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The rates of temporal and spatial species turnover have been compared in different organisms and scales, revealing that both are not independent but, rather, associated. However, the knowledge is limited for the association between spatial turnover and temporal turnover. Here, we performed two investigations of the phytoplankton composition in the lakes of the Yangtze River catchment in China in the spring and summer of 2012, which covered regional spatial scale and two‐season temporal scale. We analysed the association between temporal and spatial species turnover in phytoplankton. The results showed that 1) the two‐season temporal turnover of phytoplankton varied based on the mean values and the coefficient of variance of environmental variables, and pH was the most important variable negatively affecting the temporal turnover; 2) the spatial beta diversity of phytoplankton in summer was higher than that in spring, and the distance decay pattern was significant in summer, but not in spring; 3) the variation in spatial turnover in spring and summer was attributed to the primary environmental variables (nitrogen, phosphorus and underwater available light) and broader‐scale spatial variables; 4) the proportion of jointly explained variation of spatial Bray–Curtis dissimilarity by the environment and space increased from ~38% (spring) to ~55% (summer), which was mainly due to the variation in spatially structured environmental variables during the two‐season temporal turnover, such as pH and ion concentrations; 5) the community compositions in summer were more similar between the lakes with similar two‐season temporal turnover. These results indicate that the spatial turnover of phytoplankton composition in summer was partially predetermined by the variation in environmental variables and phytoplankton composition during the process of two‐season temporal turnover, and highlight the understanding of temporal variations in spatial beta diversity as well as the underlying assembly mechanisms in phytoplankton. [ABSTRACT FROM AUTHOR]

The diversity and composition of photosynthetic picoeukaryotes (PPEs) in two large shallow lakes in China (Lake Taihu and Lake Chaohu) were investigated from flow cytometry sorted samples using Miseq high-throughput sequencing. We collected 65 samples covering different regions of the two lakes over four seasons to unveil spatial and temporal patterns of PPEs community composition. The use of flow cytometry sorting largely improved the efficiency of detecting PPEs sequences and over 70% of the retrieved reads belonged to PPEs. Chlorophyta and Bacillariophyta dominated PPEs in most of the samples. A distinct but complex seasonality of PPEs composition emerged at the OTUs level. NGS-based Miseq sequencing facilitates an in-depth view of numerous rare OTUs. Nearly 80% of the PPEs OTUs were rare and lots of them were detected only in one season, whereas most of the abundant OTUs were frequently detected in all seasons but only changed in relative abundances. Besides, a close relative of the marine PPEs species Ostreococcus sp. (OTU_1144, 99% identity) was discovered in freshwater systems for the first time and was abundant especially in winter. The diversity and community composition of PPEs were more dependent on season rather than sampling sites. Temperature, phycocyanin and NO3AN concentrations in Lake Taihu explained the PPE composition variations, whereas in Lake Chaohu TN/TP ratios, temperature, pH and nephelometric turbidity units (NTU) seemed to be the most important factors. In addition, a great number of OTUs belong to nonpigmented picoeukaryotes, especially Chytridiomycota, Perkinsozoa, Ciliophora and Cercozoa, which are known to include algae parasites as well as predators. The results of mantel test also showed that the community of photosynthetic and nonpigmented picoeukaryotes were significantly correlated in both lakes. [ABSTRACT FROM AUTHOR]

Quantitative evidence of sudden shifts in ecological structure and function in large shallow lakes is rare, even though they provide essential benefits to society. Such 'regime shifts' can be driven by human activities which degrade ecological stability including water level control ( WLC) and nutrient loading. Interactions between WLC and nutrient loading on the long-term dynamics of shallow lake ecosystems are, however, often overlooked and largely underestimated, which has hampered the effectiveness of lake management. Here, we focus on a large shallow lake (Lake Chaohu) located in one of the most densely populated areas in China, the lower Yangtze River floodplain, which has undergone both WLC and increasing nutrient loading over the last several decades. We applied a novel methodology that combines consistent evidence from both paleolimnological records and ecosystem modeling to overcome the hurdle of data insufficiency and to unravel the drivers and underlying mechanisms in ecosystem dynamics. We identified the occurrence of two regime shifts: one in 1963, characterized by the abrupt disappearance of submerged vegetation, and another around 1980, with strong algal blooms being observed thereafter. Using model scenarios, we further disentangled the roles of WLC and nutrient loading, showing that the 1963 shift was predominantly triggered by WLC, whereas the shift ca. 1980 was attributed to aggravated nutrient loading. Our analysis also shows interactions between these two stressors. Compared to the dynamics driven by nutrient loading alone, WLC reduced the critical P loading and resulted in earlier disappearance of submerged vegetation and emergence of algal blooms by approximately 26 and 10 years, respectively. Overall, our study reveals the significant role of hydrological regulation in driving shallow lake ecosystem dynamics, and it highlights the urgency of using multi-objective management criteria that includes ecological sustainability perspectives when implementing hydrological regulation for aquatic ecosystems around the globe. [ABSTRACT FROM AUTHOR]

