Spatial Variability in Water Quality and Surface Sediment Diatom Assemblages in a Complex Lake Basin: Lake of the Woods,Ontario, Canada

Lake of the Woods (LOW) is an international waterbody spanning the Canadian provinces of Ontario and Manitoba, and the U.S. state of Minnesota. In recent years, there has been a perception that water quality has deteriorated in northern regions of the lake, with an increase in the frequency and intensity of toxin-producing cyanobacterial blooms. However, given the lack of long-term data these trends are difficult to verify. As a first step, we examine spatial and seasonal patterns in water quality in this highly complex lake on the Canadian Shield. Further, we examine surface sediment diatom assemblages across multiple sites to determine if they track within-lake differences in environmental conditions. Our results show that there are significant spatial patterns in water quality in LOW. Principal Component Analysis divides the lake into three geographic zones based primarily on algal nutrients (i.e., total phosphorus, TP), with the highest concentrations at sites proximal to Rainy River. This variation is closely tracked by sedimentary diatom assemblages, with [TP] explaining 43% of the variation in diatom assemblages across sites. The close correlation between water quality and the surface sediment diatom record indicate that paleoecological models could be used to provide data on the relative importance of natural and anthropogenic sources of nutrients to the lake.     

Concentrations and loads of nutrients and major ions in the Niagara River, 1975–2018

Environment and Climate Change Canada has monitored Niagara River water quality in support of the Great Lakes Water Quality Agreement since establishing a fixed site at Niagara-on-the-Lake in 1975. Using over 40 years of data from this site along with the Fort Erie location added in 1983, we examine the status and trends of concentrations and loadings of nutrients and major ions and assess evidence of sources between the two stations. Trends were observed for the majority of measured parameters and there is strong agreement between trends in concentrations and loadings which are generally higher at the downstream site; however, upstream/downstream differences indicate relatively little loading occurs along the length of the river itself. For total phosphorus (TP), inputs from Lake Erie via the Niagara River account for the majority of loading to Lake Ontario and, in some years, exceed the 7000 MTA Lake Ontario target. Between 2014 and 2018, we calculate the mean Niagara River TP loading to be 5275 MTA. We highlight the major changes in water quality constituents over time, including TP, and reveal increased seasonal consumption of TP and SiO₂, reflecting potential increases in the biological productivity in Lake Erie. The long and rich Niagara River dataset, which comprises year round sampling (including rare winter data), provides detailed tracking of changing Great Lakes water quality and could be further utilized to assess the impacts of climate change, improve understanding of diatom and harmful algal bloom dynamics, and enhance knowledge of in-lake major ion and nutrient dynamics.     

淀山湖富营养化现状及生态修复措施分析

根据2008—2010年淀山湖水质监测数据,评价该湖富营养化趋势。结果显示,2008年后,淀山湖的富营养化程度呈改善趋势,但水体富营养化存在区域差异。主要表现在:①2008—2010年的整个湖区水体中主要的富营养化指标TN、TP、SS、Ch-la质量浓度均呈下降趋势;②淀山湖上游来水区水体中TN、TP质量浓度比下游出水区高,综合营养状态指数同样也高;③淀山湖每年夏季暴发蓝藻水华的水域面积在不断减小。结果表明,淀山湖东南区域富营养化程度最轻,水生植物对水体净化、生态系统恢复具有重要价值;淀山湖的生态系统在逐步恢复。为使生态系统能巩固并形成良性循环,一些影响因素亟待解决。     

DISTRIBUTION AND ABUNDANCE OF STREAM FISHES IN RELATION TO BARRIERS: IMPLICATIONS FOR MONITORING STREAM RECOVERY AFTER BARRIER REMOVAL

