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Changes in water temperature and ice cover are important for controlling the biogeochemical and food-web processes of dimictic lakes in temperate regions. To investigate these changes, we applied a...

Lentic ecosystems are important agents of local and global carbon cycling, but their contribution varies along gradients of dissolved organic matter (DOM) and productivity. We investigated how contrasting summer and autumn precipitation can shape annual and inter-annual variation in ecosystem carbon (C) flux (gross primary production (GPP), ecosystem respiration (ER), and CO2 efflux) in two subtropical lakes differing substantially in trophic state and water color. Instrumented buoys recorded time series of free-water DO, terrestrial DOM (tDOM), chlorophyll a, water temperature profiles, and meteorological measurements over five years (2009–2011 and 2014–2015). Reduced precipitation caused immediate and prolonged effects on C flux in both lakes. During the drought year (2014) GPP and ER declined by 60 to 80% and both lakes were either CO2 sinks or neutral. In the subsequent wet year (2015), GPP and ER increased by 40 to 110%, and both lakes shifted to strong net CO2 emitters. Higher ecosystem R resulted from larger GPP while higher tDOM contributed to a dramatic increase in dissolved inorganic carbon, which intensified CO2 emission in both lakes. C flux was more responsive in the clear mesotrophic lake, declining by approximately 40% in the cumulative GPP and ER, and increasing by >400% in CO2 efflux whereas changes in the oligotrophic colored lake were more modest (approximately 30% and 300% for metabolic declines and efflux increases, respectively). Temporal variation and magnitude of C flux were governed by tDOM-mediated changes in epilimnetic nutrient levels and hypolimnetic light availability. This study demonstrated terrestrial loads of DOM strongly influence the inter-annual response and sensitivity of ecosystem C flux to variation in inter-annual precipitation. Our findings have important implications for predicting the trend, magnitude, duration, and sensitivity of the response of C flux in subtropical lakes/reservoirs to future changes in precipitation patterns under altered climatic conditions.

The Global Lake Ecological Observatory Network (GLEON) has built an international, grassroots network of scientists and citizens, data, and lake observatories to advance understanding of lake ecosystems. Through careful attention to the professional needs and aspirations of a community, GLEON has formed as its foundation the trust and respect essential to product-based network science. As a consequence, GLEON is making significant advancements in lake ecosystem understanding through all “five legs of the table that support scientific understanding”—natural history, multiscale data, experiments, theory, and comparative studies—with particular emphasis on multiscale data and comparative studies. Technical products, such as cyberinfrastructure in support of network data and operations, software tools for calculating lake physical metrics (e.g., thermocline depth, buoyancy frequency, Schmidt stability), and lake metabolism, as well as ecosystem-scale numerical simulation software, have derived from GLEON collaborations and have become community resources catalyzing interdisciplinary science. Education and outreach initiatives have served to engage citizens from outside the traditional boundaries of academia directly in research. Moreover, these cross-boundary collaborations have provided essential links to lake and reservoir stakeholders who have informed how science is prioritized and communicated within GLEON. As a grassroots network, GLEON derives its momentum, flexibility, and impact from its talented members, who are committed to the future sustainability of lakes and reservoirs and the services they provide.

The cycling of organic carbon is fundamental to aquatic ecosystems, reflecting processes that extend from terrestrial watersheds to fish. Here, we use embedded fluorescence sensors that sample at high frequency to investigate the daily dynamics of a proxy for the major pool of organic carbon (chromophoric dissolved organic matter, CDOM) in a clear-water Wisconsin lake (∼3 mg C·L−1). We compare the diel CDOM cycle in this lake with cycles observed previously in two dark-water lakes (10 to 20 mg C·L−1). Despite differences in DOM quality and quantity, diel fluorescence cycles were evident in the epilimnia and hypolimnia of all three lakes. The amplitude differed among lakes, but the timing of the diel cycles was similar, with increases in fluorescence during nighttime and decreases during daylight (except in the aphotic hypolimnion of the darkest lake). The amplitude of the diel cycle increased with increasing DOM concentration, and estimates of DOM turnover based on the magnitude of oscillation ranged from 0.28 mg C·L−1·day−1 in the darkest lake to 0.14 mg C·L−1·day−1 in the clear lake. Independent estimates of free water metabolism based on the daily dynamics of O2 or CO2 were in general agreement, ranging from 0.32 to 0.06 mg C·L−1·day−1. Although absolute rates of turnover varied directly with DOM concentration, relative rates were highest in clear waters (∼5%·day−1). We conclude that these daily oscillations may be a common property of lakes and that they may provide insights into internal DOM processing over short time scales.

We investigated how the lack of usual winter and spring monsoons, effectively representing consecutive drought events, affected the dynamics of ecosystem metabolism in a shallow mesotrophic seepage lake in northeastern Taiwan. An instrumented buoy provided high-frequency free-water dissolved oxygen measurements, water temperature profiles, and meteorological data, which we used to estimate daily values of gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP). Results revealed that the disappearance of monsoons decreased lake level and volume, concentrated dissolved nutrients, stimulated the development of algal biomass, promoted stratification, and resulted in a major shift in lake metabolism. Offshore GPP and R were both initially stimulated but then decreased due to shallower mixing depths in the water column. The lake rapidly shifted from a heterotrophic state to a highly autotrophic status when the water level dropped to the lowest level. A return to autotrophy was caused by a greater decline in R than an increase in GPP. This study demonstrates the dramatic effect that drought events can have on lake ecosystem function and suggests that nutrient control may be important in mitigating the effects of a predicted warmer and drier climate and increased water withdrawal in this region.

Studying mixing processes in a stratified lake is important for understanding the biological, chemical and physical processes occurring there. Statistical analyses were performed of data from a small, shallow, stratified lake in a subtropical alpine region (Yuan-Yang Lake in Taiwan) to determine the predominant physical factors in heavy-rainfall-induced mixing. This study focused on both vertical mixing in the entire water column and surface-layer mixing extending to the upper thermocline. The effects of meteorological driving forces, such as wind, heating/cooling and inflow on vertical mixing and surface layer mixing, were evaluated using the relationships between each driving force and the change in thermal stability between the pre-mixing and mixing periods. For surface layer mixing, a comparison between penetrative convection related to heating/cooling and wind-related friction velocity was conducted for each heavy rainfall event. A heat content parameter measuring thermal potential energy was introduced to further investigate inflow effects (e.g. effects of changes in discharge volume and temperature) on vertical mixing during heavy rainfall events. Results show that wind input affected vertical mixing more significantly than did other meteorological forcing factors during storm-dominant events. Indeed, wind energy input in the surface layer was more pronounced than was energy of heating/cooling for surface layer mixing. Furthermore, inflow effect was shown to be crucial during large scale and extreme weather events (i.e. lower air pressure events) in the vertical mixing process. Forcing by heating/cooling likely contributes less to mixing because it is likely less dynamic than the wind and inflow inputs with respect to internal response of the lake. In addition, a principal component analysis (PCA) modified by partial correlation was performed to verify the results quantitatively. The first and second components, which accounted for more than 90% of the total variance in the PCA, showed that the intensity of vertical mixing was affected primarily by wind-induced turbulence and inflow intrusion and was only weakly associated with the effect of net heat balance. Considering the interactions between chemical and physical processes, inflow intrusion may have an effect on dissolved oxygen concentration in the lake.