The patterns of spatial and temporal shifts in bloom-forming cyanobacteria and the driving factors for these patterns were determined by analyzing the distribution of these cyanobacteria in Lake Chaohu using data from satellite images and field samples collected during 2012 and 2013. The cyanobacterial blooms primarily occupied the western region of Lake Chaohu, and the direction and speed of the prevailing wind determined the spatial distribution of these blooms. The cyanobacteria in Lake Chaohu were dominated by species of M icrocystis and A nabaena. M icrocystis reached its peak in June, and Anabaena had peaks in May and November, with an overall biomass that was higher than that of M icrocystis. M icrocystis generally occupied the western region of the lake in summer, whereas Anabaena dominated in other regions and seasons. Temperature may be responsible for these seasonal shifts. However, total phosphorus ( TP), pH, temperature, turbidity and nitrate/nitrite nitrogen determined the coexistence of the two genera in different regions in summer. TP was correlated with M icrocystis dominance, and pH and light availability were correlated with A nabaena dominance. Our results contribute to the understanding of shifts in bloom-forming cyanobacteria and are important for the control of cyanobacterial blooms. [ABSTRACT FROM AUTHOR]

The diversity of buoyant Microcystis colony-associated, settling particle-associated and free-living freshwater bacteria in the eutrophic Lake Taihu in China was compared by the polymerase chain reaction-denaturing gradient gel electrophoresis ( PCR-DGGE) of 16S rRNA genes and by clone library analysis. The cluster analysis of DGGE profiles revealed that the buoyant Microcystis colony associates collected in summer and winter were clustered together and were distinct from settling particle-associated and free-living bacteria. In contrast, the bacterial communities of the latter two populations collected in summer were clustered together and varied from those collected in winter. The diversity indices of the Microcystis-associated bacterial population were significantly lower than those of the other two bulk bacterial communities ( P < 0.05). Clone library analysis revealed that no shared operational taxonomic units were found in the three populations. Phylogenetic analysis confirmed that buoyant Microcystis-associated bacteria were distinct from the other communities and that they were dominated by Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria. Proteobacteria, Bacteroidetes and Actinobacteria were prevalent in all three habitats. Other bacterial taxa such as Planctomycetes, Verrucomicrobia, Gemmatimonadetes and Acidobacteria were associated with settling particles and with the water column. Buoyant Microcystis colonies, settling particles and the water column in the eutrophic lake are thus inhabited by different bacterial flora. In addition, specific bacterial communities are associated with buoyant Microcystis colonies. [ABSTRACT FROM AUTHOR]

SUMMARY Phytoplankton is sensitive to rapidly increasing temperature in spring. However, studies on the effect of temperature on phytoplankton have mainly focused on constant temperatures. It is necessary to clarify the determining parameters of phytoplankton shifts during temperature increases, as temperatures are predicted to fluctuate more intensively and frequently in the future. In the study, we analyzed the responses of photosynthetic properties and growth in a cyanobacterium ( Microcystis aeruginosa) and a green alga ( Chlorella pyrenoidosa), the dominant species in Taihu, to rapid increasing-temperature process in the laboratory and in the field. The results show that gradually increasing temperature inhibited photosynthesis and the growth of C. pyrenoidosa and had almost no effect on M. aeruginosa. Elevated increasing temperature range also had more significant effects on the photosynthetic properties and growth rates of C. pyrenoidosa than those of M. aeruginosa in the laboratory and in the field. All of these results suggest that the photosynthetic performance of M. aeruginosa is more suitable to gradually increasing temperature and relatively strong temperature variations than that of C. pyrenoidosa, which might partially contribute to Microcystis excluding Chlorella competitively in aquatic ecosystem. Our findings point out the possible importance of the rapid and dramatic increasing-temperature process to the formation of cyanobacterial blooms. [ABSTRACT FROM AUTHOR]

