River-connected Lake Research Articles

Effects of loss of lateral hydrological connectivity on fish functional diversity

Loss of lateral hydrological connectivity (LHC) is a major cause of biodiversity decline in river floodplains, yet little is known about its effects on aquatic functional diversity in these ecosystems. We quantified functional alpha and beta diversity of fish assemblages in Yangtze River floodplain lakes and explored their responses to loss of LHC with generalized linear mixed models. Functional richness was much lower in lakes that were not connected to the Yangtze River (i.e., disconnected lakes), where functional evenness and divergence were higher. LHC was the most important factor shaping fish diversity patterns in this region. Predicted reductions in functional richness and taxonomic richness due to LHC loss were higher for functional richness (0.47-0.82) than taxonomic richness (0.32) for all species assemblages except nonmigratory species. The distribution of functional strategies of migratory and nonmigratory fishes was highly uneven throughout the floodplain. Taxonomic beta diversity was much higher than functional beta diversity. The former was due mainly to spatial turnover (73.6-83.8%), which suggested that dissimilarity of diversity among fish assemblages was largely induced by species replacement. The latter was induced by the nestedness-resultant component of overall beta diversity (70.7-86.0%), which indicated a high degree of function loss without replacement. Both taxonomic and functional beta diversity were higher in disconnected lakes, where they were significantly correlated with fishing activity and water quality, than in river-connected lakes. We showed for the first time the effects of loss of LHC on fish functional diversity in large river floodplains. We found a serious decline of fish functional richness in the Yangtze floodplain, and functional diversity remained highly vulnerable to loss of LHC even though this is a species-rich ecosystem.

Assessing Aquatic Ecological Health for Lake Poyang, China: Part II Index Application

As one of the largest Yangtze river-connected lakes in China, Lake Poyang’s heath status has increasingly become a cause for concern. This study evaluated Lake Poyang’s heath status using a newly developed index (ecosystem health index for Lake Poyang, LP-EHI). The evaluation results revealed a declining tendency in the lake’s health status from 2010 to 2014. The health conditions for Lake Poyang were “good”, “fair”, and “fair” in 2010, 2013, and 2014, respectively. The individual indices for water quality, nutrient conditions, and benthic macroinvertebrate showed a deteriorating trend from 2010 to 2014 mainly due to human activities, such as industrial pollution. Further analysis revealed that the health status of Lake Poyang was strongly affected by hydrological processes. This study demonstrated the potential of LP-EHI in evaluating the ecological health of Lake Poyang. Such a heath status evaluation can be applied to other lakes, and can benefit lake management, restoration, and conservation.

Water level management of lakes connected to regulated rivers: An integrated modeling and analytical methodology

Reservoir operations significantly alter the hydrological regime of the downstream river and river-connected lake, which has far-reaching impacts on the lake ecosystem. To facilitate the management of lakes connected to regulated rivers, the following information must be provided: (1) the response of lake water levels to reservoir operation schedules in the near future and (2) the importance of different rivers in terms of affecting the water levels in different lake regions of interest. We develop an integrated modeling and analytical methodology for the water level management of such lakes. The data-driven method is used to model the lake level as it has the potential of producing quick and accurate predictions. A new genetic algorithm-based synchronized search is proposed to optimize input variable time lags and data-driven model parameters simultaneously. The methodology also involves the orthogonal design and range analysis for extracting the influence of an individual river from that of all the rivers. The integrated methodology is applied to the second largest freshwater lake in China, the Dongting Lake. The results show that: (1) the antecedent lake levels are of crucial importance for the current lake level prediction; (2) the selected river discharge time lags reflect the spatial heterogeneity of the rivers’ impacts on lake level changes; (3) the predicted lake levels are in very good agreement with the observed data (RMSE ≤ 0.091 m; R2 ≥ 0.9986). This study demonstrates the practical potential of the integrated methodology, which can provide both the lake level responses to future dam releases and the relative contributions of different rivers to lake level changes.