Dams are ubiquitous in coastal regions and have altered stream habitats and the distribution and abundance of stream fishes in those habitats by disrupting hydrology, temperature regime and habitat connectivity. Dam removal is a common restoration tool, but often the response of the fish assemblage is not monitored rigorously. Sedgeunkedunk Stream, a small tributary to the Penobscot River (Maine, USA), has been the focus of a restoration effort that includes the removal of two low‐head dams. In this study, we quantified fish assemblage metrics along a longitudinal gradient in Sedgeunkedunk Stream and also in a nearby reference stream. By establishing pre‐removal baseline conditions and associated variability and the conditions and variability immediately following removal, we can characterize future changes in the system associated with dam removal. Over 2 years prior to dam removal, species richness and abundance in Sedgeunkedunk Stream were highest downstream of the lowest dam, lowest immediately upstream of that dam and intermediate farther upstream; patterns were similar in the reference stream. Although seasonal and annual variation in metrics within each site was substantial, the overall upstream‐to‐downstream pattern along the stream gradient was remarkably consistent prior to dam removal. Immediately after dam removal, we saw significant decreases in richness and abundance downstream of the former dam site and a corresponding increase in fish abundance upstream of the former dam site. No such changes occurred in reference sites. Our results show that by quantifying baseline conditions in a small stream before restoration, the effects of stream restoration efforts on fish assemblages can be monitored successfully. These data set the stage for the long‐term assessment of Sedgeunkedunk Stream and provide a simple methodology for assessment in other restoration projects. Copyright © 2011 John Wiley & Sons, Ltd.     

Multi‐dimensional effects on Cladoceran (Crustacea,Anomopoda) assemblages in two cascade reservoirs in Southeast Brazil

The Cladocera assemblages in two cascade reservoirs located in the Paranapanema River in Brazil were studied during two consecutive years. Upstream Chavantes Reservoir is an accumulation system, with a long water retention time, high depth and oligo‐mesotrophic status. The downstream Salto Grande Reservoir is a small, run‐of‐river reservoir, with a short water retention time, shallow depth and meso‐eutrophic status. The goal of this study was to determine the inter‐ and intra‐reservoir limnological differences with emphasis on the Cladocerans assemblages. The following questions were posed: (i) what are the seasonal dynamics of the reservoir spatial structures; (ii) how dynamics, seasonally, is the reservoirs spatial structure; and (iii) are the reservoir independent systems? A total of 43 Cladoceran species were identified in this study. Ceriodaphnia silvestrii was the most abundant and frequent species found in Chavantes Reservoir, while C. cornuta was most abundant and frequent in Salto Grande Reservoir. The Cladoceran species richness differed significantly among sampling sites for both reservoirs. In terms of abundance, there was a significant variation among sampling sites and periods for both reservoirs. A cluster analysis indicated a higher similarity among the deeper compartments, and the intermediate river‐reservoir zones was grouped with the riverine sampling sites. For the smaller Salto Grande Reservoir, the entrance of a middle size tributary causes major changes in the system. A distinct environment was observed in the river mouth zone of another small tributary, representing a shallow environment with aquatic macrophyte stands. A canonical correlation analysis between environmental variables and Cladoceran abundance explained 75% of the data variability, and a complementary factorial analysis explained 65% of the variability. The spatial compartmentalization of the reservoirs, as well as the particular characteristics of the two study reservoirs, directly influenced the structure of the Cladoceran assemblages. The conditions of the lacustrine (dam) zone of the larger Chavantes Reservoir were reflected in the upstream zone of the smaller downstream Salto Grande Reservoir, highlighting the importance of plankton exportation in reservoir cascade systems. The comparative spatial–temporal analysis indicated conspicuous differences between the two reservoirs, reinforcing the necessity of considering tropical/subtropical reservoirs as complex, multi‐compartmental water systems.     