Vertical mixing affects the vertical transport of biochemical materials and modifies the characteristics of basin-scale internal waves in subtropical, subalpine Yang Yuan Lake (YYL), in the north central region of Taiwan. Vertical mixing in YYL is generally caused by typhoons (strong winds and heavy rainfalls) during the spring, summer and early fall, or by cooling of the water column during late fall and winter. Vertical mixing caused by typhoons significantly affects internal thermal dynamics and biochemical processes, with basin-scale internal waves enhanced before complete mixing occurs, and temporal variations of water temperature dramatically changing after typhoon events. This study quantitatively determines when the complete mixing (defined as homogenous temperature in a water column) occurs on the basis of the balance between kinetic energy and potential energy and also shows what meteorological conditions contribute to the vertical mixing. Data on water column thermal profiles and meteorological variables were collected by a wireless, instrumented buoy in the deepest location of YYL and from a nearby meteorological station, from spring 2004 to summer 2006. This study also investigated how a physical process (internal waves) is affected by the mixing associated with typhoons. Signal processing and a two-layer model of the water column are used to understand the characteristics of basin-scale internal waves.

Long-term research on freshwater ecosystems provides insights that can be difficult to obtain from other approaches. Widespread monitoring of ecologically relevant water-quality parameters spanning decades can facilitate important tests of ecological principles. Unique long-term data sets and analytical tools are increasingly available, allowing for powerful and synthetic analyses across sites. Long-term measurements or experiments in aquatic systems can catch rare events, changes in highly variable systems, time-lagged responses, cumulative effects of stressors, and biotic responses that encompass multiple generations. Data are available from formal networks, local to international agencies, private organizations, various institutions, and paleontological and historic records; brief literature surveys suggest much existing data are not synthesized. Ecological sciences will benefit from careful maintenance and analyses of existing long-term programs, and subsequent insights can aid in the design of effective future long-term experimental and observational efforts. Long-term research on freshwaters is particularly important because of their value to humanity.

We evaluated the predictive ability of a one-dimensional coupled hydrodynamic-biogeochemical model across multiple temporal scales using wavelet analysis and traditional goodness-of-fit metrics. High-frequency in situ automated sensor data and long-term manual observational data from Lake Mendota, Wisconsin, USA, were used to parameterize, calibrate, and evaluate model predictions. We focused specifically on short-term predictions of temperature, dissolved oxygen, and phytoplankton biomass over one season. Traditional goodness-of-fit metrics indicated more accurate prediction of physics than chemical or biological variables in the time domain. This was confirmed by wavelet analysis in both the time and frequency domains. For temperature, predicted and observed global wavelet spectra were closely related, while observed dissolved oxygen and chlorophyll fluorescence spectral characteristics were not reproduced by the model for key time scales, indicating that processes not modeled may be important drivers of the observed signal. Although the magnitude and timing of physical and biological changes were simulated adequately at the seasonal time scale through calibration, time scale-specific dynamics, for example short-term cycles, were difficult to reproduce, and were relatively insensitive to the effects of varying parameters. The use of wavelet analysis is novel to aquatic ecosystem modeling, is complementary to traditional goodness-of-fit metrics, and allows for assessment of variability at specific temporal scales. In this way, the effect of processes operating at distinct temporal scales can be isolated and better understood, both in situ and in silico. Wavelet transforms are particularly well suited for assessment of temporal and spatial heterogeneity when coupled to high-frequency data from automated in situ or remote sensing platforms.

Disturbances act as powerful structuring forces on ecosystems. To ask whether environmental microbial communities have capacity to recover after a large disturbance event, we conducted a whole-ecosystem manipulation, during which we imposed an intense disturbance on freshwater microbial communities by artificially mixing a temperate lake during peak summer thermal stratification. We employed environmental sensors and water chemistry analyses to evaluate the physical and chemical responses of the lake, and bar-coded 16S ribosomal RNA gene pyrosequencing and automated ribosomal intergenic spacer analysis (ARISA) to assess the bacterial community responses. The artificial mixing increased mean lake temperature from 14 to 20°C for seven weeks after mixing ended, and exposed the microorganisms to very different environmental conditions, including increased hypolimnion oxygen and increased epilimnion carbon dioxide concentrations. Though overall ecosystem conditions remained altered (with hypolimnion temperatures elevated from 6 to 20°C), bacterial communities returned to their pre-manipulation state as some environmental conditions, such as oxygen concentration, recovered. Recovery to pre-disturbance community composition and diversity was observed within 7 (epilimnion) and 11 (hypolimnion) days after mixing. Our results suggest that some microbial communities have capacity to recover after a major disturbance. © 2012 International Society for Microbial Ecology All rights reserved.

For lake microbes, water column mixing acts as a disturbance because it homogenizes thermal and chemical gradients known to define the distributions of microbial taxa. Our first objective was to isolate hypothesized drivers of lake bacterial response to water column mixing. To accomplish this, we designed an enclosure experiment with three treatments to independently test key biogeochemical changes induced by mixing: oxygen addition to the hypolimnion, nutrient addition to the epilimnion, and full water column mixing. We used molecular fingerprinting to observe bacterial community dynamics in the treatment and control enclosures, and in ambient lake water. We found that oxygen and nutrient amendments simulated the physical-chemical water column environment following mixing and resulted in similar bacterial communities to the mixing treatment, affirming that these were important drivers of community change. These results demonstrate that specific environmental changes can replicate broad disturbance effects on microbial communities. Our second objective was to characterize bacterial community stability by quantifying community resistance, recovery and resilience to an episodic disturbance. The communities in the nutrient and oxygen amendments changed quickly (had low resistance), but generally matched the control composition by the 10th day after treatment, exhibiting resilience. These results imply that aquatic bacterial assemblages are generally stable in the face of disturbance.

