An algorithm to retrieve chlorophyll,dissolved organic carbon,and suspended minerals from Great Lakes satellite data

An algorithm that utilizes individual lake hydro-optical (HO) models has been developed for the Great Lakes that uses SeaWiFS, MODIS, or MERIS satellite data to estimate concentrations of chlorophyll, dissolved organic carbon, and suspended minerals. The Color Producing Agent Algorithm (CPA-A) uses a specific HO model for each lake. The HO models provide absorption functions for the Color Producing Agents (CPAs) (chlorophyll (chl), colored dissolved organic matter (as dissolved organic carbon, doc), and suspended minerals (sm)) as well as backscatter for the chlorophyll, and suspended mineral parameters. These models were generated using simultaneous optical data collected with in situ measurements of CPAs collected during research cruises in the Great Lakes using regression analysis as well as using specific absorption and backscatter coefficients at specific chl, doc, and sm concentrations. A single average HO model for the Great Lakes was found to generate insufficiently accurate concentrations for Lakes Michigan, Erie, Superior and Huron. These new individual lake retrievals were evaluated with respect to EPA in situ field observations, as well as compared to the widely used OC3 MODIS retrieval. The new algorithm retrievals provided slightly more accurate chl values for Lakes Michigan, Superior, Huron, and Ontario than those obtained using the OC3 approach as well as providing additional concentration information on doc and sm. The CPA-A chl retrieval for Lake Erie is quite robust, producing reliable chl values in the reported EPA concentration ranges. Atmospheric correction approaches were also evaluated in this study.     

Satellite remote sensing reveals impacts from dam‐associated hydrological changes on chlorophyll‐a in the world's largest desert lake

We present an approach that uses satellite products to derive models for predicting lake chlorophyll from environmental variables, and for investigating impacts of changing environmental flows. Lake Turkana, Kenya, is the world's largest desert lake, and environmental flows from the Omo River have been modified since 2015 by the Gibe III dam in Ethiopia. Using satellite remote sensing, we have evaluated the influence of these altered hydrological patterns on large‐scale lake phytoplankton concentrations for the first time. Prior to dam completion, strong seasonal cycles and large spatial gradients in chlorophyll have been observed, related to natural fluctuations in the Omo River's seasonal discharge. During this period, mean lake chlorophyll showed a strong relationship with both river inflows and lake levels. Empirical models were derived which considered multiple hydro‐climatic drivers, but the best model for predicting chlorophyll‐a was a simple model based on Omo River discharge. Application of this model to data for 2015–2016 estimated that during the filling of Gibe III annual mean Lake Turkana chlorophyll declined by 30%. Future water management scenarios based on Gibe III operations predict reduced seasonal chlorophyll‐a variability, while irrigation scenarios showed marked declines in chlorophyll‐a depending on the level of abstraction. These changes demonstrate how infrastructure developments such as dams can significantly alter lake primary production. Our remote sensing approach is easy to adapt to other lakes to understand how their phytoplankton dynamics may be affected by water management scenarios.     

基于卫星测高和实测数据的博斯腾湖水位变化分析

湖泊水位是湖泊水文观测必不可缺的要素,直接关系到湖泊物质交换和能量平衡,对研究湖泊运动和区域气候环境变化至关重要。为了掌握内陆湖泊水位的变化过程和空间特征,以新疆博斯腾湖为例,综合Jason-1&2、ENVISat&ERS、ICESat-1、ICESat-2等卫星测高资料,提取博斯腾湖湖泊水域瞬时水位和日均水位,并根据Hydroweb水位记录和1975-2020年博斯腾湖湖泊水位观测及水域面积数据,检验Jason-1&2、ENVISat&ERS、ICESat-1、ICESat-2测高数据的估计精度。借助趋势面分析方法,分析博斯腾湖水域水位变化的空间差异和特征。结果表明：Hydroweb水位记录、Jason-1&2、ENVISat&ERS、ICESat-1、ICESat-2卫星资料估计湖泊日均水位的绝对误差分别为0.24、0.34、0.28、0.18、0.08 m; 2020年博斯腾湖年均水位为1 048.10±0.12 m,与1975年年均水位相比增加了0.70±0.15 m;湖泊瞬时水位在空间尺度上存在一定水位差,ICESat-2测高数...     