Effect of dam removal on habitat use by spawning Atlantic salmon

By impeding migration and degrading habitat downstream, dam construction has caused population declines in many migratory fish populations. As part of the landlocked Atlantic salmon (Salmo salar) restoration program in Lake Champlain, the Willsboro Dam was removed from the Boquet River, NY in 2015 providing an opportunity to study the effects of dam removal on spawning habitat quality and availability. Spawning habitat surveys were conducted downstream of the dam site in 2014, 2016 and 2017, and in historical spawning grounds upstream in 2016 and 2017. The habitat used was characterized by measuring depth, water velocity, and substrate size at each redd. Mean habitat use did not differ between upstream and downstream sites for any variables in 2016 and only differed for depth in 2017. However, the variance in depth and substrate used for spawning were lower at the upstream site in 2016, likely due to an abundance of habitat. In the downstream site, the mean and variance in depth at redds decreased after dam removal as did the variance in substrate size, increasing the habitat suitability of redds. When compared to literature data, habitat used upstream of the former dam was of medium quality in both 2016 and 2017, and improved downstream from low to medium quality in both column velocity and substrate size after dam removal. This study illustrates that positive shifts in the quality of habitat used can occur rapidly following dam removal by allowing access to suitable spawning habitat upstream and improving habitat downstream.     

THE MANAGEMENT MODES OF SEASONAL FLOODS AND THEIR IMPACT ON THE RELATIONSHIP BETWEEN CLIMATE AND STREAMFLOW DOWNSTREAM FROM DAMS IN QUEBEC (CANADA)

The goal of the study was to compare the modes of management of seasonal floods for different dams and to constrain their impact on the relationship between climate variables and streamflow downstream from the dams. At the Rawdon dam, downstream from which the Ouareau River is characterized by a natural‐type regulated flow regime, a ‘type A’ flood management mode prevails, in which the same rainfall and/or snowmelt events account for seasonal floods both in the unregulated (natural) stretch of river upstream from the dam and in the river downstream from the dam. As a result, seasonal floods in the natural setting and downstream from the dam are nearly synchronous. In contrast, downstream from the Matawin dam (Matawin River), which produces an inversion‐type regulated flow regime, the prevalent flood management modes are of types B and D, whereby seasonal floods observed upstream and downstream from the dam are not caused by the same rainfall and/or snowmelt events and, as a result, are not synchronous. This difference in seasonal flood management modes affects the interannual variability of the magnitude of seasonal daily maximum flows related to the seasonal floods. Thus, the interannual variability of these flows downstream from the Matawin dam differs significantly from that of flows upstream. No correlation is observed between climate variables and streamflow downstream from the Matawin dam. This absence of correlation disappears gradually at the annual scale, at which streamflow is correlated with rainfall, as is observed upstream from the dam. Copyright © 2013 John Wiley & Sons, Ltd.     

Influence of extreme drawdown on water quality downstream of a large water storage reservoir

The Serial Discontinuity Concept (SDC) proposes that dams have the potential to affect the downstream ecological condition of rivers. While the SDC was developed principally around changes to physical habitat or temperature, reservoirs also have the potential to impact on downstream water quality, including algal community structure. In the current study we examined the impacts of an extreme drawdown event on nutrient loads and algal community structure downstream of a large water storage reservoir in south‐eastern Australia—Lake Hume. The lake was a net exporter of carbon, nitrogen, phosphorus and iron during the study period and was a net sink for manganese. Most of the carbon, nitrogen and phosphorus exported from the lake was in the form of algal biomass. Processes in the lake also influenced the downstream algal community structure. Upstream of the reservoir green algae were the most dominant species; within and downstream of the reservoir cyanobacteria dominated. Much of the algal biomass found at the downstream sites appeared to originate in Lake Hume and was physically transported downstream. Copyright © 2009 John Wiley & Sons, Ltd.     

Characterization of Surface-Associated Protozoan Communities in a Lake Erie Coastal Wetland (Old Woman Creek,Ohio)

