Timothy K. Kratz

Complexity of Coupled Human and Natural Systems

Integrated studies of coupled human and natural systems reveal new and complex patterns and processes not evident when studied by social or natural scientists separately. Synthesis of six case studies from around the world shows that couplings between human and natural systems vary across space, time, and organizational units. They also exhibit nonlinear dynamics with thresholds, reciprocal feedback loops, time lags, resilience, heterogeneity, and surprises. Furthermore, past couplings have legacy effects on present conditions and future possibilities.

Carbon dioxide supersaturation in the surface waters of lakes

Data on the partial pressure of carbon dioxide (CO2) in the surface waters from a large number of lakes (1835) with a worldwide distribution show that only a small proportion of the 4665 samples analyzed (less than 10 percent) were within �20 percent of equilibrium with the atmosphere and that most samples (87 percent) were supersaturated. The mean partial pressure of CO2 averaged 1036 microatmospheres, about three times the value in the overlying atmosphere, indicating that lakes are sources rather than sinks of atmospheric CO2. On a global scale, the potential efflux of CO2 from lakes (about 0.14 x 1015 grams of carbon per year) is about half as large as riverine transport of organic plus inorganic carbon to the ocean. Lakes are a small but potentially important conduit for carbon from terrestrial sources to the atmospheric sink.

Coupled Human and Natural Systems

Abstract 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 societys unprecedented challenges.

DISSOLVED ORGANIC CARBON AS AN INDICATOR OF THE SCALE OF WATERSHED INFLUENCE ON LAKES AND RIVERS

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.

Randomized Intervention Analysis and the Interpretation of Whole‐Ecosystem Experiments

Randomized intervention analysis (RIA) is used to detect changes in a ma- nipulated ecosystem relative to an undisturbed reference system. It requires paired time series of data from both ecosystems before and after manipulation. RIA is not affected by non-normal errors in data. Monte Carlo simulation indicated that, even when serial au- tocorrelation was substantial, the true P value (i.e., from nonautocorrelated data) was <.05 when the P value from autocorrelated data was <.01. We applied RIA to data from 12 lakes (3 manipulated and 9 reference ecosystems) over 3 yr. RIA consistently indicated changes after major manipulations and only rarely indicated changes in ecosystems that were not manipulated. Less than 3% of the data sets we analyzed had equivocal results because of serial autocorrelation. RIA appears to be a reliable method for determining whether a nonrandom change has occurred in a manipulated ecosystem. Ecological argu- ments must be combined with statistical evidence to determine whether the changes dem- onstrated by RIA can be attributed to a specific ecosystem manipulation.

Lakes and streams as sentinels of environmental change in terrestrial and atmospheric processes

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.

Wireless Sensor Networks for Ecology

Abstract Field biologists and ecologists are starting to open new avenues of inquiry at greater spatial and temporal resolution, allowing them to “observe the unobservable” through the use of wireless sensor networks. Sensor networks facilitate the collection of diverse types of data (from temperature to imagery and sound) at frequent intervals—even multiple times per second—over large areas, allowing ecologists and field biologists to engage in intensive and expansive sampling and to unobtrusively collect new types of data. Moreover, real-time data flows allow researchers to react rapidly to events, thus extending the laboratory to the field. We review some existing uses of wireless sensor networks, identify possible areas of application, and review the underlying technologies in the hope of stimulating additional use of this promising technology to address the grand challenges of environmental science.

Species Compensation and Complementarity in Ecosystem Function

Functional complementarity occurs when ecosystem processes are maintained at constant levels despite stresses that induce shifts in the populations driving those processes. Understanding when such complementarity occurs depends on an integration of perspectives from population and ecosystem ecology. Here, we examine the extent to which functional complementarity is linked with compensatory dynamics among species that carry out a particular ecosystem function. Our approach combines analyses of long-term zooplankton data from a whole-lake acidification experiment with a theoretical treatment of species compensation. Results from the acidification of Little Rock Lake, WI indicated that the biomass of cladocerans, copepods, rotifers, and total zoo-plankton remained at high levels despite the loss of component species from each group. Compensatory increases by other taxa were responsible for this complementarity of function. Theoretical considerations indicated that the degree of compensation occurring among species in response to environmental change increased in response to two different factors: the functional similarity of interacting species and the degree to which an environmental change acts nonuniformly on their interactions. Finally, we tested the extent to which compensation in unperturbed systems could predict functional complementarity. We analyzed a 7-year record from the reference basin of Little Rock Lake and found that substantial compensation occurred only in the natural dynamics of rotifers and cladocerans during some seasons. Complementarity in response to acidification was evident, however, among copepods and total zooplankton in addition to rotifers and cladocerans, and could not have been predicted solely on the basis of compensation prior to stress. Taken together, our analyses reveal how functional complementarity is linked to a variety of compensatory interactions among species responding to stress.

Temporal Patterns in Bacterial Communities in Three Temperate Lakes of Different Trophic Status

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.

Spatial Variation among Lakes within Landscapes: Ecological Organization along Lake Chains

ABSTRACT 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.