A band-ratio algorithm for retrieving open-lake chlorophyll values from satellite observations of the Great Lakes

The U.S. Environmental Protection Agency's Great Lakes National Program Office (GLNPO) has collected water quality data from the five Great Lakes annually since 1993. We used the GLNPO observations made since 2002 along with coincident measurements made by the Sea-viewing Wide Field-of-View Sensor (SeaWiFS) and the Moderate-resolution Imaging Spectroradiometer (MODIS) to develop a new band-ratio algorithm for estimating chlorophyll concentrations in the Great Lakes from satellite observations. The new algorithm is based on a third-order polynomial model using the same maximum band ratios employed in the standard NASA algorithms (OC4 for SeaWiFS and OC3M for MODIS). The sensor-specific coefficients for the new algorithm were obtained by fitting the relationship to several hundred matched field and satellite observations. Although there are some seasonal variations in some lakes, the relationship between the observed chlorophyll values and those modeled using the new coefficients is fairly stable from lake to lake and across years. The accuracy of the satellite chlorophyll estimates derived from the new algorithm was improved substantially relative both to the standard NASA retrievals and to previously published algorithms tuned to individual lakes. Monte-Carlo fits to randomly selected subsets of the observations allowed us to estimate the uncertainty associated with the retrievals purely as a function of the satellite data. Our results provide, for the first time, a single simple band ratio method for retrieving chlorophyll concentrations in the offshore “open” waters of the Great Lakes from satellite observations.     

Stimulation of Lake Michigan Plankton Metabolism by Sediment Resuspension and River Runoff

Previous work during a major sediment resuspension event (March 1988) in southern Lake Michigan demonstrated that nutrients and carbon derived from resuspended sediment stimulated intense winter heterotrophic production while simultaneously decreasing light availability and autotrophic biomass. However, the role of riverine inputs on plankton metabolism remained unclear. Here we present results from a simulated enrichment experiment (March 2000) designed to examine the influence of resuspended sediments and riverine inputs on Lake Michigan plankton dynamics. Lake water amended with realistic levels of river water, coastal resuspended sediment and river water + sediment all showed enhanced heterotrophic bacterial production and plankton respiration rates, relative to the lake water control. Bacterial production increased by approximately 4× in river water treatments and by a factor of 2.5× for the sediment only treatment compared to lake water controls. Rates of net primary production were stimulated by river water (8.5×) and resuspended sediment (3×), but most by a combination of river water + sediments (11×). Community respiration showed a similar response with rates approximately 8x higher in river water amendment treatments and 3.5× higher in the sediment treatment. Extrapolating experimentally determined production rates to the southern Lake Michigan basin indicated that heterotrophic and autotrophic production in this nearshore region may be enhanced by as much as 3× and 5.2× due to these source inputs. Indeed, field measurements throughout southern Lake Michigan from 1998–2000 support these experimental results. Experimental and field observations suggest that both seasonal riverine inputs and episodic resuspended sediments influence the regional scale ecosystem metabolism and biogeochemistry in Lake Michigan.     

Using satellite observations to assess the spatial representativeness of the GLNPO water quality monitoring program

The U.S. EPA's Great Lakes National Program Office (GLNPO) annual water quality survey (WQS) collects data at a relatively small number of stations in each lake. The survey was designed to measure conditions in the open-water regions of the lakes where an assumption of spatial homogeneity was thought likely to be met and the measured variables could be characterized by simple statistics. Here we use satellite observations to assess how well statistics based on samples collected in the GLNPO sampling network represent the lake-wide values of two variables, surface chlorophyll concentration and Secchi depth. We find strong linear relationships between the mean values calculated from the samples and the corresponding averages based on the subsets of the full satellite images. Although overall the means of the values from the sample locations agree well with means calculated from most of the non-coastal regions of the lakes, in terms of water depth, the GLNPO station averages best represent the regions of Lake Huron deeper than 30?m, of Lakes Michigan and Superior deeper than 90?m, and of Lake Ontario deeper than 60?m. When the lake regions are defined by distance offshore rather than by depth, the GLNPO station chlorophyll means in Lakes Huron, Ontario, and Superior are closest to the means for the area of the lakes >10?km offshore. In Lake Michigan the closest correspondence is with the >20?km offshore region. On a whole-lake basis in Lake Erie the GLNPO station chlorophyll averages are closest to the average calculated from the entire lake.     