Examination of surface-associated protozoa in Great Lakes coastal marshes has been minimal, although they play crucial roles in nutrient and energy transfer in similar systems. Spatio-temporal patterns of artificial substrate-associated protist and bacterial abundances are related to physico-chemical conditions at this “freshwater estuary.” Assemblages were sampled monthly (May-October) at upstream, downstream, and at two mid-estuarine sites. A total of 278 protistan taxa were observed, and greatest richness occurred in late June. Richness at the estuary mouth was consistently higher than upstream, possibly due to the influence of Lake Erie assemblages and variability in hydrology-driven habitat characteristics. PCA yielded two components accounting for 90% of total variance in species by sites data. Species abundances at the two mid-estuarine sites were most highly correlated, and taxa seasonally abundant at any one site tended to be abundant at others. Presence/absence data were used to calculate community similarity indices, and a permutation-computed test statistic compared the mean similarity of triplicate substrates in one month (at a given site) to the between-month similarity. Significant differences (α = 0.05) between some months (site by site) were observed, with a unique assemblage noted in October. Following classification into six functional feeding groups, bactivores (mainly heterotrophic microflagellates) and primary producers (mainly cryptomonads and chlamydomonads) dominated assemblages. Organisms in other functional groups comprised only 10–15% of observed taxa. A preponderance of bactivorous taxa is consistent with observations of a dense and metabolically active bacterial community associated with abundant paniculate organic matter in easily -resuspended surficial sediments.     

贵州草海水质时空变化和水体营养状况

为了解和改善草海水质现状,于2017年分季度监测了12个样点的11项水质指标,并分别采用综合营养状态指数法(TLI)和ArcGIS软件对全湖水体进行了营养状况评价和分区,根据《地表水环境质量标准》(GB 3838—2002)对水质进行了类别划分。研究表明:①除pH值和溶氧(DO)外,其他指标季节差异显著,悬浮物(SPM)在秋季最高,TN,TP浓度在春季最高,COD_Mn,NH₄⁺-N和Chl.a浓度在夏季最高;TN,TP,NH₄⁺-N和SPM浓度从上游到下游逐渐降低;②草海水体呈中营养-轻度富营养化(45.3≤TLI(∑)≤57.7);根据TLI(∑)值将全湖分为入水口(轻度富营养化)、近县城和入水口(主要为轻度富营养化)、湖中部(中营养-轻度富营养化)、中下游至近出水口(中营养水平)4个区;③草海水质主要为 Ⅱ— Ⅳ类,TN和COD_Mn是影响水质类别的主要指标。因此,草海水质保护的关键是周边污水排放治理,重点区域是湖区上游和入湖河流。     

Peaking hydropower and fish assemblages: an example from the Tallapoosa River,AL

Dams alter many aspects of riverine environments and can have broad effects on aquatic organisms and habitats both upstream and downstream. While dams and associated reservoirs can provide many services to people (hydropower, recreation, flood control, and navigation), they can also negatively affect riverine ecosystems. In particular, hydropeaking dams affect downstream fish habitats by increasing variability in discharge and temperature. To assess the effects of Harris Dam on the Tallapoosa River, AL, operating under an adaptive management plan implemented in 2005, we sampled fish for community analyses from four sites on the river: three in the regulated reach downstream of the dam, and one unregulated site upstream. Fish were collected every other month using boat/barge electrofishing. We used Shannon's H, nonmetric multidimensional scaling (NMDS), a multiresponse permutation procedure (MRPP), and indicator species analysis to quantify patterns in fish assemblage structure and determine how assemblages varied among sites. NMDS and MRPP indicated significant fish assemblage differences among sites, with the tailrace fish assemblage being distinct from the other downstream sites and sites becoming more similar to the upstream, unregulated site (relative to fish assemblages) with distance downstream of the tailrace. The tailrace fish assemblage included higher proportions of rheophilic species that may be better suited to variable and/or high flows. Altered fish assemblages demonstrated continued effects of Harris Dam on the downstream aquatic systems, particularly close to the dam. These effects may indicate that further mitigation should be considered depending on conservation and management goals.     