We compared lakes and streams in northern Wisconsin to ask how regional patterns of water chemistry differed, and how ecosystem size relates to water chemistry. By applying traditional statistical approaches in novel ways, we identified factors influencing regional chemistry of lakes and streams. Chemistry differed significantly among the two ecosystem types (p 5 0.01), and two distinct variable groups were apparent. Almost two thirds of the variation in 16 constituents was associated with concentration gradients related to groundwater inputs (group 1, represented by Ca) and adjacent wetlands (group 2, represented by dissolved organic carbon [DOC]). Largest differences were observed in groundwater-associated variables. Median Ca concentrations in lakes were an order of magnitude lower than in streams (lake 5 1.43 mg L21, stream 5 12.45 mg L21), and indicate that regional lake chemistry is strongly influenced by precipitation or short groundwater flow paths, whereas streams are more affected by groundwater with substantial mineral water contact. In contrast, variables reflecting wetland influence (e.g., DOC) did not differ between streams and lakes. Despite these differences, as lake and stream size increased, their biogeochemical attributes converged, likely reflecting increased diversity of flow paths to larger systems, greater biogeochemical processing, and changes in landscape composition. Our results have identified critical differences (groundwater inputs) and similarities (organic carbon dynamics) between lakes and streams and emphasize the importance of ecosystem size in interpreting these observations at the regional scale. Studies such as this will provide essential pieces of information for regional and global biogeochemical studies of freshwater ecosystems.

We studied how typhoon strength affects the daily dynamics of ecosystem metabolism of a subtropical alpine lake in Taiwan. We identified proximal agents of typhoon disturbance and assessed the resistance (the extent of change induced by a disturbance) and resilience (the rate of recovery after a disturbance) of lake metabolism to them. Gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production were estimated from high-frequency dissolved oxygen data provided by an instrumented buoy. Typhoons resulted in significantly lower GPP (3%–81% decrease), and higher ER (7%–828% increase) compared with immediately before the events, and thus the lake became more heterotrophic (28%–852% increase in heterotrophy). The resistance and resilience of lake metabolism depended on the intensity of the typhoon. Smaller typhoons (with average daily accumulated precipitation (ADAP) < 200 mm·day–1) had greater effects on lake metabolism than medium (ADAP = 200–350 mm·day–1) and large (ADAP > 350 mm·day–1) typhoons. However, metabolism also recovered more quickly after smaller typhoons than after medium or larger typhoons. Typhoon effects on ecosystem metabolism is likely mediated by the magnitude and duration of typhoon-induced changes in lake mixing, the quantity and quality of dissolved organic carbon, and the biomass of primary producers.

A nowcasting system integrating observations and a numerical model was developed to better understand the spatial distributions and temporal variations in the dynamics of a small, subtropical lake occasionally influenced by severe rainstorms. The nowcasting system was used to monitor real-time meteorological conditions and thermal structures, to provide spatial information on the thermal and flow dynamics from model predictions, and to compare the predictions with the observations. The system was also able to determine whether the instruments collecting field data were performing correctly using telemetry operations. The nowcasting system was initially operated in the spring of 2009 in Yuan-Yang Lake, Taiwan. Rainstorm-induced mixing occurred due to Typhoon Morakot during August 7–9, 2009. The mixing was observed by the instruments, and the spatial distributions and temporal variations during the mixing were successfully predicted by the three-dimensional hydrodynamic lake model. A quantitative comparison of the energy balances among the heat, wind, and water inflow inputs to the lake implied that the typhoon-induced mixing was primarily caused by strong winds. The model predictions showed that the lake experienced mixing and flooding (large amounts of inflow/outflow discharges), resulting in homogenous temperatures and flows that moved to the outlet of the lake.

A simple method to automatically measure the date of ice-on, the date of ice-off, and the duration of lake ice cover is described. The presence of ice cover is detected by recording water temperature just below the ice/water interface and just above the lake bottom using moored temperature sensors. The occurrence of ice-on rapidly leads to detectible levels of inverse stratification, defined as existing when the upper sensor records a temperature at least 0.1°C below that of the bottom sensor, whereas the occurrence of ice-off leads to the return of isothermal mixing. Based on data from 10 lakes over a total of 43 winter seasons, we found that the timing and duration of inverse stratification monitored by recording temperature sensors compares well with ice cover statistics based on human observation. The root mean square difference between the observer-based and temperature-based estimates was 7.1 d for ice-on, 6.4 d for ice-off, and 10.0 d for the duration of ice cover. The coefficient of determination between the two types of estimates was 0.93, 0.86, and 0.91, respectively. The availability of inexpensive self-contained temperature loggers should allow expanded monitoring of ice cover in a large and diverse array of lakes. Such monitoring is needed to improve our ability to monitor the progression of global climate change, and to improve our understanding of the relationship between climate and ice cover over a wide range of temporal and spatial scales.

Significant improvements have been made in estimating gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP) from diel, “free-water” changes in dissolved oxygen (DO). Here we evaluate some of the assumptions and uncertainties that are still embedded in the technique and provide guidelines on how to estimate reliable metabolic rates from high-frequency sonde data. True whole-system estimates are often not obtained because measurements reflect an unknown zone of influence which varies over space and time. A minimum logging frequency of 30 min was sufficient to capture metabolism at the daily time scale. Higher sampling frequencies capture additional pattern in the DO data, primarily related to physical mixing. Causes behind the often large daily variability are discussed and evaluated for an oligotrophic and a eutrophic lake. Despite a 3-fold higher day-to-day variability in absolute GPP rates in the eutrophic lake, both lakes required at least 3 sonde days per week for GPP estimates to be within 20% of the weekly average. A sensitivity analysis evaluated uncertainties associated with DO measurements, piston velocity (k), and the assumption that daytime R equals nighttime R. In low productivity lakes, uncertainty in DO measurements and piston velocity strongly impacts R but has no effect on GPP or NEP. Lack of accounting for higher R during the day underestimates R and GPP but has no effect on NEP. We finally provide suggestions for future research to improve the technique.