Trends in Water Clarity of the Lower Great Lakes from Remotely Sensed Aquatic Color

Satellite observations of aquatic colour enable environmental monitoring of the Great Lakes at spatial and temporal scales not obtainable through ground-based monitoring. By merging data from the Coastal Zone Color Scanner (CZCS) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), monthly binned images of water-leaving radiance over the Great Lakes have been produced for the periods 1979–1985 and 1998–2006. This time-series can be interpreted in terms of changes in water clarity, showing seasonal and inter-annual variability of bright-water episodes such as phytoplankton blooms, re-suspension of bottom sediments, and whiting events. Variations in Secchi disk depth over Lakes Erie and Ontario are predicted using empirical relationships from coincident measurements of water transparency and remotely-sensed water-leaving radiance. Satellite observations document the extent to which the water clarity of the lower Great Lakes has changed over the last three decades in response to significant events including the invasion of zebra mussels. Results confirm dramatic reductions in Lake Ontario turbidity in the years following mussel colonization, with a doubling of estimated Secchi depths. Evidence confirms a reduction in the frequency/intensity of whiting events in agreement with suggestions of the role of calcium uptake by mussels on lake water clarity. Increased spring-time water clarity in the eastern basin of Lake Erie also corroborates previous observations in the region. Despite historical reports of localised increases in transparency in the western basin immediately following the mussel invasion, image analysis shows a significant increase in turbidity between the two study periods, in agreement with more recent reports of longer term trends in water clarity. Through its capacity to provide regular and readily interpretable synoptic views of regions undergoing significant environmental change, this work illustrates the value of remotely sensing water colour to water clarity monitoring in the lower Great Lakes.     

Impacts of the Great Lakes on Regional Climate Conditions

Estimates of lake-induced spatial changes of six climate variables (precipitation, mean minimum and mean maximum temperatures, cloud cover, vapor pressure, and wind speed) were derived for the entire Great Lakes basin. These patterns were estimated by a comparison of maps of each weather variable using: (1) all regional climate data, and (2) regional data when observations within an 80-km zone around the lakes were removed. Results generally confirm expectations and prior findings, but point to inadequacies in data collection that limit a highly precise analysis. Lake effects are most noticeable in precipitation and temperature and vary considerably by season, time of day, and lake size. Greatest lake influences are found near Lake Superior where up to 100% more precipitation falls downwind of the lake in winter compared to that expected without its presence. During summer, all lakes cause a downwind decrease in rainfall of 10% to 20%. Mean minimum temperatures in the basin are higher in all seasons and over all lakes. Lake-induced reductions in mean maximum temperatures in the region are observed during spring and summer. Effects on cloud cover are greatest during winter and show increases of approximately 25% in areas downwind of Lakes Superior and Michigan. Conversely, the cool summertime waters of Lakes Michigan and Huron reduce cloudiness roughly 10%. Variations in vapor pressure are consistent with observed changes in temperature. Amounts in winter are estimated to be 10% to 15% higher across the center of the basin, but decrease by roughly 5% to 10% at many lake shore sites in summer. Seasonal wind speed data were considered to lack an appropriate number of quality long-term climate stations to determine spatial lake effects. Surface elevations, increasing east of the basin, complicated detection of effects due solely to the lakes.     

Chronicles of hypoxia: Time-series buoy observations reveal annually recurring seasonal basin-wide hypoxia in Muskegon Lake – A Great Lakes estuary

We chronicled the seasonally recurring hypolimnetic hypoxia in Muskegon Lake – a Great Lakes estuary over 3?years, and examined its causes and consequences. Muskegon Lake is a mesotrophic drowned river mouth that drains Michigan's 2nd largest watershed into Lake Michigan. A buoy observatory tracked ecosystem changes in the Muskegon Lake Area of Concern (AOC), gathering vital time-series data on the lake's water quality from early summer through late fall from 2011 to 2013 (www.gvsu.edu/buoy). Observatory-based measurements of dissolved oxygen (DO) tracked the gradual development, intensification and breakdown of hypoxia (mild hypoxia <4?mg DO/L, and severe hypoxia <2?mg DO/L) below the ~6?m thermocline in the lake, occurring in synchrony with changes in temperature and phytoplankton biomass in the water column during July–October. Time-series data suggest that proximal causes of the observed seasonal hypolimnetic DO dynamics are stratified summer water-column, reduced wind-driven mixing, longer summer residence time, episodic intrusions of cold DO-rich nearshore Lake Michigan water, nutrient run off from watershed, and phytoplankton blooms. Additional basin-wide water-column profiling (2011–2012) and ship-based seasonal surveys (2003–2013) confirmed that bottom water hypoxia is an annually recurring lake-wide condition. Volumetric hypolimnetic oxygen demand was high (0.07–0.15?mg DO/Liter/day) and comparable to other temperate eutrophic lakes. Over 3?years of intense monitoring, ~9–24% of Muskegon Lake's volume experienced hypoxia for ~29–85?days/year – with the potential for hypolimnetic habitat degradation and sediment phosphorus release leading to further eutrophication. Thus, time-series observatories can provide penetrating insights into the inner workings of ecosystems and their external drivers.     