The phytoplankton lysis rates in the different eutrophic regions of Lake Taihu were determined based on the activities of particle and dissolved esterase, as well as the decay rate of the latter, from August 2009 to March 2010. Two peaks were observed for the chlorophyll a concentration, one in September 2009 and another in January 2010. The highest phytoplankton lysis rates observed in October were associated with the decay of the summer bloom, whereas the minimum lysis rates observed in January were associated with the winter bloom. The highest cell lysis rates in Meiliang Bay, Lake centre, and Gonghu Bay were 0.67, 0.77, and 0.68 d-1, respectively, whereas the lowest lysis rates in these regions were 0.03, 0.01, and 0.05 d-1, respectively. Water temperature showed an apparent indirect effect on lysis rate. In addition, a significant inverse relationship was observed between lysis rates and nitrate concentration in the three lake regions, which suggests that phytoplankton cell lysis is associated with changes in nitrate concentration. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Microcystis aeruginosa (Kütz.) Kütz. commonly occurs as single cells at early recruitment but forms large colonies in summer. Colony formation will induce many acclimative changes. In this study, we demonstrated the photochemical changes before and after colony formation. In the laboratory, light curves showed that colonies were more responsive to high light than single cells. The values of the maximal slope of electron transport rate (ETR)-light curve (α), relative maximal electron transport rate (rETR), and onset of light saturation ( I) of colonies were significantly higher than those of single cells ( P < 0.05), indicating that colonies have higher photosynthetic capability than single cells, especially in high light, where values of rETR and I of colonies were 2.32 and 2.41 times those of single cells. Moreover, the dark-light experiments showed that colonial cells can more effectively resist darkness damage. In addition, pigments of colonial cells were higher than those of single cells ( P < 0.05). The higher pigment contents probably contribute to higher photosynthetic capability. In the field, the inhibition rate of F/ F in single cells increased significantly faster than that of colonies as light increased ( P < 0.05), but nonphotochemical quenching (NPQ) value of colonies was higher (32.4%) than that of single cells at noon, which indicated colonial cells can more effectively resist high-light inhibition than single cells ( P < 0.05). Polysaccharides of colonies were significantly higher compared to those in unicellular cells ( P < 0.05) based on their contents and ultrastructural characteristics. This finding implies that colonies could not effectively decrease photoinhibition by negative buoyancy regulation. In fact, NPQ may be an important mechanism for avoiding photodamage. All of these phenomena can help explain the ecological success of colonial M. aeruginosa in eutrophic water. [ABSTRACT FROM AUTHOR]

The dynamics of nutrients, such as phosphorus, nitrogen, and carbohydrates, during cyanobacteria cell lysis was investigated under darkness incubation in the laboratory. The cell lysis rate of cyanobacteria sampled from Lake Taihu was measured using an esterase assay. Based on particulate esterase activity, the calculated cyanobacteria lysis rate was 0.094 d–1. During 30 days of darkness incubation, Chlorophyll aconcentration decreased from 56 μg L–1to 2.0 μg L–1. Parallel to this, total particulate carbohydrate concentration decreased rapidly. The fluctuation of dissolved organic carbon concentration was a function of the production of noncarbohydrate by cyanobacteria and the decomposition of carbohydrate by bacteria. Total dissolved carbohydrates and dissolved polysaccharides concentrations showed a similar pattern, declining at the beginning of the experiment and keeping relatively stable, thereafter. In contrast, the concentration of dissolved monosaccharides remained constant during the entire process. The concentrations of NH4and PO43–increased at the early stage, and then decreased afterwards. A gradual decrease in NO3–concentration after day 8 indicated that anaerobic conditions might be produced during the cell lysis process. The present results demonstrated cyanobacteria cell lysis has a big influence on the nutrient status of the surrounding water. © 2010 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim [ABSTRACT FROM AUTHOR]