Remote observation of water clarity patterns in Three Gorges Reservoir and Dongting Lake of China and their probable linkage to the Three Gorges Dam based on Landsat 8 imagery

The Secchi disk depth (ZSD) plays a critical role in describing water clarity. Several studies have shown linkages between Three Gorges Dam (TGD) and the downstream lacustrine ecosystem in the middle and lower Yangtze River basin. However, the potential influence on the ZSD fluctuation in the entire anthropogenic reservoirs of Three Gorges (ER) and Dongting Lake (DTL) has not been reported, possibly due to technical obstacles in obtaining statistically significant spatial and temporal results. We addressed this challenge by using remote sensing technology: the Landsat 8 Operational Land Imager (OLI). We proposed a new, robust remote-sensing algorithm to estimate ZSD from OLI imagery using red and green band-ratio, leading to MAPE of 21.68% and RMSE of 0.076m for ZSD ranging from 0.1m to 1.05m. After satisfactory image-based validation, the algorithm was implemented on OLI data to derive ZSD patterns over ER and DTL from 2013 to 2017. Several crucial findings can be drawn: 1) Spatial-temporal patterns of ZSD exhibited notable fluctuations over both ER and DTL, and they also demonstrated a significant correlation with each other because of the opposite temporal cycle of ZSD fluctuations between ER and DTL; 2) Temporally, monthly fluctuations of ZSD between ER and DTL had opposite temporal cycles, which was mainly attributed to the surface runoff and sediment discharge driven by the outbound runoff variations of TGD. Spatially, the heterogeneity of the ZSD pattern in ER might have resulted from the different geographical regions being divided by large anthropologic hydrological facilities, such as TGD; 3) The relationship between ZSD and total suspended matter (TSM) showed a significant negative correlation, as did the relationship between ZSD and Kd(490). These findings demonstrate that TSM often plays a principal role in light attenuation of extremely turbid inland waters; 4) An inversed phenomenon of water clarity was observed at the intersection of DTL and the Yangtze River around Chenglingji site (YRAC), which was due to the opposite temporal cycle of ZSD fluctuations between DTL and ER after the impoundment of TGD; and 5) Owing to the analysis of noise-equivalent ZSD, OLI data can be used to derive ZSD, since the imagery uncertainty is 0.07m by means of our band-ratio algorithm, which demonstrates similar results to MODIS. The proposed ZSD-derived algorithm in this study could be suitable for other turbid lakes or reservoirs to formulate related strategies of water quality management in the middle and lower Yangtze River basin, and the unveiled findings here improve our understanding of ZSD spatiotemporal fluctuations in large river-connected lakes, such as Poyang Lake.

Suspended sediment distribution under varied currents in the largest river-connected lake of China

Abstract Poyang Lake was selected as the research area. Based on laboratory experiment, field investigation and numerical simulation, the spatial distributions of suspended sediment (SS) under the gravity-flow, jacking-flow and back-flow patterns were quantitatively analysed. An annular flume experiment was conducted to determine the critical starting shear stresses of the sediments in the flood and dry seasons. By numerical experiment, the SS transport under different flow patterns was explored. Several results stand out. (1) The critical starting shear stresses of the sediments in the flood and dry seasons were 0.35 N·m−2 and 0.29 N·m−2, respectively. (2) Due to the strongest flow disturbance and scouring effect, SS under the gravity-flow pattern was characterized by the highest loads. The lowest SS was observed during the jacking-flow pattern, which could be attributed to the lowest water level gap between the lake and external rivers. The loads ranged from 0.053 kg·m−3 to 0.068 kg·m−3. (3) Under the back-flow pattern, SS in the north lake was evidently influenced by the Yangtze River, and the mean value was approximately 0.12 kg·m−3. With the gradually weakened back-flow impact, the SS load was decreased from the north to the middle of the lake.