Patterns of Lake Balkhash water level changes and their climatic correlates during 1992–2010 period

The variation in Lake Balkhash water levels during the period from 1992 to 2010 and their relationship with climate dynamics were investigated in this study, using satellite altimetry data and meteorological records from climate stations located in the lake catchment basin. The altimetry‐derived water level demonstrated a general water level increase, reaching a mean value of 8.1 cm year^?1 in July 2005, with a maximum value of 342.52 m. The increased Lake Balkhash water level was accompanied by an overall upward trend in precipitation and temperature in the catchment basin during the study period. A strong increase in the winter and spring temperature was the main contributor to the general upward temperature trend, whereas a significant change of summer and autumn precipitation was the major contributor to the annual precipitation trend. Neither precipitation nor temperature increased uniformly across the entire lake drainage basin. The study results identified the most pronounced climate change occurring in the mountainous part (>2000 m above sea level) of the basin, in the upper reaches of the Ili river, which is the main water inflow to the lake. Statistical analysis indicated the Lake Balkhash water level is strongly correlated with both precipitation and temperature. The correlations were investigated for three altitudinal strata (<1000 m, 1000–2000 m, >2000 m) corresponding to the lower, middle and upper reaches of the Ili river. The best correlations were obtained for the upper reaches of the Ili river, indicating a changing snow cover and glacier equilibrium are the main factors controlling the water level trends in Lake Balkhash.     

SeaPRISM observations in the western basin of Lake Erie in the summer of 2016

In the summer of 2016, a robotic sun photometer called the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) Photometer Revision for Incident Surface Measurements (SeaPRISM), was deployed at a Coast Guard channel marker in western Lake Erie, measuring atmospheric properties and spectral water-leaving radiance. The instrument was deployed by the National Oceanic and Atmospheric Administration (NOAA) to support remote sensing validation and harmful algal bloom (HAB) satellite products. The Lake Erie SeaPRISM is also part of the international federated AERONET program maintained by the National Aeronautics and Space Administration (NASA), and more specifically is part of the AERONET Ocean Color (AERNOET-OC) network. The main purpose of this component of AERONET is specific to calibration/validation efforts for ocean color. The AERONET-OC network currently consists of 23 field radiometers at aquatic sites around the world. The Lake Erie site is the second freshwater lake location world-wide after the Palgrunden site in Sweden. During its operating period from mid-July to early September 2016, various environmental conditions were observed including a cyanobacteria bloom. Water-leaving radiance observations were generated on 43 out of 51 days, and varied by a factor of five. The variability in the above-water radiometry tracked that of in-water measurements made by a nearby buoy. During this brief operating window, satellite matchups were generated for several satellites. We highlight the first year's observations in relation to remote sensing validation and report on observations of cyanobacteria blooms from hourly to weekly time scales.     

基于Landsat卫星数据的洪湖水体遥感监测研究

2011年洪湖遭遇了70 a一遇的干旱,湖区水体面积发生了明显的变化。为了解影响洪湖水体面积变化的因素,利用1973年以来的多期Landsat卫星遥感图像数据,提取洪湖湖区水域面积,结合同期逐月日平均降雨量数据,对洪湖水体面积变化进行了探讨。分析表明：洪湖水体面积呈下降趋势;1973~2000年洪湖水体面积和年降雨量数据、汛期降雨量数据的相关性都很高,水体面积的变化趋势和年降雨量的变化趋势基本一致;而1973~2010年洪湖水体面积和年降雨量数据、汛期降雨量数据的相关性都有所降低,说明2000年后,影响洪湖水体面积的因素可能存在降雨量之外的其他因素,具体情况还有待进一步深入研究。     

Coastal geomorphic response to seasonal water-level rise in the Laurentian Great Lakes: An example from Illinois Beach State Park,USA