Phytoplankton biomass and primary production dynamics in Lake Kariba

The present study examined spatial, seasonal and depth variations in phytoplankton biomass and primary production (PP), compared with those reported for other tropical African lakes, to determine whether or not measured phytoplankton changes might be linked to climate warming. The biomass of three major phytoplankton classes (Cyanophyceae; Chlorophyceae; Bacillariophyceae) and net PP were measured during the midwinter and midsummer at six different depths at 35 sampling sites distributed across the lake’s five basins. A more rigorous sampling regime was used in the fifth basin, with phytoplankton biomass and PP rates measured every second month over a 24 month period at six different depths at ten sampling sites located in riverine and lacustrine waters. Cyanophyceae, which displayed a gradient of decreasing biomass from Basins 2 to 5, contributed 69% of the total phytoplankton biomass in the lake’s five basins during summer. This percentage was approximately four times greater than that contributed by the Bacillariophyceae and about ten times greater than that contributed by the Chlorophyceae. During winter, Bacillariophyceae biomass was equivalent to that of the Cyanophyceae, but displayed an opposing gradient of increasing biomass from Basins 1 to 3, with a subsequent biomass decline from Basins 3 to 5. Chlorophyceae exhibited no distinct biomass gradient across the five lake basins, being undetectable during winter. The biomass of all three phytoplankton classes and the net PP varied in magnitude and direction monthly between the lacustrine and riverine waters, with increasing water depth and with no distinct seasonal patterns being evident. The monthly variations in biomass were related to the thermal stratification cycle, hydrological gradients and the extent of water mixing, being similar to those reported for other tropical African lakes. It is noteworthy that total phytoplankton biomass and PP in Lake Kariba have declined by about 95% and 50%, respectively, since the 1980s. These declines correspond to an upward shift in the depth of the thermocline, associated with an average temperature increase of 1.9 °C and a 50% reduction in the depth of the euphotic zone, since the 1960s.     

Multi-run migratory behavior of adult male lake sturgeon in a short river

Lake sturgeon (Acipenser fulvescens) can migrate long distances to spawn, but many populations currently spawn in systems where the length of accessible riverine migratory habitat has been greatly reduced by dam construction. With the increased prevalence of shortened rivers, focusing on migratory dynamics in short rivers (<30 km) is beneficial to understanding the migratory needs of lake sturgeon populations. Here we document male lake sturgeon movements during the spawning period in the Winooski River, Vermont, USA; a river with only 17 km to the first natural upstream barrier. Male lake sturgeon were acoustically tagged (n = 25, 1215–1470 mm TL) and tracked using five to nine stationary receivers from 2017 to 2019. River discharge, temperature, the lagged effect of temperature (3-day), and time of day were significant factors describing upstream movements of tagged fish. Migrating male lake sturgeon (n = 10 in 2017, n = 18 in 2018, and n = 17 in 2019) displayed general movement patterns during the spawning period that included a single run upstream to the spawning site (60%), upstream and downstream movements throughout the river during the season (20%), or multiple runs made up the entire length of the spawning tributary to the spawning site (20%). No multi-run males were observed during 2018 when discharge was less flashy (i.e., fewer steep increases and declines in discharge) than in 2017 and 2019. These results suggest that the prevalence of multi-run spawning behavior of male lake sturgeon is related to flow conditions.     

An analysis of MODIS-derived algal and mineral turbidity in Lake Erie

Satellite-derived estimates of chlorophyll concentrations based on colour ratio algorithms traditionally fail in turbid waters such as those found in Lake Erie, resulting in chlorophyll concentrations often orders of magnitude in error and spatial distributions mirroring that of known suspended sediment distributions. Methods are presented here that were used to simultaneously extract algal and mineral suspended particulate matter for Lake Erie from the red and near-infrared bands of NASA's MODIS-Aqua sensor. Results produced spatially and temporally distinct seasonal cycles in agreement with bio-geo-physical processes on the lake. Derived imagery was used to monitor seasonal cycles of both algal and mineral particulate matter on the lake and determine areas of persistently elevated concentrations that may highlight regions of potential water quality concern.     