Spatial distribution of crustacean zooplankton in a large river-connected lake related to trophic status and fish

<p>It has been reported that community structure of crustacean zooplankton is related to environment factors and abundance of predacious fishes. Most studies, however were based on observation on independent water bodies, which may complicate the results with uninterested factors. A large river-connected lake has partitioned regions with the characteristics of either a river or a lake albeit allowing exchanging of zooplankton of different regions. Thus, a river-connected lake is an ideal system for studying distribution of crustacean zooplankton and investigating its affecting factors. In this study, 11 stations of a large river-connected lake, Lake Gaoyou, were sampled in all four seasons. Taxonomic diversity and species abundance were recorded and their correlation with environmental factors was investigated. A total of 26 and 22 species of crustacean zooplankton were recorded in the river and lake region, respectively. The abundance of crustacean in the river region was significantly higher than that in the lake region. The relative abundance of calanoids and <em>Bosmina</em> <em>fatalis</em> in the river region was lower than that in the lake region, whereas the abundance of cyclopoids and other cladocera species was lower in the lake region than in the river region. Cluster analysis showed that crustacean assemblages could be divided into two, one cluster grouping all sites from the river region, and the other cluster included all sites in the lake region. According to Pearson correlation and canonical correspondence analysis, the distribution of crustacean was positively correlated with water temperature and chlorophyll a gradient. Chlorophyll a and fish abundance are excellent predictors on spatial distribution of crustacean zooplankton in Lake Gaoyou. Our findings may also help in planning of bighead carp stocking in Lake Gaoyou.</p>

Microsatellite development and characterization for Saurogobio dabryi Bleeker, 1871 in a Yangtze river-connected lake, China

Microsatellite development and characterization for Saurogobio dabryi Bleeker, 1871 in a Yangtze river-connected lake, China

Water transparency distribution under varied currents in the largest river-connected lake of China.

Water transparency is an important ecological indicator for shallow lakes. The largest shallow lake, Poyang Lake, as well as the most typical river-connected lake in China was selected as the research area. In view of the complicated water-sediment conditions induced by its frequent water exchange with external rivers, the dominant factors driving water transparency were determined against the field investigated data from 2003 to 2013 and a specific driving function was established. A numerical model coupling suspended sediment, Chl-a and chemical oxygen demand was developed and validated, and the spatial water transparency distributions under three typical current structures in Poyang Lake, Gravity-style, Jacking-style and Backflow-style, were quantitatively estimated. The following results stood out: water transparency in the lake varied distinctly with the current status; Backflow-style current was basically characterized by the lowest water transparency, while that under Jacking-style was the highest due to the lower sediment carrying capacity. In some outlying regions in the lake, where the water current is hardly influenced by the mainstream, the water transparency was always kept at a stable level.

Fluctuations in Cd release from surface-deposited sediment in a river-connected lake following dredging

Lakes connected to rivers have highly fluctuating hydrological conditions and therefore processes involving the retention and release of heavy metals are complex. Jinshan Lake is a typical river-connected lake in China, was selected to estimate Cd distribution in the surface-deposited sediment after dredging. Sediment samples with low, medium and high Cd contents were collected for annular flume experiments. The shear stress-induced Cd releases from these sediments were explored and incorporated into a numerical model using the finite volume method (FVM). The process of Cd release was studied for different temporal scales by numerical simulation. We observed that, because of the adequate water exchange between Jinshan Lake and the Yangtze River, the high-water year featured the largest annual release of Cd at 8777kg, which was 2722kg and 2033kg higher than that of normal and low-water years, respectively. A balance between the upstream runoff and downstream tide resulted in the lowest Cd release in a normal-water year. The Cd release during a tidal cycle fell from 48kg during the flood season to 11kg during the dry period, whereas the variation coefficient increased from 0.59 to 0.67 during this period. Dredging project generally reduced the release of Cd from sediment in Jinshan Lake and this impact became more prominent in the presence of sufficient water exchange and strong bed shear stress.

Macroinvertebrate communities and the relationships with biotic factors in river-connected lakes in the lower reaches of Yangtze River, China.