Summary 1. Exchange of material across habitat boundaries is a key process in riparian zones. The movement of coarse wood from lakeshore forests to the littoral zone, where it provides habitat for aquatic organisms, is not well understood. In 2003, we resampled coarse wood within the littoral zone of four lakes in Northern Wisconsin (U.S.A.), that had originally been surveyed in 1996, to quantify the spatial arrangement of littoral coarse wood and estimate input, loss and movement rates. 2. All four lakes had a clumped pattern of littoral coarse wood, and the locations of clusters were similar in both census years. Littoral coarse wood was more abundant than expected on moderate to steep slopes, on southern shorelines, and in areas with sparse residential development. 3. All four lakes had a net accumulation of coarse wood; rates of wood input ranged from 0.5 to 1.9 logs km−1 year−1, i.e. there was more wood in 2003 than 1996. Movement rates of tagged logs varied 14-fold among lakes, with a maximum in one lake of 42% of logs recovered in 2003 more than 20 m from their 1996 location. Median distance moved ranged from 22 to 323 m among the four lakes. 4. Areas of persistently high wood density may be keystone habitats whose presence enables the persistence of populations of certain aquatic organisms. Conservation of locations with high wood density may be important to maintain target densities of coarse wood of lakes with human development.

Innovative uses of advanced sensors and sensor networks are starting to be translated into new ecological knowledge. These sensors are providing a new set of “eyes” through which researchers may observe the world in new ways, extend spatial and temporal scales of observation, more accurately estimate what cannot be observed, and, most important, obtain unexpected results or develop new paradigms. Automated sensors are widely deployed by members of the Organization of Biological Field Stations, yet some needs—particularly for chemical and biological sensors—are not currently being met. There are additional opportunities for developing sensor networks at synoptic, regional, continental, and global scales. Although we are seeing more uses of sensor systems and, in particular, sensor networks, the opportunities for these systems are just beginning to be realized, with much more work to be done, including formulation of new questions, development of new sensors, better software, and new ways for researchers to...

The Intergovernmental Panel on Climate Change predicts dramatic changes in precipitation patterns over the next century. One potential method for inferring how these changes in annual precipitation and intensity of storm events will influence aquatic ecosystems is to study and model present-day lakes that share climatic characteristics with future climate scenarios. A small lake in north-central Taiwan provided an excellent opportunity to explore the influence of intense meteorological events on CO2 exchange between surface waters and the atmosphere. Each year Yuan Yang Lake (YYL) is influenced by multiple typhoons that pass near the island of Taiwan. This lake has been instrumented with a sensor network that monitors water column and meteorological parameters at a high temporal resolution (2–10 min intervals). Using this high-resolution data and manually collected CO2 samples, a mass-balance model of CO2 dynamics in YYL was developed. In addition, a generalized simulation model was used to explore how typhoon frequency, intensity, and timing impact CO2 efflux to the atmosphere. Our findings suggest that increased annual precipitation and frequency of storm events results in greater epilimnetic interaction with the watershed and hypolimnion. These interactions resulted in elevated epilimetic CO2 concentrations and therefore greater evasion of CO2 to the atmosphere.

SUMMARY 1. We used high-frequency in situ dissolved oxygen measurements to investigate the seasonal variability and factors regulating metabolism in a subtropical alpine lake in Taiwan between May 2004 and October 2005, specifically exploring how the typhoon season (from June or July to October) affects lake metabolism. 2. Gross primary production (GPP) and ecosystem respiration (R) both peaked in early summer and mid-autumn but dropped during the typhoon season and winter. Yuan-Yang Lake is a net heterotrophic ecosystem (annual mean net ecosystem production )39.6 lmole O2 m )3 ). 3. Compared to the summer peaks, seasonal averages of GPP and R decreased by approximately 50% and 25%, respectively, during the typhoon season. Ecosystem respiration was more resistant to external disturbances than GPP and showed strong daily variation during typhoon seasons. 4. Changes in the quality and quantity of dissolved organic carbon controlled the temporal dynamics and metabolic regulation. External disturbances (typhoons) caused increased allochthony, increasing DOC and water colour and influencing lake metabolism. 5. Seasonal winter mixing and typhoon-induced water mixing in summer and autumn play a key role in determining the extent to which the lake is a seasonal carbon sink or source to the atmosphere.

Free-water measurements of dissolved oxygen (DO) in lakes are becoming common and provide opportuni- ties for estimating ecosystem processes, such as gross primary production (GPP) and ecosystem respiration (R). The models used to estimate metabolism often subsume biological processes into one parameter each for GPP and R. However, high-frequency DO observations made over days show diverse patterns at multiple time scales, suggesting a complex suite of processes controls DO dynamics. Can we improve metabolism estimates and pre- dictive ability for DO at diel scales by adding complexity to the models? In this study, we use data from two north temperate lakes to test a variety of metabolism models representing a suite of non-linear metabolic processes. To test whether alternative models can be discriminated, we simulated DO with assumed parameter values and auto-regressive noise, and fit the models to the simulated DO. The most complex model could be discriminated from simpler models and provided the most accurate and precise predictions. However, when models were fit to observed DO data from the sensor network, the simplest model predicted DO as well as the most complex one. The added complexity did not improve model performance. An analysis of the model resid- uals indicates that physics may explain some of the DO pattern not predicted, especially high-frequency oscil- lations and anomalies that appear to coincide with weather patterns. Under reasonably stable weather condi- tions and at scales of a few days, simple metabolism models explain the bulk of diel DO variability.

Recent advances in our understanding of the importance of continental- to global-scale connectivity among terrestrial and aquatic ecosystems make consideration of aquatic–terrestrial linkages an urgent ecological and environmental issue. Here, we describe the role of inland waters as sentinels and integrators of the impact of humans on terrestrial and aquatic ecosystems. The metabolic responses of lakes and streams (ie the rates at which these systems process carbon) are proposed as a common metric to integrate the impacts of environmental change across a broad range of landscapes. Lakes and streams transport and alter nutrients, contaminants, and energy, and store signals of environmental change from local to continental scales over periods ranging from weeks to millennia. A carefully conceived and well-integrated network that includes monitoring and experimental approaches to terrestrial–aquatic connectivity is critical to an understanding of basic ecosystem-level processes and to forecasting and mitigating future environmental impacts at the continental scale.

Humans have continuously interacted with natural systems, resulting in the formation and development of coupled human and natural systems (CHANS). Recent studies reveal the complexity of organizational, spatial, and temporal couplings of CHANS. These couplings have evolved from direct to more indirect interactions, from adjacent to more distant linkages, from local to global scales, and from simple to complex patterns and processes. Untangling complexities, such as reciprocal effects and emergent properties, can lead to novel scientific discoveries and is essential to developing effective policies for ecological and socioeconomic sustainability. Opportunities for truly integrating various disciplines are emerging to address fundamental questions about CHANS and meet society's unprecedented challenges.