Hydrodynamic processes, such as fluctuating water levels, waves, and currents, shape coastlines across timescales ranging from minutes to millennia. In large lacustrine systems, such as the Laurentian Great Lakes, the role of water level in driving long-term (centuries to millennia) coastal evolution is well understood. However, additional research is needed to explore short-term (weeks to months) beach geomorphic response to fluctuating water level. Developing a process-focused understanding of how water level fluctuations shape coastal response across these shorter time scales is imperative for coastal management. Here, we present measurements of geomorphic response along a lacustrine beach ridge plain to seasonal water level fluctuations during a decadal high-stand in Lake Michigan water level. Frequent topographic change measurements revealed high spatial and temporal variability in geomorphic response to rising lake level. Sites immediately downdrift of shore protection began to erode immediately as lake level increased. The co-occurrence of peak seasonal lake levels and a modest increase in wave energy resulted in erosion and overwash at sites that resisted erosion during the initial seasonal rise in lake level. None of the sites in this study returned to their initial morphology following seasonal lake level rise. Given that peak water levels were nearly identical in 2017 and 2018, yet the majority of erosion at our sites occurred in 2017, we postulate that erosion associated with seasonal lake level rise is primarily a function of the change in annual maximum water level from year to year, rather than solely the elevation of the water level.     

Cold and wet: Diatoms dominate the phytoplankton community during a year of anomalous weather in a Great Lakes estuary

As sentinels of climate change and other anthropogenic forces, freshwater lakes are experiencing ecosystem disruptions at every level of the food web, beginning with the phytoplankton, a highly responsive group of organisms. Most studies regarding the effects of climate change on phytoplankton focus on a potential scenario in which temperatures continuously increase and droughts intersperse heavy precipitation events. Like much of the conterminous United States in 2019, the Muskegon River watershed (Michigan, USA) experienced record-breaking rainfall accompanied by unusually cool temperatures, affording an opportunity to explore how an alternate potential climate scenario may affect phytoplankton. We conducted biweekly sampling of environmental variables and phytoplankton in Muskegon Lake, a Great Lakes Area of Concern that connects to Lake Michigan. We compared environmental variables in 2019 to the previous eight years using long-term data from the Muskegon Lake Observatory buoy, and annual monitoring excursions provided historical phytoplankton data. Under cold and wet conditions, diatoms were the single dominant division throughout the entire growth season – an unprecedented scenario in Muskegon Lake. In 10 of the 13 biweekly sampling days in 2019, diatoms comprised over 75% of the phytoplankton community in the lake by count, indicating that the spring diatom bloom persisted through the fall. Additionally, phytoplankton seasonal succession and abundance patterns typically seen in this lake were absent. In a world experiencing reduced predictability, increased variability, and regional climate anomalies, studying periods of extreme weather events may offer insight into how natural systems will be affected and respond under future climate scenarios.     

Convergence of trophic state and the lower food web in Lakes Huron,Michigan and Superior

Signs of increasing oligotrophication have been apparent in the open waters of both Lake Huron and Lake Michigan in recent years. Spring total phosphorus (TP) and the relative percentage of particulate phosphorus have declined in both lakes; spring TP concentrations in Lake Huron are now slightly lower than those in Lake Superior, while those in Lake Michigan are higher by only about 1 μg P/L. Furthermore, spring soluble silica concentrations have increased significantly in both lakes, consistent with decreases in productivity. Transparencies in Lakes Huron and Michigan have increased, and in most regions are currently roughly equivalent to those seen in Lake Superior. Seasonality of chlorophyll, as estimated by SeaWiFS satellite imagery, has been dramatically reduced in Lake Huron and Lake Michigan, with the spring bloom largely absent from both lakes and instead a seasonal maximum occurring in autumn, as is the case in Lake Superior. As of 2006, the loss of cladocerans and the increased importance of calanoids, in particular Limnocalanus, have resulted in crustacean zooplankton communities in Lake Huron and Lake Michigan closely resembling that in Lake Superior in size and structure. Decreases in Diporeia in offshore waters have resulted in abundances of non-dreissenid benthos communities in these lakes that approach those of Lake Superior. These changes have resulted in a distinct convergence of the trophic state and lower food web in the three lakes, with Lake Huron more oligotrophic than Lake Superior by some measures.     

A comparison of chlorophyll a values obtained from an autonomous underwater vehicle to satellite-based measures for Lake Michigan

Accurate methods to track changes in lake productivity through time and space are critical to fisheries management. Chlorophyll a is the most widely studied proxy for ecosystem primary production and has been the topic of many studies. The main sources of chlorophyll a measurements are ship-based measures or multi-spectral satellite data. Autonomous underwater vehicles can survey large spatial extents approaching the scale of satellite data, but with the accuracy of ship-based water sampling methods. We use several statistical measures to compare measures of chlorophyll a collected in Lake Michigan with spatiotemporally matched satellite-derived measures of chlorophyll a from the MODIS Aqua multi-spectral sensor using NASA's OC3 and the Great Lakes Fit algorithms. Our findings show a near one to one relationship between AUV data and both satellite-derived data sets when the AUV data are coarsened to the resolution of the satellite data. A comparison of satellite-based chlorophyll a to AUV-derived chlorophyll summarized in discrete water depth bins suggested that, based on decreasing coefficients of determination, satellite estimates of chlorophyll accounted for the most variability in chlorophyll a concentrations in the upper 10 m of the water column, even though satellite sensors may detect past this depth.     