A spatio‐temporal water quality assessment of the Beas and Sutlej Rivers at the Harike Wetland: A Ramsar site in Punjab,India

Lakes play a vital role in regulating water storage, flow of river water, and ultimately maintaining a balanced ecosystem. Spatial and temporal variations in physicochemical parameters of water in Harike Wetland, a Ramsar site in the northwestern state of Punjab, India, were studied. This study was conducted on a monthly basis from January to December 2015. The water quality was studied at ten locations from sites 1 to 10 upstream, central and downstream from Harike Lake for ten physicochemical parameters, including temperature, pH, electrical conductivity, turbidity, dissolved oxygen concentration biological oxygen demand, nitrate and phosphate concentrations and salinity. The findings of this study revealed that, except for temperature and pH, all parameters exhibited relatively higher values for the Sutlej River, compared with the Beas River, with sampling sites 5 to site 7 exhibiting intermediate results. The mean seasonal temperature variations ranged from 16.9 to 26.6 °C, the pH from 7.7 to 8.2, electrical conductivity from 223 to 303 μS cm^?1 and TDS concentration from 148.7 to 180.4 ppm. Correlation analysis was conducted to assess the relations between the variables. The electrical conductivity exhibited a high positive correlation with salinity and biological oxygen demand, whereas it correlated negatively with the dissolved oxygen concentration. Box and whisker plots were also plotted for the study results to better examine the data distribution.     

Implications of an invasive fish barrier for the long‐term recovery of native fish assemblages in a previously degraded northeastern Illinois River system

Barriers to fish movement have been used to prevent the spread of invasive fishes but may also limit the movements of native fishes. We evaluated the potential consequences of a proposed barrier on the Illinois River Waterway, meant to inhibit the spread of silver and bighead carps, to the continued recovery of native fishes in the Des Plaines River following water quality improvements. We compared changes in upstream cumulative species richness and community structure from 1983 to 2013 in the DuPage River, an adjacent tributary with an impassable dam, to the area upstream of a newly proposed barrier on the Des Plaines River where fish can currently pass through a navigational lock. Fewer species displayed truncated distributions upstream of the passable lock and dam (n = 18) compared with the impassable dam (n = 23). Due to water quality improvements in the Illinois River as a whole, cumulative species richness downstream of both dams steadily increased over time. Richness also increased upstream of the passable dam but plateaued upstream of the impassable dam. Fifteen to 18 species accounted for differences in community structure between areas downstream and upstream of either dam. Most species (78–100%) were found in greater relative abundance downstream of the impassable dam, and only 53% were found in greater relative abundance downstream of the passable dam. The truncation in species richness and abundance at the impassable dam foreshadows the potential consequences of an indiscriminate barrier on native fishes and the continued recovery of native assemblages.     

Zooplankton partitioning in a tropical alkaline–saline endorheic Lake Nakuru,Kenya: Spatial and temporal trends in relation to the environment

Spatial and temporal zooplankton variations were studied for 1 year in tropical alkaline–saline Lake Nakuru to determine how they partition in the habitat, relative to environmental variables. Monthly samples were collected at 10 sampling sites, with subsurface tows, using 33.5‐μm mesh plankton nets. Physicochemical parameters displayed clear seasonal variations associated with precipitation patterns. Nine species, belonging to two main zooplankton taxonomic groups (ciliates; rotifers), were identified in the samples. Brachionus dimidiatus dominated the samples, accounting for 80% of the total zooplankton abundance. Kruskall–Wallis tests indicated significant (P < 0.05) temporal and spatial variations among all taxonomic groups. Different zooplankton species displayed a clear succession throughout the year. The total abundance of the rotifers and ciliates peaked at sampling sites near inlets during the long rainy seasons, while those in the inshore sites displayed variable succession patterns. Spatiotemporal structure of the zooplankton assemblages, and its correlation with environmental variables, indicated each species displayed distinct niche‐based partitioning. The ciliates niche was associated with increasing soluble reactive phosphorus, total phosphorus and nitrite–nitrogen (NO₂–N) concentrations. Niche partitioning in rotifers was associated with nitrate–nitrogen (NO₃–N), conductivity and pH. These results indicate physical niche separation, even in a small, relatively homogenous lake among species of rotifers and ciliates, providing information from which future changes in their abundance and spatial distributions can be predicted, given continuous water quality changes.     