We compared long-term change in two lake districts, one in a forested rural setting and the other in an urbanizing agricultural region, using lakes as sentinel ecosystems. Human population growth and land-use change are important drivers of ecosystem change in both regions. Biotic changes such as habitat loss, species invasions, and poorer fishing were prevalent in the rural region, and lake hydrology and biogeochemistry responded to climate trends and landscape position. Similar biotic changes occurred in the urbanizing agricultural region, where human-caused changes in hydrology and biogeochemistry had conspicuous effects. Feedbacks among ecosystem dynamics, human uses, economics, social dynamics, and policy and practice are fundamental to understanding change in these lake districts. Sustained support for interdisciplinary collaboration is essential to build understanding of regional change.

Dissolved inorganic carbon (DIC) and dissolved oxygen (DO) are commonly measured to compute metabolism of aquatic ecosystems. However, concentrations of DIC and DO depend on many factors in addition to ecosystem metabolism, such as water temperature, gas exchange with the atmosphere, abiotic chemical reactions, and inputs in precipitation, groundwater, and surface water. We used 20-year time series from seven lakes to understand how DIC and DO concentrations are controlled as a function of time scale. Diel cycles of both solutes are controlled primarily by metabolism, exchange with the atmosphere, and temperature. At seasonal and annual scales, metabolism is important, but physical processes associated with spring and autumn mixing, as well as solute loading from the watershed, have comparably large effects. At decadal scales, effects of metabolism are negligible. Controls of the two solutes diverge, with variance in DIC explained largely by solute inputs and variance in DO explained largely by water temperature. Like other indicators in many ecosystems, variability of DIC and DO is strongly scale dependent and associated with different drivers depending on the time scale of the analysis.

Net ecosystem production (NEP) trends among lakes have been ascribed to differences in nutrient and allochthonous carbon inputs, but little is known on how different habitats within lakes contribute to these trends. We sampled pelagic and littoral surface waters using sonde (i.e., free-water) and bottle methods concurrently in lakes spanning a range of trophic conditions. We considered whether the typically higher metabolism estimates found with sonde methods are due to contributions from littoral habitats not reflected by bottle estimates. We sought the source of littoral contributions by selecting sites with maximum differences in macrophyte abundance. Sonde estimates for pelagic primary production and respiration were two–three times greater than bottle estimates. Sonde/bottle ratios were higher in productive lakes and lakes with more littoral area. Bottle estimates were similar among all sites, and sonde estimates in macrophyte-poor sites were similar to pelagic sondes. However, sonde estimates in macrophyte-rich areas were four–nine times greater than bottle estimates. Results suggest littoral zones increase whole-lake NEP in eutrophic systems, whereas the Sphagnum mat surrounding dystrophic lakes decreases NEP. Non-planktonic organisms associated with macrophytes provide important littoral contributions to whole-lake metabolism and to understanding NEP trends among lakes.

Summary 1 Residential development is often concentrated near aquatic ecosystems and understanding riparian–littoral zone interactions is therefore critical for assessing its ecological effects. 2 Introduction of wood from riparian forests into the littoral zone, where it becomes habitat for aquatic organisms, is an important, but poorly understood, process. We related the density of littoral coarse wood both among and within 45 lakes in Vilas County, Wisconsin, USA, to forest structure, abiotic drivers and land use. 3 Among all lakes and among a subset of low-development lakes, the best predictor of the density of littoral coarse wood was the density of riparian coarse wood. At the within-lake (site) level, two alternative models explain variability in coarse wood abundance: as a function of exposure to wind and amount of riparian coarse wood or as a function of exposure to wind and land-use intensity. 4 Both among and within lakes, areas more modified by humans had a lower density of littoral coarse wood. Conversely, areas with little (current) human impact were tremendously variable; some sites and lakes had abundant wood and others had virtually none. 5 Contrary to previous studies, there was no relationship between living trees and coarse wood density, suggesting that riparian and littoral coarse wood densities may be strongly influenced by past disturbance, both human and natural. 6 This study highlights the importance of cross boundary subsidies in understanding the impact of development on ecosystems. The concentration of residential development at the boundary between terrestrial and aquatic ecosystems appears to reduce the flow of coarse wood from forests to lakes. Loss of this resource may have negative consequences for lake biota and the aquatic food web.

The cycle of waterborne methylmercury (meHg) in Little Rock Lake is characterized by a period of accumulation during summertime (when the lake is warm and open to the atmosphere) and a period of decline during winter (when the lake is sealed by ice). We followed this cycle for 16 yr, during which time the lake was acidified with H2SO4 and then allowed to recover naturally as part of a long-term field experiment on acidic rain. Mass balance was used to quantify meHg sources and sinks during acidification and recovery. Although year-to-year variability in the summertime accumulation of meHg was high during both acidified and de-acidified years (C.V. 0.7 and 0.5, respectively), on average 65% more meHg accumulated in the water column during acidification. Most of the meHg mass accumulated in the anoxic hypolimnion (.70%), even though the hypolimnion constituted ,5% of the lake volume. In hypolimnetic waters, we observed a direct correlation between the maximum meHg concentration and the sulfate deficit for each year (r 2 5 0.5‐0.9) and a direct correlation between meHg and sulfide concentrations (r 2 5 0.7). Sulfide was directly related to dissolved organic carbon at concentrations between 300 and 600 mmol L 21 carbon (C). Seasonal changes in waterborne Hg (II) , meHg, and sulfate reduction covaried with the atmospheric deposition of Hg (II) and SO . Across all years, the interaction term [SO 3 Hg (II) ] explained 70% of the variation 22 22 44 in the meHg accumulation rate during summer. These results indicate that meHg production was co-mediated by several simultaneously occurring processes that affect the supply of Hg (II) substrate to the anoxic hypolimnion and the activity of methylating bacteria that are present there. They imply that meHg levels in lakes may respond to future changes in atmospheric Hg deposition in a rapid but complex way, modulated by environmental variables that can interact synergistically with Hg (II) supply. Such variables include sulfate in acid rain, organic carbon in terrestrial runoff, and temperature.