冰川亏损对哈拉湖流域湖泊水位波动的影响

青藏高原的内陆湖泊水位和冰川变化和其流域内冰川质量亏损对湖泊水位波动的影响及其贡献对水量平衡研究具有重要意义。以哈拉湖流域冰川为例,基于2000-2015年星载雷达测高资料和Landsat卫星多光谱遥感资料分别提取湖泊水位和面积变化;结合附近的托勒台站气象观测资料,进一步分析其水位波动变化原因和冰川亏损对湖泊水量贡献。结果表明:受年降水量和夏季降水量增加影响,哈拉湖水位呈增加趋势,但哈拉湖流域冰川亏损加速趋势不明显;与2000年相比,湖泊面积增加了(21.4±4.8)km~2,湖泊水位增加了(1.68±0.26)m,相应的湖泊水容量增加了(16.1±0.3)×10~8m~3水当量。流域冰川亏损量达对哈拉湖水量的贡献率为39.65%,降水量增加对湖泊水量贡献了22.82%。     

Distribution and foraging patterns of common loons on Lake Michigan with implications for exposure to type E avian botulism

Common loons (Gavia immer) staging on the Great Lakes during fall migration are at risk to episodic outbreaks of type E botulism. Information on distribution, foraging patterns, and exposure routes of loons are needed for understanding the physical and ecological factors that contribute to avian botulism outbreaks. Aerial surveys were conducted to document the spatiotemporal distribution of common loons on Lake Michigan during falls 2011–2013. In addition, satellite telemetry and archival geolocator tags were used to determine the distribution and foraging patterns of individual common loons while using Lake Michigan during fall migration. Common loon distribution observed during aerial surveys and movements of individual radiomarked and/or geotagged loons suggest a seasonal pattern of use, with early fall use of Green Bay and northern Lake Michigan followed by a shift in distribution to southern Lake Michigan before moving on to wintering areas. Common loons tended to occupy offshore areas of Lake Michigan and, on average, spent the majority of daylight hours foraging. Dive depths were as deep as 60 m and dive characteristics suggested that loons were primarily foraging on benthic prey. A recent study concluded that round gobies (Neogobius melanostomus) are an important prey item of common loons and may be involved in transmission of botulinum neurotoxin type E. Loon distribution coincides with the distribution of dreissenid mussel biomass, an important food resource for round gobies. Our observations support speculation that energy transfer to higher trophic levels via gobies may occur in deep-water habitats, along with transfer of botulinum neurotoxin.     

Spatio-temporal trends in the density and condition of a secondary consumer,Bythotrephes, in southern Lake Michigan

Natural gradients in temperature, nutrient loading, and primary productivity contribute to broad scale regional differences in the food web structure of large lakes, such as Lake Michigan. These factors influence spatial patterns of primary production and resource reliance of high trophic level consumers. Secondary consumers, such as larval fish and predatory zooplankton represent an important intermediate link within large lake food webs, but spatial patterns in their density and physiological condition have not been as thoroughly assessed. To analyze the spatial and temporal condition of secondary consumers in Lake Michigan, we sampled the ubiquitous spiny water flea, Bythotrephes cederströmii during the 2015 Cooperative Science and Monitoring field year in Lake Michigan. Monthly estimates of density, instar frequency, length-at-age, and RNA content of Bythotrephes were compared between the eastern and western shores of southern Lake Michigan. Condition indices differed seasonally between the eastern and western shores. During June-August, Bythotrephes were more abundant and in better condition along the eastern shore of Lake Michigan, but in September, all indices were comparable between transects. While this study focused on a single year, other studies across multiple years have demonstrated consistent upwellings and cooler temperatures along the western shore of southern Lake Michigan relative to the eastern shore. A temporal lag in preferable environmental conditions along the western shore may be common and lead to delayed growth and reduced physiological condition of secondary consumers, such as predatory zooplankton and late spring emerging larval fish.     

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.