytoplankton was high. The slope of trophic-levels spectrum was low (18.54%), and the biomass distribution of the community was balanced (less ‘up-and-down’). The energy flow transfer efficiency of food web was as high as 14.1% and the ratio of grazing food chain and detrital food chain was close to 1:1. The ascendency was better than other scenarios. These data indicated that the ecosystem after integrated manipulation was more stable and mature than that other single-biomanipulation. Moreover, integrated biomanipulation could substantially increase fishery income. Integrated biomanipulation can therefore be used to meet the requirements of sustainable development, especially in tropical and subtropical eutrophic lakes used as drinking water source.

The adaptations to tube-dwelling life of Propsilocerus akamusi (Diptera: Chironomidae) larvae and its eutrophication-tolerant mechanisms

Propsilocerus akamusi (Diptera: Chironomidae) is a dominant macroinvertebrate species in many eutrophic lakes in subtropical and temperate zones. P. akamusi larvae can migrate deep into the sediment (>30 cm) during summer where is no oxygen. However, to our knowledge, the specific adaptive tactics of its tube-dwelling life (>30 cm) and underlying mechanisms why this species is favored by nutrient-rich lakes remain limited. With the understanding above-mentioned issues, we can provide important information for the development of sensitive biomonitoring. We examined monthly morphological dynamics and physiological adaptations of P. akamusi to anoxic conditions, and environmental relationships of the species abundance in a subtropical eutrophic lake. Results of our experiments indicated that this species presented extremely low fuel consumption at a state of estivation when the larvae dwelt in deep sediment, and conducted anaerobic respiration, with ethanol as a major metabolite, to withstand long term anoxic conditions. Additionally, in our field study, the abundance of P. akamusi correlated positively with most nutrient-related variables, indicating that this species can serve as sentinel organisms of nutrient enrichment. We suggest that the high tolerance to anoxia and detritivorous diet of P. akamusi larvae may allow the species to benefit from nutrient enrichment. The vertical niche differentiation and the avoidance of benthivorous predator by vertical sediment migration behavior may also contribute to its high dominance in eutrophic freshwaters.

Rapid daily change in surface water pCO2 and CO2 evasion: A case study in a subtropical eutrophic lake in Southern USA

Evasion of carbon dioxide (CO2) from lakes is a significant component in the continental carbon balance, but most current CO2 evasion estimates ignore daily CO2 fluctuations. To test the hypothesis that partial pressure of CO2 (pCO2) and CO2 evasion vary throughout a day due to biological processes driven by solar radiation, we conducted in-situ pCO2 and ambient water measurements over eleven 10-h periods in a subtropical, eutrophic shallow lake from November 2017 to May 2018. In-situ measurements were performed at 7:00, 10:00, 14:00, and 17:00 Central Standard Time of the United States (CST), and CO2 evasion rates were estimated based on the field pCO2 records. Strong daily declining trends of pCO2 and CO2 flux were found throughout the seasons except for one winter day with unusually low temperatures. At 7:00, 10:00, 14:00, and 17:00 CST of a day, average pCO2 were 1131, 839, 345 and 205 µatm, respectively, while average CO2 fluxes were 80, 67, −10, and −34 mmol m2 h−1. Significant differences were found in average pCO2 between any two measured time points in a day, while significant reductions in CO2 flux were observed between 10:00 and 14:00 CST and between 14:00 and 17:00 CST. pCO2 and CO2 flux dynamics were most likely driven by the air-water exchanges during nighttime hours and mainly driven by aquatic metabolism in the daytime. These findings suggest possible large uncertainties in the estimation of carbon emitted from trophic lakes, highlighting the need for further research on diurnal pCO2 fluctuation from different aquatic ecosystems to improve CO2 evasion estimation.

Factors affecting annual occurrence, bioaccumulation, and biomagnification of polycyclic aromatic hydrocarbons in plankton food webs of subtropical eutrophic lakes.

The biological pump plays a critical role in the occurrence and fate of hydrophobic organic contaminants (HOCs) mostly in temperate and frigid oligotrophic waters. However, the factors for the long-term occurrence and fate of HOCs in subtropical eutrophic waters remain largely unknown. This study provides novel insights into biogeochemical and physical factors on the annual occurrence, bioaccumulation, and biomagnification of 16 polycyclic aromatic hydrocarbons (PAHs) in the plankton food webs of four Chinese subtropical eutrophic lakes by one-year simultaneous field observations for five compartments. The annual mean ΣPAH16 in the water columns ranged from 359.69 ± 31.52 ng L-1 to 682.69 ± 65.41 ng L-1, and increased with the annual mean trophic state index, and phytoplankton biomass of these lakes, but was independent on the proximity of the lakes to urban areas. Biodilution effect played an important role in the occurrence of the PAHs in both phytoplankton and zooplankton. In contrast to previous studies in oligotrophic waters, not only the biological pump but also the equilibrium partitioning and the indirect influence of eutrophication (high pH induced by phytoplankton, and phytoplankton life cycling) modulated the annual occurrence of the PAHs in the water columns of these eutrophic lakes. Biphasic correlations were found between the bioaccumulation factors of the PAHs by plankton and the temperature (n = 97-136, R2 = 0.06-0.24, p ≤ .008), and were related to plankton phenology. Bioaccumulation factors by plankton were dependent on the hydrophobicity of the PAHs (n = 16, R2 = 0.27-0.31, p ≤ .023), and decreased with plankton biomass (n = 94-103, R2 = 0.09-0.27, p ≤ .010). Trophic transfer of the PAHs from phytoplankton to zooplankton increased with phytoplankton biomass (n = 26, R2 = 0.27, p = .004), and the temperature (n = 102-135, R2 = 0.06-0.13, p ≤ .004), but decreased with lake trophic state index. Biomagnification only occurred during phytoplankton bloom periods.