The relationships of macroinvertebrate communities with abiotic factors in lake and river system were well documented, but their relationships with biotic factors were few explored. The macroinvertebrate communities in two river-disconnected lakes (Poyang Lake and Shijiu Lake) and the rivers connected to Shijiu Lake were investigated to reveal the differences in communities and explore the relationships with biotic factors. A total of 34 species were recorded in the three water bodies. Combined with the previous study results, the dominant species in Poyang Lake is almost unchanged since 1997. Mollusks were dominated in abundance in Poyang Lake and the rivers connected to Shijiu Lake, while oligochaetes dominated the communities in Shijiu Lake. The alpha diversity indices of macroinvertebrates in Poyang Lake and the rivers connected to Shijiu Lake were distinctly higher than that of Shijiu Lake, and the beta diversity index of Shijiu Lake was the highest, supporting the rules that alpha diversity of macroinvertebrates achieves a maximum at a moderate level of connectivity and beta diversity seems to be higher when the connectivity is weaker. The differences in communities were significant between the Poyang Lake and Shijiu Lake, while the differences in the Poyang Lake and the rivers connected to Shijiu Lake were not significant. According to the results of canonical correspondence analysis (CCA), protozoon densities and phytoplankton biomass affected the distribution of macroinvertebrates in Shijiu Lake. Species number of planktonic crustaceans influenced the distribution of macroinvertebrates in rivers connected to Shijiu Lake, while species number of rotifer and densities of phytoplankton distinctly impacted on the distribution of macroinvertebrates in Poyang Lake.

Characteristics of phosphorus sorption at the sediment–water interface in Dongting Lake, a Yangtze-connected lake

Phosphorus in sediments plays an important role in lake eutrophication. This study investigated the phosphorus sorption characteristics of sediments in a river-connected lake and identified the phosphorus source or sink areas in the lake. Sediment samples with different physicochemical characteristics were collected in 2011 and 2013 from Dongting Lake, which is connected to the world's third longest river, the Yangtze. Batch incubation experiments were conducted to determine the phosphorus sorption characteristics of the sediment samples. The sediment sample from Luzui (D3) exhibited the highest sorption capacity in 2013 (0.75 mg g−1) because of its large proportion of fine particles (<63 μm). Results of sorption isotherms illustrated that the modified Langmuir model can effectively describe the sorption isotherms (R2 > 0.91). The calculated results (equilibrium phosphorus concentration, 0.025–0.032 mg L−1) indicated that the estuary from Dongting Lake to the Yangtze River (D1) served as a phosphorus sink because of the large proportion of fine particles and low total phosphorus content in the sediments. The east Dongting Lake (D2 and D3) with a high phosphorus concentration served as a phosphorus source. Yugong Miao (D4) and Nanzui (D5) had a low phosphorus concentration in water because of their location and flow field, respectively.

Mapping Dynamics of Inundation Patterns of Two Largest River-Connected Lakes in China: A Comparative Study

Poyang Lake and Dongting Lake are the two largest freshwater lakes in China. The lakes are located approximately 300 km apart on the middle reaches of the Yangtze River and are differently connected through their respective tributary systems, which will lead to different river–lake water exchanges and discharges. Thus, differences in their morphological and hydrological conditions should induce individual lake spatio-temporal inundation patterns. Quantitative comparative analyses of the dynamic inundation patterns of Poyang Lake and Dongting Lake are of great importance to basic biogeochemical and ecological studies. In this study, using Moderate Resolution Imaging Spectoradiometer (MODIS) satellite imagery and a geographic information system (GIS) analysis method, we systematically compared the spatio-temporal inundation patterns of the two river-connected lakes by analyses of the lake area, the inundation frequencies (IFs) and the water variation rates (WVRs). The results indicate that there was a significant declining trend in the lakes’ inundation area from 2000 to 2011. The inundation areas of Poyang Lake and Dongting Lake, decreased by 54.74% and 40.46%, with an average annual decrease rate of 109.74 km2/y and 52.37 km2/y, respectively. The alluvial regions near Dongting Lake expressed much lower inundation frequencies, averaged over multiple years, than the alluvial regions near Poyang Lake. There was an obvious spatial gradient in the distribution of water inundation for Poyang Lake; the monthly mean IF slowly increased from north to south during the low-water, rising, and high-water periods. However, Dongting Lake expressed a clear zonal distribution of water inundation, especially in the low-water and rising periods. In addition, the WVRs of the two lakes differently changed in space throughout the year, but were in good agreement with the changing processes of water expansion or shrinkage. The different inundation frequencies and water variation rates in the two lakes may possibly depend on many intrinsic factors, including surface discharges from their respective tributaries, river–lake water exchanges and the lakes’ topographical characteristics. These findings are valuable for policymakers because they may lead to different decisions and policies for these two complex river–lake systems.