Variations in lake evaporation have a significant impact on the energy and water budgets of lakes. Understanding these variations and the role of climate is important for water resource management as well as predicting future changes in lake hydrology as a result of climate change. This study presents a comprehensive, 10-year analysis of seasonal, intraseasonal, and interannual variations in lake evaporation for Sparkling Lake in northern Wisconsin (USA). Meteorological and lake temperature measurements are made at a raft on the lake and are supplemented by radiation measurements from a nearby airport. The data are analyzed over 14-day periods from 1989 to 1998 (during the ice-free season) to provide bi-weekly energy budget estimates of evaporation rate (along with uncertainty estimates). The mean evaporation rate for Sparkling Lake over the study period is 3.1 mm day−1, with a coefficient of variation of 25%. Considerable variability in evaporation rates is found on a wide range of timescales, with seasonal changes having the highest coefficient of variation (18%), followed by the intraseasonal (15%) and interannual timescales (12%; for summer means). Intraseasonal changes in evaporation are primarily associated with synoptic weather variations, with high evaporation events tending to occur during incursions of cold, dry air (due, in part, to the thermal lag between air and lake temperatures). Seasonal variations in evaporation are largely driven by temperature and net radiation, but are out-of-phase with changes in wind speed. This presents challenges when calculating evaporation rates by means of the simpler mass-transfer technique. On interannual timescales, changes in summer evaporation rates are strongly associated with changes in net radiation and show only moderate connections to variations in temperature or humidity. Nonetheless, we are able to identify a simple, empirical relationship for estimating interannual evaporation rates that is more accurate than the mass transfer technique.

We evaluated several factors influencing the taxonomic richness of macrophytes, benthic invertebrates, snails, and fish in a series of northern Wisconsin lakes. We chose the study lakes to decouple the potential effects of ionic strength of lake water and stream connection, two factors that are usually highly correlated and therefore have been confounded in previous studies. In addition, our study lakes covered a wide range in a variety of characteristics, including residential development, abundance of exotic species, nutrient concentrations, predator abundance, and lake size. Species richness within each of the four taxonomic groups was significantly positively related to ionic strength (as measured by specific conductance); we also found secondary associations with other variables, depending on the specific group of organisms. The relationship between richness and lake area was dependent on the specific conductance of the lake and the vagility of the organisms; less vagile groups of organisms showed stronger and steeper species–area relationships in low-conductivity lakes. Further, after variance owing to specific conductance was removed, the presence of stream connections was positively related to species richness for fish, snails, and macrophytes as well as familial richness in benthic invertebrates. Our results indicate that lakes with relatively more groundwater input have lower extinction rates for all four groups of taxa and that lakes with stream inlets and outlets have enhanced immigration rates for fish, snails, benthic invertebrate families, and macrophytes. These findings link processes of immigration and extinction of four groups of organisms of varying vagility to landscape-level hydrologic characteristics related to the glacial history of the region.

Rusty crayfish (Orconectes rusticus) were first observed in Trout Lake, Wisconsin, in 1979 and took 19 years to completely disperse around the littoral zone, advancing at an average rate of 0.68 km·year–1. With the invasion of rusty crayfish, we found that fishes that share prey taxa with crayfish declined in numbers over time, but piscivorous fish species did not change in abundance. Snails declined from >10 000 to –2 in one of the first invaded areas. Mean abundance of Odonata, Amphipoda, and Trichoptera decreased significantly lake-wide. Resident crayfish species nearly disappeared, although total crayfish abundance, driven by high abundances of rusty crayfish, continued to rise. Submerged macrophyte species richness declined by as much as 80% at some locations. Together these responses demonstrate dramatic long-term changes in the littoral zone biota of Trout Lake. Continued invasions of similar lakes in the region suggest that these impacts are occurring on a region-wide basis with potentially irreversible effects on communities and ecosystems. Only through long-term natural experiments such as this study can researchers ascertain the full extent of invasions and their impacts on community and ecosystem process that respond at spatial and temporal scales not captured in mesocosm studies.

Despite considerable attention in recent years, the composition and dynamics of lake bacterial communities over annual time scales are poorly understood. This study used automated ribosomal intergenic spacer analysis (ARISA) to explore the patterns of change in lake bacterial communities in three temperate lakes over 2 consecutive years. The study lakes included a humic lake, an oligotrophic lake, and a eutrophic lake, and the epilimnetic bacterial communities were sampled every 2 weeks. The patterns of change in bacterial communities indicated that seasonal forces were important in structuring the behavior of the bacterial communities in each lake. All three lakes had relatively stable community composition in spring and fall, but summer changes were dramatic. Summertime variability was often characterized by recurrent drops in bacterial diversity. Specific ARISA fragments derived from these lakes were not constant among lakes or from year to year, and those fragments that did recur in lakes in different years did not exhibit the same seasonal pattern of recurrence. Nonetheless, seasonal patterns observed in 2000 were fairly successful predictors of the rate of change in bacterial communities and in the degree of autocorrelation of bacterial communities in 2001. Thus, seasonal forces may be important structuring elements of these systems as a whole even if they are uncoupled from the dynamics of the individual system components.

Recent literature has suggested that for many lakes and rivers, the respiratory breakdown of organic matter (R) exceeds production of organic matter by photosynthesis (gross primary production [GPP]) within the water body. This metabolic balance (GPP , R; ‘‘heterotrophy’’) implies that allochthonous organic matter supports a portion of the aquatic ecosystem’s respiration. Evidence that many lakes are heterotrophic comes from diverse approaches, and debate remains over the circumstances in which heterotrophy exists. The methods used to estimate GPP and R and the limited extent of lake types studied, especially with respect to dissolved organic carbon (DOC) and total phosphorus (TP) concentrations, are two reasons for differing conclusions. We deployed O2 and CO2 sondes to measure diel gas dynamics in the surface waters of 25 lakes. From these data, we calculated GPP, R, and net ecosystem production (NEP 5 GPP 2 R). Over the broad range in TP and DOC among the lakes, diel CO2 and O2 changed on a near 1 : 1 molar ratio. Metabolism estimates from the two gases were comparable, except at high pH. Most lakes in our data set had negative NEP, but GPP and R appeared to be controlled by different factors. TP correlated strongly with GPP, whereas DOC correlated with R. At low DOC concentrations, GPP and R were nearly equal, but, at higher DOC, GPP and R uncoupled and lakes had negative NEP. Strong correlations between lake metabolism and landscape related variables suggest that allochthonous carbon influences lake metabolism.