Paleolimnological evidence of the effects of recent cultural eutrophication during the last 200 years in Lake Malawi,East Africa

To assess the long-term water quality changes in Lake Malawi, paleolimnological reconstructions of four radiometrically dated sediment cores collected in 1997/98 along a longitudinal transect of the lake were based on preserved diatom assemblages, as well as stratigraphies of organic carbon, total nitrogen, total phosphorus and biogenic silica concentrations. Population growth, deforestation and intensive agriculture, especially in the southern catchments, have accelerated soil erosion causing rivers to transport greater sediment and nutrient loads into Lake Malawi. Southern cores contain evidence of nutrient enrichment starting around 1940, as indicated by increased silica, carbon and nitrogen influxes. By 1980, increased rates of sedimentation, phosphorus influx, diatom influx, and relative abundances of eutrophic diatom taxa are attributable to accelerated nutrient enrichment accompanying soil erosion. The succession of diatoms in southern Lake Malawi begins with dominance by Aulacoseira nyassensis and Fragilaria africana, followed by a shift in 1980 towards Stephanodiscus, Cyclostephanos and Nitzschia, diatom taxa that have reduced silica requirements. Paleoecological results indicate that patterns of diatom assemblage change are not uniform lake-wide. Evidence of eutrophication extends to central Lake Malawi, but is not observable in the paleo-record from the deeper northern basin. The recent cultural eutrophication of the southern lake has impacted the biogeochemical cycling of silica, the available silica to phosphorus ratios, and the diatom communities of a large portion of Lake Malawi. These results provide a warning that eutrophication of this great lake is underway and will continue unless changes are made to current land use practices within the lake's catchment.     

Long-term Trends in the Seasonal Cycle of Great Lakes Water Levels

Numerous long-term trends in the rate-of-change in monthly mean Great Lakes water levels are identified for the period 1860 to 1998. Statistically significant trends are found for 2, 4, 5, and 7 months of the year for Lakes Superior, Michigan-Huron, Erie, and Ontario, respectively. Many of the trends translate into large changes in net water flux (600 to 1,700 m³/s). In each case, significant positive trends are roughly offset by negative trends during other times of the year. Together with similar trends in monthly lake level anomalies (deviations from the annual mean), these trends indicate important changes in the seasonal cycle of Great Lakes water levels. Specifically, Lakes Erie and Ontario are rising and falling (on an annual basis) roughly one month earlier than they did 139 years ago. Maximum lake levels for Lake Superior are also slightly earlier in the year, and the amplitude of the seasonal cycle of Lake Ontario is found to increase by 23% over the 139-year period. Some of the changes are consistent with the predicted impacts of global warming on spring snowmelt and runoff in the Great Lakes region. Other potential contributors to the observed trends include seasonal changes in precipitation and humaninduced effects such as lake regulation and changes in land use.     

Hydrological alterations as the major driver on environmental change in a floodplain Lake Poyang (China): Evidence from monitoring and sediment records

Due to the lack of long-term records on shallow lake environmental change, knowledge of the processes and mechanisms behind the limnological response of many shallow floodplain lakes to hydrological alterations and nutrient loading is often limited. We examined seasonal monitoring data and a dated sediment core from Lake Poyang, a large floodplain lake located on the Yangtze floodplain in the SE China. Multivariate analysis based on contemporary data (diatoms and water quality) revealed that the seasonal changes in the diatom assemblage of the lake were correlated with water temperature and Secchi depth (SD), although the weak spatial effect was not negligible. During the dry winter season, low water temperature, low SD, and high nutrient levels, were accompanied by high abundances of planktonic Aulacoseira species along with Stephanodiscus hantzschii, a species well adapted to cold and eutrophic waters. During the summer wet season, however, when water temperature and SD were high and nutrient levels low, benthic and epiphytic diatoms, such as the genus Achnanthes, dominated. Sediment records of diatoms and geochemistry were used to estimate long-term variation in the ecological condition of the lake. During the past ~60 years, the lake has shifted from a natural hydrologically connected, oligotrophic lake dominated by benthic and epiphytic diatoms to a poorly hydrologically connected, eutrophic state driven by nutrient-tolerant planktonic and eutrophic diatoms. Furthermore, our results indicate that the proposed Poyang dam may severely affect the water quality and ecosystem of the lake by altering its seasonal hydrology.