Browsing by Author "Heffernan, James B"

An abstract of a dissertation: As humans increasingly alter the surface geomorphology of the Earth, a multitude of artificial aquatic systems have appeared, both deliberately and accidentally. Human modifications to the hydroscape range from alteration of existing waterbodies to construction of new ones. The extent and ecosystem services of these systems are underexplored, but likely substantial and changing. Instead of simply accepting that artificial ecosystems have intrinsically low values, environmental scientists should determine what combination of factors, including setting, planning and construction, subsequent management and policy, and time, impact the condition of these systems. Scientists, social scientists, and policymakers should more thoroughly evaluate whether current study and management of artificial aquatic systems is based on the actual ecological condition of these systems, or judged differently, due to artificiality, and consider resultant possible changes in goals for these systems. The emerging recognition and study of artificial aquatic systems presents an exciting and important opportunity for science and society.

Irrigation ditches are ubiquitous features of water networks in rural and urban settings in drylands, and are thus potentially important habitats within the modern hydroscape. The habitat value of ditches and other artificial systems depends on whether these systems respond to local and watershed-scale land use in similar ways to natural features, or whether artificial origin inherently constrains a system's ecological condition. The ditches and creeks of Bishop, California are fed by water from the same minimally developed watershed on the Eastern slope of the Sierra Nevada Mountains, and so served to test whether artificial and natural waters in the same watershed setting and with shared water can provide similar habitat. We sampled benthic macroinvertebrates at 52 sites within the town, stratified by substrate and season. Communities varied by substrate and season as expected, but did not differ significantly between artificial and natural streams. Instead, both types of streams changed as water flowed from undeveloped desert through town, suggesting that irrigation ditches respond to local urbanization in much the same way that natural streams do. Differences in finer-scale spatial structure of community similarity suggest that community assembly processes may differ between natural and artificial channels, but potential mechanisms for these differences are unclear. This study demonstrates that artificial aquatic systems may have substantial ecological value, and suggests that the poor condition of many artificial aquatic systems may reflect stressful watershed settings rather than something intrinsic to their artificiality.

The drainage ditches of the North Carolina Coastal Plain do not merely degrade wetlands; they themselves have ecological structural characteristics of wetlands. We surveyed 32 agricultural, freeway, and forested ditch reaches across this region for hydrologic indicators, soil organic matter, and plants. All showed some hydrologic indicators and had some soil organic matter, with easterly, swampy forests having the most, though with substantial variation across all and few significant differences between types. All had hydrophytic herbaceous plant communities in the sense of at least half their percent cover belonging to obligate, facultative wetland, or facultative taxa. These herbaceous communities differed significantly across site types (F=3.25, d.f.= 2, p=0.001), and responded to both landscape-level factors like nearby development coverage and local-level factors like apparent mowing. Sample sites were not well mapped in well-used federal aquatic databases; the National Hydrography Dataset only included one on a “CanalDitch” flow line, and the National Wetlands Inventory only included two within “partially drained/ditched” areas, and none as individual “excavated” features. Others were mis-categorized, but neither database included any highway sampling sites. Despite this limited information about extent, variation and management impact suggests that human potential to impact wetland structure of these manmade aquatic ecosystems throughout the North Carolina Coastal Plain, and beyond, could be large.

Artificial lakes are a dominant aquatic ecosystem type, but the processes controlling their condition are under-explored. Here we use structural equations modeling to compare the formation of algal blooms and associated water quality issues in 1,045 artificial and 870 natural lakes in the United States using the U.S. Environmental Protection Agency’s National Lakes Assessment data. We compare chemical and physical measurements associated with water quality and the relationships between them, and find that the processes are significantly different between natural and artificial lakes, in a way that suggests impacts of interference with thermal stratification through dam management in reservoirs. However, both the overall processes and the distributions of the data are roughly similar between the two origin types, and between 2007 and 2012 sample years. Artificial lakes are lakes, and process-based explorations of their behavior can help us better know management options.