Optimal operation of a multi-reservoir system for environmental water demand of a river-connected lake

Dongting Lake, a large river-connected lake in the Yangtze River watershed, plays important roles in flood control, drought mitigation, and biodiversity conservation. Its ecosystem has recently been severely affected by upstream water resource development such as reservoir operations. In this study, an optimization model is developed for the operation of a multi-reservoir system, including the Three Gorges Reservoir (TGR) on the upper Yangtze River and 8 major reservoirs on the tributaries to Dongting Lake. The optimal target in pursuit of the ecological objective is to maximize the environmental water demand (EWD) satisfaction of the lake. A support vector regression-based model is used to predict the response of the lake level to reservoir operations. The optimization is carried out under different scenarios for both normal and dry conditions, and the results show that: (i) the existing operation policy could result in significant hydrologic alteration; (ii) in the normal condition, the proposed optimal joint operation policy could increase the general EWD satisfaction rate of Dongting Lake from 85.4% to 95.7%; and (iii) the improvement of EWD satisfaction in the normal condition is mainly affected by the TGR, while in the dry condition, the southern and western lake regions are more sensitive to the tributaries' reservoirs.

Development of the alluvial and lacustrine terraces on the northern margin of the Hetao Basin, Inner Mongolia, China: Implications for the evolution of the Yellow River in the Hetao area since the late Pleistocene

Terrace formation processes can reflect the evolution of large rivers. The Hetao Basin is a floodplain of the Yellow River, and there are two to three terraces that extend E–W on the northern margin. Based on a detailed field geological survey and measurement of the terrace sections, combined with sedimentological and chronological analyses, we reconstructed the developmental processes of the three terraces and inferred that three river terraces were formed by the Yellow River. The OSL dating results show that the time of deposition of terrace 3 was prior to 118kaBP. Subsequently, the terrace began to experience incision and formed the geomorphic surface of terrace T3. Similarly, terrace 2 and terrace 1 were mainly deposited from ~109 and 59ka, respectively; and began to be incised prior to 71 and 46ka, respectively. The three terraces in the northern Hetao Basin are the products of the combined actions of climatic shifts and tectonic uplift. The Yellow River's evolutionary history in the Hetao area since the Late Pleistocene is discussed. Before about 120kaBP, the Yellow River may have flowed through the northern Hetao Basin. Between 110 and 70kaBP, the Yellow River experienced at least two changes between river and lake sedimentation. From 60 to 50ka, a very large river-connected lake of the Yellow River was developed in the Hetao Basin. After 50ka, the Hetao river-connected paleolake of the Yellow River gradually dried.

Algal growth simulation in fluctuating water–sediment circumstances of the largest river-connected lake in China

In this paper, an improved simulation method was developed to investigate the mechanisms of algal growth in Poyang Lake, which is the largest freshwater lake and the most typical river-connected lake in China. In view of its frequent water and sediment exchange with the external rivers, the influences of flow disturbance and suspended sediment concentration on algal growth were quantitatively analyzed based on field investigation and laboratory experiment, and the controlling equations of l(u) and g(S) were established and embedded into the traditional algal growth model for more accurate simulation in Poyang Lake. The improved model was calibrated and validated against the consecutive field monitoring data from March to November in 2012. By simulation, the algal concentration fluctuation in a common-water year and the spatial distribution under two typical current structures: “Gravity Style” and “Jacking Style,” were quantitatively estimated. The results showed that the improved model gave a good approximation of the algal growth processes under the varied water–sediment environment in Poyang Lake. Algal concentrations in the middle area were evidently higher than that in the south and north lake area, while the variation trends were basically the same. Due to the slower current, the algal concentration under “Jacking Style” flow structure was averagely increased by 18.8 % than that under the “Gravity Style.”