Effects of floating-leaved macrophytes on water quality and phytoplankton: an in situ experiment in a Chinese shallow lake

In this study, in situ experiments were conducted to evaluate the effects of floating-leaved macrophytes (FLMs, i.e. Trapa uncinata and Nymphaea tetragona) on water quality and algal growth and to investigate the potential inhibition of phytoplankton growth by FLMs. Enclosures were established in a shallow subtropical eutrophic lake, and FLMs were planted therein. Open water areas in the enclosure with FLMs, an enclosure without FLMs, and the lake water surrounding the enclosures served as the control groups. Physical parameters, nutrient contents, and algae were evaluated for six sampling sites at 10-d intervals. Treatment with FLMs significantly promoted water transparency and reduced turbidity. However, the concentrations of nutrients were similar in the different sites, despite the nutrient absorption by FLMs. The overall algal abundance was reduced in the presence of FLMs, and this was shown in chlorophyll-a, algal density, and biomass. FLM treatments promoted changes in the algal community composition, benefitting metaphytic species adapted to higher transparency. A principal components analysis showed that algal attributes were higher in the control areas than in the FLM-covered areas, despite the similar nutrient concentrations. This indicated that the inhibition of algae in this study was based not on nutrient competition but instead on the shading effect. Using FLMs as a management tool to create a clear water state may be one solution for turbid waters in subtropical eutrophic shallow lakes.

The response of phytoplankton in a subtropical lake to the cessation of aquaculture: a case study of Lake Dianshan, China

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 chlorophyll-a 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.

Bottom-up versus top-down effects on ciliate community composition in four eutrophic lakes (China)

Previous studies have shown that ciliate plankton is generally controlled by food resources (e.g., algae) and predators (e.g., metazooplankton). Among lakes with similar trophic levels but different distributions of phyto- and metazooplankton, the main forces acting on ciliate assemblages may be different. We investigated the relationship between ciliate communities and bottom-up versus top-down variables based on a survey of four subtropical eutrophic lakes (China). Two of the lakes (Chaohu, Taihu) are located on the Mid-lower Yangtze Plain near sea level, and the other two (Dianchi, Xingyunhu) on the Yunnan-Kweichow Plateau at 1700m above sea level. Blooms of cyanobacteria developed during summer in Lakes Chaohu and Taihu and throughout the year in Lakes Dianchi and Xingyunhu. Ciliate functional feeding groups differed significantly between lakes. The results of canonical correspondence analysis (CCA) and variation partitioning showed that cyanobacteria significantly influence ciliate species, whereas ‘edible’ algae (cryptophytes, diatoms) and cladocerans were the important variables in explaining the ciliate community structure of Lakes Dianchi and Xingyunhu compared with Lakes Taihu and Chaohu. Our results highlight the importance of consistent cyanobacterial blooms in shaping the ciliate community in subtropical eutrophic shallow lakes by interacting with top-down and bottom-up factors.

Alternative food sources of native and non-native bivalves in a subtropical eutrophic lake

The aim of this study was to evaluate the consumption of alternative food sources (water column and sediment) and the potential competition under different conditions between Diplodon parallelopipedon (native) and Corbicula fluminea (non-native). We evaluated filtration and ingestion rates of water column at 6 and 24 h, held with and without organic matter and different phytoplankton types: needle-shaped green algae (Ankistrodesmus sp.) and filamentous cyanobacteria (Planktothrix agardhii) dominated communities. Our results confirmed higher filtration and ingestion rates per biomass unit for C. fluminea in the presence of sediment without organic matter. However, when we compared the filtration of bivalves held in sediment with organic matter, D. parallelopipedon rates were not significantly different from C. fluminea values. Moreover, in the presence of filamentous cyanobacteria, only C. fluminea filtration and ingestion rates decreased significantly. Our experimental results and previous field evidence concur that C. fluminea were not able to outcompete the native bivalves in organic matter rich sediment and cyanobacteria blooms conditions. The differential responses to the eutrophication process between exotic and native bivalves, apparently favoring the later, might buffer the potential competition interactions allowing their coexistence.