Hg and SO4 are priority pollutants in fresh waters throughout the northern hemisphere; both have atmo - spheric sources associated with anthropogenic emissions to the lower troposphere. Although depositional change has had a demonstrable impact on lakes over decadal time scales, effects over shorter time scales remain uncertain. To better understand the responsiveness of lakes to changing pollutant inputs, we examined the coupling of water chemis- try to atmospheric deposition over time scales of weeks to years in Little Rock Lake. The results indicate that the lake is tightly coupled to its air shed with respect to Hg. The annual cycle of Hg in surface waters closely tracks the annual cycle of Hg in precipitation. For SO4, the annual cycle in lake water is damped and not in phase with the cycle of at- mospheric deposition because of more intense internal recycling. These observations are in keeping with the residence times of Hg and S in lake water (Hg in months; S in years). Nevertheless, because atmospheric deposition is the domi- nant source of both pollutants, trends in lake water track trends in precipitation over a decadal time scale. Thus, remote lakes may be highly responsive to short term changes in Hg deposition, whereas responses to SO4 deposition may be more gradual. Resume :H g et SO 4 constituent des polluants des eaux douces de toute premiere importance dans l'hemisphere nord; tous les deux proviennent de sources atmospheriques associees aux emissions anthropiques dans la troposphere infe- rieure. Bien que les changements dans ces precipitations aient eu des effets quantifiables sur les lacs a l'echelle tempo- relles de decennies, leurs effets sur des echelles temporelles plus courtes restent mal connus. Pour mieux comprendre la reaction des lacs aux apports changeants de polluants, nous avons etudie conjointement la chimie des eaux et les precipitations atmospheriques a l'echelle de semaines et d'annees au lac Little Rock. Le Hg du lac est couple de facon tres serree a l'atmosphere environnant et le cycle annuel du Hg dans les eaux de surface suit de tres pres le cycle an- nuel du Hg dans les precipitations. Le cycle annuel du SO4 dans l'eau est tamponne et n'est pas en phase avec le cycle des precipitations atmospheriques a cause d'un recyclage interne plus important. Ces observations sont compatibles avec les durees de sejour du Hg (en mois) et du S (en annees) dans l'eau de lac. Neanmoins, puisque les precipitations atmospheriques sont la source principale des deux polluants, les tendances dans l'eau du lac suivent celles des precipi - tations a l'echelle des decennies. Ainsi, les lacs isoles peuvent reagir fortement a des changements a court terme dans les precipitations de Hg, alors que leur reaction aux precipitations de SO 4 peuvent etre plus graduelles.

(2002). Human influences on the spatial distribution of coarse woody debris in lakes in Northern Wisconsin, USA. SIL Proceedings, 1922-2010: Vol. 28, No. 2, pp. 545-548.

(2002). Effects of atmospheric nutrient inputs and climate change on the trophic status of Crystal Lake, Wisconsin. SIL Proceedings, 1922-2010: Vol. 28, No. 1, pp. 467-470.

Landscapes result from the interaction between nature and society. To understand current landscapes, it is essential to disentangle this interaction; to accomplish this, an historical approach is necessary. Here we focus on the interaction between humans and lakes during the last century in two sites in Wisconsin, USA, a state where lakes, in excess of 10,000, have played an important role in the evolution of the landscapes they belong to. We draw contrasts between the two localities, which are set in landscapes that differ in their physiographic setting, environmental history, and current structure. One, in northern Wisconsin, is a forested recreational lake district; the other, in southern Wisconsin, is dominated by agricultural uses and urban and suburban development. We contend that a common development in both localities has been the increase in the value that society attaches to lakes. As a consequence, lakes are playing a bigger role in the evolution of the terrestrial landscapes surrounding them.

Long-term monitoring of a precipitation-dominated lake in northern Wisconsin indicates decreasing mercury concentrations over the past decade. In the surface waters of Little Rock Lake, aqueous HgT has decreased at an average rate of 0.04 ng/L/yor roughly 40% from 1988 to 1999. The concentration of aqueous meHgT in Little Rock Lake has decreased tooat an average rate of 0.004 ng/L/y (roughly 50% since 1988). Atmospheric monitoring also indicates decreased mercury in bulk precipitation. For those years when we have a continuous data record (1994 through 1999), the annual Hg cycle in bulk precipitation has been damped and the average rate of bulk atmospheric Hg deposition decreased by roughly 50% (from 11.2 ug/m2/y to 5.5 ug/m2/y). These trends suggest that the lake waters have responded to decreased atmospheric Hg depositionperhaps driven by reduced anthropogenic emissions in the region. Ancillary data and mass balance modeling are used to more fully investigate recent changes in the aquatic mercury cycle.

(2000). Lakes in the landscape: approaches to regional limnology. SIL Proceedings, 1922-2010: Vol. 27, No. 1, pp. 74-87.

The degree to which a variable's dynamics are synchronized among a set of neighboring lakes indicates the relative influence of regional climate variability. Furthermore, by determining which lakes tend to be most synchronous in their response to climate we can deduce the primary ways that climate influences the dynamics of the variable in question. In this study, we look for evidence of synchrony in the temperature, calcium, and chlorophyll dynamics of seven neighboring northern Wisconsin lakes. Our goals were to assess the influence of climate on dynamics of these variables and the effect of lake basin properties on their response to climate signals. Synchrony was measured as the r2 value from a regression relating observations of a variable in one lake against simultaneous observations of the same variable in another. Temperature was the most synchronous variable, with all pairs displaying significant synchrony. Calcium was also highly synchronous, with 15 of the 21 lake pairs exhibiting significant sy...

SUMMARY 1. We tested the degree to which a lake’s landscape position constrains the expression of limnological features and imposes a characteristic spatial pattern in a glacial lake district, the Northern Highland Lake District in north-central Wisconsin. 2. We defined lake order as a metric to analyze the effect of landscape position on limnological features. Lake order, analogous to stream order, is based solely on geographical information and is simple to measure. 3. We examined the strength of the relationship between lake order and a set of 25 variables, which included measures of lake morphometry, water optical properties, major ions, nutrients, biology, and human settlement patterns. 4. Lake order explained a significant fraction of the variance of 21 of the 25 variables tested with ANOVA. The fraction of variance explained varied from 12% (maximum depth) to 56% (calcium concentration). The variables most strongly related to lake order were: measures of lake size and shape, concentrations of major ions (except sulfate) and silica, biological variables (chlorophyll concentration, crayfish abundance, and fish species richness), and human-use variables (density of cottages and resorts). Lake depth, water optical properties, and nutrient concentrations (other than silica) were poorly associated with lake order. 5. Potential explanations for a relationship with lake order differed among variables. In some cases, we could hypothesize a direct link. For example, major ion concentration is a function of groundwater input, which is directly related to lake order. We see these as a direct influence of the geomorphic template left by the retreat of the glacier that led to the formation of this lake district. 6. In other cases, a set of indirect links was hypothesized. For example, the effect of lake order on lake size, water chemistry, and lake connectivity may ultimately explain the relation between lake order and fish species richness. We interpret these relationships as the result of constraints imposed by the geomorphic template on lake development over the last 12 000 years. 7. By identifying relationships between lake characteristics and a measure of landscape position, and by identifying geomorphologic constraints on lake features and lake evolution, our analysis explains an important aspect of the spatial organization of a lake district.