Taken together, this dissertation examines an artificial version of each of the major aquatic ecosystem types: stream, wetland, and lake. It examines the processes controlling their ecological condition with increasing intricacy with each chapter, and finds ways that artificial aquatic ecosystems are both similar to and different from natural ones. This dissertation provides a new way of looking at the constraints and opportunities that artificial waterbodies afford those in charge of them and interested in their conservation potential.

Rivers transform more than half of the organic inputs they receive from terrestrial systems through metabolic processes. In addition to providing the energetic base to sustain stream food webs, these transformations are linked to multiple elemental cycles and result in the release of greenhouse gasses to the atmosphere, oxygen depletion, and the mobilization of trace metals. Despite this central importance we lack a sufficient understanding of what drives processes within these ecosystems to make broad scale predictions about metabolic rates and biogeochemical cycles. This dissertation addresses how local controls interact with hydrologic and geomorphic constraints to determine the nature and magnitude of stream carbon and energy cycling. Through field observations and ecosystem process models, this study characterizes the spatial and seasonal biogeochemical dynamics of a single study system, New Hope Creek, a low gradient 3rd order river in the North Carolina Piedmont. I present a mechanistic explanation of stream ecosystem functioning from three different perspectives and use the insights gained to confront existing conceptual models and methods in river science. In chapter two, I examined the frequency and dynamics of river hypoxia, or depleted dissolved oxygen, in the North Carolina Piedmont by synthesizing state monitoring records since the 1960s and by collecting high resolution measurements of oxygen along a 20 km section of New Hope Creek. In chapter three, I monitored dissolved oxygen concentrations and modeled the metabolic regimes across six river segments over the course of three years. I compared the rates and seasonality of annual metabolism across space and time and examined the impact of climatic, hydrologic, and geomorphic drivers. Using the scales of this variation for context, I compared the metabolic regime in New Hope Creek today to a historical study of annual metabolism collected fifty years ago in the same site. In chapter four, I measured dissolved greenhouse gas concentrations from the autumn to the following spring at the six study locations from chapter three. I calculated the rate of exchange of CO2, CH4, N2O and O2 with the atmosphere, estimated the fraction of this flux that was attributable to instream metabolic processes, and determined the best predictors for each concentration and flux rate. The results from this research have implications for both conceptual models and methodological approaches. First, I found that hypoxia is widespread throughout New Hope Creek. It can arise through oxygen supply limitation due to seasonal low flows and warm water, even in the absence of high organic matter and nutrient loading. Hypoxia is most frequent at night and in pools, and is systematically underrepresented by state-wide monitoring records which rely on point samples and avoid both night and pools. Second, warmer water temperatures shift stream ecosystems toward more net heterotrophy. The effects of rising temperatures on stream ecosystem respiration are contingent upon organic matter inputs and storage, each of which are strongly constrained by flow regime and local geomorphology. Hydrologic settings in which storms are more frequent or more severe will drive high variability in the transport and fate of organic matter processing in streams. Third, the geomorphology and hydrology of New Hope Creek create conditions under which stream metabolic cycling is the dominant control on greenhouse gas concentrations and flux rates. These findings challenge preconceived notions about how rivers work and encourage us to reconsider conceptual models. Rivers are lotic ecosystems, which are defined by their advective flow, with the implication that they are well aerated and rarely hypoxic. Methods in stream ecology are not suited to study the conditions that arise during lentic, or non-flowing, time periods and as a result, the geomorphologies that create these conditions are avoided and understudied. However, these results suggest that they are control points of biogeochemical activity and that they may be more susceptible to change in response to anthropogenic drivers. Collectively, this study calls into question the binary distinction between lotic and lentic ecosystem dynamics with which we tend to categorize and study freshwater ecosystems. To understand the full range of variability of inland waters we must broaden our conceptual frameworks and adapt our methods of study to encompass ecosystems that span this divide.