Fluctuation of the water environmental carrying capacity in a huge river-connected lake.

A new method, with the non-fully mixed coefficient (NFMC) considered, was put forward to calculate the water environmental carrying capacity (WECC) for huge river-connected lakes, of which the hydrological conditions always vary widely during a year. Poyang Lake, the most typical river-connected lake and the largest freshwater lake in China, was selected as the research area. Based on field investigations and numerical simulation, the monthly pollutant degradation coefficients and non-fully mixed coefficients of different lake regions were determined to explore the WECCs of COD, TN and TP of Poyang Lake in a common water year. It was found that under the hydrological conditions of a common water year the total WECCs of COD, TN and TP in the lake were respectively 181.9 × 104 t, 33.3 × 104 t and 1.86 × 104 t. Due to the varied lake water volume and self-purification ability, an evident temporal fluctuation of WECCs in Poyang Lake was observed. The dry seasons were characterized by a higher NFMCs but lower WECCs owing to the lower water level and degradation ability. Variation coefficients of COD and TN WECC were close to each other, of which the average level was about 58.5%, a little higher than that of TP.

Effects of water level fluctuations on lakeshore vegetation of three subtropical floodplain lakes, China

Vegetation communities in floodplain lakes are adapted to natural water regimes. Construction of sluices or dams between rivers and their floodplain lakes will inevitably lead to great changes in plant communities. In order to evaluate the effects of water level fluctuations (WLFs) on lakeshore vegetation in floodplain lakes of the Yangtze River, a field investigation was conducted in two river-disconnected lakes (Wuchang Lake and Shengjin Lake) and one river-connected lake (Shimen Lake). The results showed that plant species richness was highest in the disconnected Shengjin Lake with intermediate amplitude of WLFs, and lowest in the connected Shimen Lake. Species composition differed among the three lakes and the two disconnected lakes with more similar WLFs exhibiting the greatest similarity. Six plant communities were classified using TWINSPAN. Multivariate analyses showed that the amplitude of WLFs was the most important factor in determining the distribution of lakeshore plants, followed by relative elevation and duration of submergence. Our results provide an eco-hydrological basis for plant restoration in the Yangtze floodplain lakes. We suggest that fluctuating amplitude should be increased in lakes with small WLFs, and small habitats with various hydrological regimes should be created in lakes with large WLFs.

The effects of hydrological extremes on former gravel pit lake ecology: management implications

Urban gravel pit lake ecosystems are particularly susceptible to flooding and anthropogenic nutrient loading, and are therefore likely to be significantly affected by the future projections of precipitation extremes. Six shallow ex-gravel pit lakes at Attenborough Nature Reserve, (Nottinghamshire, U.K.), three connected to the nutrient-rich River Erewash and three isolated from it, were monitored from 2005–2008 (including the highest and 6th lowest rainfall years of a 40-year record). We aimed to compare lake ecological response to hydrological extremes and to see whether river-connected gravel pits responded differently to those fed mainly by groundwater. Our results showed that flood conditions in the river-connected lakes reduced the maximum phytoplankton biomass achieved and favoured smaller taxa (cryptophytes, Scenedesmus spp. and small centric diatoms). Consequently dissolved nitrate (NO3-N) and silica (SiO2) maxima were higher during flood years because of the reduced capacity for algal uptake, but dissolved and particulate phosphorus (P) concentrations were lower because increased flushing rates led to more effective removal of sediment-released P from the lake systems. Lakes isolated from the River Erewash responded less clearly to increased rainfall, except for a delay in the timing of the phytoplankton maximum, and a pronounced rise in SiO2 concentrations after drought conditions. Spatial comparisons also showed that nutrient pollution in the River Erewash and in storm drains feeding into one of the 'isolated' lakes (Beeston Pond) led to the existence of a turbid lake ecosystem state through the increased supply of dissolved NO3-N. The other isolated lakes had lower phytoplankton biomass and extensive aquatic plant coverage, but significantly greater proportions of bloom-forming cyanobacteria (Anabaena, Aphanizomenon), because of greater water retention times. Our results show that ex-gravel pit lakes connected to rivers may be more susceptible to ecological disruption from future flooding events and are heavily influenced by river water quality, but may be less susceptible to potentially toxic cyanobacteria blooms which may be favoured in lakes isolated from rivers with long water retention times. Artificial manipulation of urban lake hydrology to increase flushing with water of a higher quality or isolation of lakes from nutrient-rich inflows may be a useful management strategy, but the effectiveness of either approach depends on the ecological quality of the lake and inflowing river. Therefore, managing urban lakes in the face of climate change and increased urbanisation requires a detailed understanding of lake ecology and hydrology. McGowan, S.; Needham, T.; Pointer, C. M.