1. Within a region with common climatic conditions, lake thermal variables should exhibit coherent variability patterns to the extent to which they are not influenced by lake specific features such as morphometry and water clarity. We tested the degree of temporal coherence in interannual variability for climatic variables (air temperature and solar radiation) among four lake districts in the Upper Great Lakes Region. We also tested the degree of coherence of lake thermal variables (near-surface temperature, eplimnetic temperature, hypolimnetic temperature and thermocline depth) for lakes within these districts. 2. Our four lake districts included the Experimental Lakes Area in north-western Ontario, the Dorset Research Centre area north of Toronto, Ontario, the Northern Highland Lake District in northern Wisconsin, and the Yahara Lakes near Madison in southern Wisconsin. Seventeen lakes were analyzed for lake thermal variables dependent on stratification. Another five lakes were added for the analysis of near-surface temperature. 3. The analysis tested whether for monthly and summer means, the climate (air temperature and solar radiation) across the four lake districts was coherent interannually and whether variables which measure the thermal structure of the lakes were coherent interannually among lakes within each lake district and across the four lake districts. 4. Temporal coherence was estimated by the correlation between lake districts for meteorological variables and between lake pairs for lake thermal variables. Mean coherence and the percentage of correlations exceeding the 5% significance level were derived both within and between lake districts for lake thermal variables. 5. Across the four lake districts, summer mean air temperature was highly coherent while summer solar radiation was less coherent. Approximately 60–80% of the interannual variation in mean summer air temperature at a site occurred across the entire region. Less than 45% of the variation in solar radiation occurred across sites. 6. Epilimnetic temperature and the near-surface temperature were highly coherent both within and between lake districts. The coherence of thermocline depth within and between lake districts was weaker. Hypolimnetic temperature was not coherent between lake districts for most lake pairs. It was coherent among lakes within some lake districts. 7. The influences of local weather and differences among lakes in water clarity are discussed in the context of differences in levels of coherence among lake thermal variables and among lake pairs for a given variable.

SUMMARY 1. Within a lake district of relatively homogeneous geomorphology, the responses of lakes to climate are influenced by the complexity of the hydrogeologic setting, position in the landscape, and lake-specific biological and physical features. We examined lake chemical responses to drought in surface water- and groundwater-dominated districts to address two general questions. (1) Are spatial patterns in chemical dynamics among lakes uniform and synchronous within a lake district, suggesting broad geomorphic controls; variable in a spatially explicit pattern, with synchrony related to landscape position, suggesting hydrologic flowpath controls; or spatially unstructured and asynchronous, suggesting overriding control by lake-specific factors? (2) Are lake responses to drought a simple function of precipitation quantity or are they dictated by more complex interactions among climate, unique lake features, and hydrologic setting? 2. Annual open-water means for epilimnetic concentrations of chloride, calcium, sulfate, ANC, DOC, total nitrogen, silica, total phosphorus, and chlorophyll a measured between 1982 and 1995 were assembled for lakes in the Red Lake and ELA districts of northwestern Ontario, the Muskoka ‐ Dorset district in south-central Ontario, and the Northern Highland district of Wisconsin. Within each district, we compared responses of lakes classified by landscape position into highland or lowland, depending on relative location within the local to regional hydrologic flow system. Synchrony, defined as a measure of the similarity in inter-annual dynamics among lakes within a district, was quantified as the Pearson product-moment correlation (r) between two lakes with observations paired by year. To determine if solute concentrations were directly related to interannual variations in precipitation quantity, we used regression analysis to fit district-wide slopes describing the relationship between each chemical variable and annual (June to May) and October to May (Oct‐May) precipitation.

Land use and land cover can have a significant impact on water chemistry, but the spatial scales at which landscape attributes exert a detectable influence on aquatic systems are not well known. This study quantifies the extent of the landscape influence using the proportion of wetlands in the watershed measured at different distances to predict dissolved organic carbon (DOC) concentrations in Wisconsin lakes and rivers, and to de- termine whether the watershed influence varies with season or hydrologic type of lake. The proportion of wetlands in the total watershed often explained the most variability of DOC in lakes when stepwise regression was used. However, best-model techniques revealed that, for lakes, r2 values often only differed 1-3% between models using the proportion of wetlands in the total watershed and models using only the proportion of wetlands in near- shore riparian areas (25-100 m). In rivers, the proportion of wetlands in the watershed always explained considerably more of the variability in DOC than did the proportion of wetlands in the nearshore riparian zone. The watershed influence also varied seasonally in rivers, as the proportion of the watershed covered by wetlands explained more of the variability in DOC in the fall than in the spring. Overall, the proportion of wetlands in the landscape explained much more of the variability of DOC concentrations in rivers than in lakes.

Although limnologists have long been interested in regional patterns in lake attributes, only recently have they considered lakes connected and organized across the landscape, rather than as spatially independent entities. Here we explore the spatial organization of lake districts through the concept of landscape position, a concept that considers lakes longitudinally along gradients of geomorphology and hydrology. We analyzed long-term chemical and biological data from nine lake chains (lakes in a series connected through surface or groundwater flow) from seven lake districts of diverse hydrologic and geomorphic settings across North America. Spatial patterns in lake variables driven by landscape position were surprisingly common across lake districts and across a wide range of variables. On the other hand, temporal patterns of lake variables, quantified using synchrony, the degree to which pairs of lakes exhibit similar dynamics through time, related to landscape position only for lake chains with lake water residence times that spanned a wide range and were generally long (close to or greater than 1 year). Highest synchrony of lakes within a lake chain occurred when lakes had short water residence times. Our results from both the spatial and temporal analyses suggest that certain features of the landscape position concept are robust enough to span a wide range of seemingly disparate lake types. The strong spatial patterns observed in this analysis, and some unexplained patterns, suggest the need to further study these scales and to continue to view lake ecosystems spatially, longitudinally, and broadly across the landscape.

A single stress, acidification with sulfuric acid, was applied to Little Rock Lake in a whole-ecosystem manipulation. We documented a wide range of responses to the acidification, including increases in the concentrations of various chemicals, shifts in microbial processes and a major increase in water clarity to UV-B radiation. Each of these changes could in itself be considered as a separate ecosystem stress that is distinct from the intended manipulation. Acidification in Little Rock Lake was accompanied by a number of substantial changes in the occurrence of organisms. A series of detailed investigations indicates that the mechanisms underlying these organismal changes are varied but cannot usually be tied to the direct effects of acidification. Overall, our results demonstrate how multiple stresses can arise from a single agent operating on an ecosystem and suggest that singly operating stresses may actually be quite rare.