A 2700 cal yr BP extreme flood event revealed by sediment accumulation in Amazon floodplains

Climatic conditions are one of the most important factors affecting hydrological processes in fluvial systems. Higher discharges are responsible for higher erosion, greater transport, and also higher deposition. Consequently, sediment accumulation in Amazonia floodplain river-connected lakes can be directly related to hydrological patterns of the Amazon River mainstream. In this context, we analyzed five sediment cores taken in two floodplain systems situated in the lower Amazon River, to investigate sediment accumulation patterns during the Holocene. Our records show abrupt fluctuations in sedimentation rates in lakes that can reach more than 2cm/yr during some periods. We find that in all cores, sediment stratigraphy is characterized by packages of sediments of uniform age, which are typically 10–80cm thick and present a variegated color. The 14C age of the upper package is about 2700calyr BP. During this abrupt event, sediment accumulation rates in floodplain lakes can be at least 200 times higher than those of “normal” periods. This sedimentation event is interpreted as being the consequence of one or several successive extreme floods. The 2700calyr BP event has been also observed in other sites in South America and other regions in the world, although different impacts can be observed in each system. This event probably corresponds to a conjunction of favorable conditions for extreme Amazon discharge associated with the Middle to Late Holocene increase of austral summer insolation and shifts of the Intertropical Convergence Zone (ITCZ) from northern to southern positions. In this context, a marked negative peak in solar irradiance at 2700calyrs BP seems to have provoked cooling on the continents and a southward shift of the ITCZ associated with a probable reduction in the Atlantic Meridian Overturning Circulation.

A floodplain-scale lake classification based on characteristics of macroinvertebrate assemblages and corresponding environmental properties

Floodplain lakes have been experiencing great pressures by human activities, and ecological functions in different types of lakes show different degrees of degradation. For facilitating conservation and management of different types of floodplain lakes, it is necessary to classify the lakes into similar groups according to certain standards. In this study, on basis of consideration of macroinvertebrate assemblages and corresponding environmental properties, the Yangtze floodplain lakes were classified into three major types grouping five groups of lakes: (1) river-disconnected lakes (algal lakes, macrophytic-algal transition lakes, and macrophytic lakes), (2) semi-connected lakes (oxbow lakes), (3) river-connected lakes. The classification of floodplain lakes mainly reflects the gradients of trophic and hydrological connectivity. The key factors structuring macroinvertebrate assemblages in the Yangtze floodplain lakes were mainly hydrological (connectivity rating, water depth), trophic (total phosphorus, macrophytes biomass) and morphometric (development of lake shoreline). Among the floodplain lakes, ecological status of river-connected lakes, where biodiversity, biomass and production of macroinvertebrates reached maxima, has been confirmed to be the best. From the view of conservation and management of the entire floodplain lakes, it is suggested that protecting the remnants of river-connected lakes, controlling eutrophication and linking disconnected lakes freely with the mainstream are crucial.