Alison Appling's paper on the theory of fine-scale nutrient dynamics is now available online at American Naturalist. The goal of the paper is to provide biological theory for the interpretation of passively-observed patterns such as diel variation and experimental manipulations such as pulsed nutrient additions. To do so, Alison developed a physiological model of nutrient uptake and growth, based on existing models that have been used to understand competition and productivity in aquatic ecosystems. The major findings are that 1) the presence of diel variation is a potential indicator of nutrient limitation status (with diel variation in a solute indicating that the varying nutrient is not limiting), 2) the shape of diel curves (or pulse responses) can vary strongly depending on physiological characteristics of biota, and 3) that differential lags among nutrients can de-couple them over diel or experimental time scales. We think this paper will inform the growing number of studies that use in situ nutrient sensors to characterize these fine-scale patterns.
The first chapter Dina Liebowitz's dissertation work is now available online in Freshwater Biology. This paper examines the relationships among environmental conditions (especially dissolved oxygen concentrations), the abundance of grazing snails, and the presence of benthic algae in Florida springs. By comparing data from field surveys both within and across springs, Dina showed that the abundance of snails (especially the genus Elimia) is the primary determinant of the abundance of algae in Florida springs. In addition, we found that algal abundance is generally either very high or very low, a pattern that is consistent with alternative stable states. Finally, our field survey also shows that oxygen concentrations, among other environmental controls, influences the abundance of snails. Putting these findings together, we suggest that even temporary declines in oxygen concentrations, which occur commonly in springs, could suppress snail populations and activity and allow algae to escape grazer control.
Most of the management attention in springs has emphasized nutrient concentrations as the major driver of algal mats, a hypothesis which we have challenged pretty strongly in an earlier paper that first articulated the oxygen-grazer-algae cascade. Dina's other chapters examine these processes experimentally, and lead to similar conclusions. Very exciting to have this work out, and congratulations Dina!
Meredith Steele's paper on the characteristics of urban water bodies in US cities has been published in this month's issue of Ecological Applications. Based on analysis of over 1 million water bodies in 100 US cities, we show that urban water bodies exhibit less variation in size than their counterparts in undeveloped lands, are generally less connected to surface channel networks, and that despite the presence of many unusually shaped water bodies in urban lands, they also tend to be more simply shaped. These patterns have implications for how urban water bodies interact with and respond to upland processes and act as habitat for biota.
We also found that city size influences whether or not the types of water bodies in cities are similar to those in surrounding undeveloped lands. In large cities, wetlands, ponds, and other shallow water bodies are present in about the same proportion as in their surroundings. But in smaller cities, these water bodies are generally absent. This pattern suggests that the processes that shape urban hydrography change as cities grow: initially, cities may be located in relatively dry land where building is easier, but as the urban footprint expands, alteration of water bodies may become more prevalent. What we don't know is how past and present protections for aquatic habitats may influence these patterns.
Members of the Heffernan Lab and our colleagues in the Duke River Center made the trek to Portland last week to participate in the first ever Joint Aquatic Sciences Meeting, which convened members of the four largest freshwater scientific societies. Jim Heffernan demonstrated that the Everglades has alternative stable landscape states and (filling in for Martin Doyle) discussed the timescales of reservoir management. Anna Braswell presented evidence that local feedbacks influence continental scale distribution of salt marshes. Chelsea Clifford presented her research on artificial streams in California. Megan Fork used an interactive poster to show how DOC concentrations and fluxes are changing across the US. Heffernan Lab Alumni were out in force as well: Ewan Isherwood talked about vegetation distinctness in the Everglades, Alison Appling talked about greenhouse gas concentrations in Northeastern Lakes, and Meredith Steele somehow wove together urban hydrography and Mermaids. Work from the Heffernan lab was also mentioned in two of the Plenary sessions, via presentations by Laurel Larsen and Pat Soranno. We capped off a succesful meeting by having lunch with members of the Sponseller lab. Now, back to work!
Our paper on the landscape pattern of cypress dome wetlands in Big Cypress National Preserve, led by Adam Watts, is available online at Earth Surface Processes and Landforms. Using a variety of lines of evidence, we show that cypress domes are regularly distributed, and that this pattern is more strongly reflected in the bedrock and vegetation than in soil elevations. We also expand on and refine our conceptual model of how this pattern arises. In brief, we propose that cypress domes are drilling into the limestone bedrock via the acidity of organic matter and CO2 production by vegetation. This local positive feedback expands wetland basins vertically and laterally. As wetlands expand, they essentially come into competition for runoff from the adjacent uplands, which ultimately limits the size and density of wetlands on the landscape. Patterns of soil phosphorus suggest that P mobilization by this dissolution may amplify this biogeomorphic feedback. This paper lays the foundation for our new NSF grant, which will support more mechanistic study of the processes that create these wetland basins and control their distribution on the landscape.
Alison Appling's paper on the physiological basis of fine-scale ecosystem nutrient dynamics has been accepted for publication in the prestigious journal The American Naturalist. The core aim of this paper is to understand the magnitude and shape of rising and falling nutrient concentrations in response to daily variation in metabolism, and in responses to other, episodic pulses of nutrients. Alison adopted physiological models of nutrient metabolism that have been used to understand competition and productivity of phytoplankton, but used them to examine how temporal variation in inputs of one resource (light, nitrogen, or phosphorous) influences temporal variation in the export of others. The model shows 1) that the presence or absence of diel nutrient variation or responses to resource pulses may be an indicator of nutrient limitation, 2) that variation in physiological traits can modify the temporal patterns of uptake in response to single or repeated nutrient pulses, and 3) that differences in the physiology of N and P metabolic pathways can de-couple their uptake in time. These findings provide an important theoretical basis for the interpretation of nutrient spiraling measurements from plateau and pulse enrichments and of high-frequency time series obtained from nutrient sensors. Congratulations Alison!
Our Urban Homogenization group has a new paper out in the Proceedings of the National Academy of Sciences. Led by Colin Polsky at Clark University, the paper shows that lawn care practices vary both among cities and among different life stages, development densities, and socio-economic groups within cities. The major conceptual contributions of the paper are to introduce the idea of scale to our previous work on homogenization, and to demonstrate that lawn care practices may be very heterogeneous, even if the biophysical structure of residential landscapes is homogenized by urbanization.
Nutrient limitation happens when nutrient supply is insufficient to meet demand by biota, but most assessments of nutrient limitation are based only on concentration. In streams, where metabolism is highly variable and streamflow continuously replenishes nutrient supply, concentration has been a poor predictor of nutrient limitation and autotroph biomass. We have a paper out in Freshwater Science that explores new predictors of nutrient limitation, based on nutrient spiraling theory and empirical measurements of metabolism and nutrient flux. The paper was led by Sean King, who finished his PhD at the University of Florida a few years ago, and is now at the Southwest Florida Water Management District (SWFWMD). Congratulations Sean!
Meredith Steele's paper on convergent surface water distributions in US cities is now available online (and open accesss) at Ecosystems. In this paper, meredith showed that the abundance and distribution of surface water (hydrography) is largely a function of land use, and relatively insensitive to climatic and geologic controls, within the boundaries of 100 US cities. In essence, we add lakes, ponds, and canals to dry places, and remove them from wet places; the end result is that cities have very similar hydrography whether they are situated in a desert, a subtropical wetland, or a temperate forest. Check it out!
This is the homepage of the Heffernan Lab at Duke University. Here you can find all sorts of information about our research, teaching, and outreach. If you have any questions, contact Dr. Heffernan.
Dr. Jim Heffernan
I am an Assistant Professor in the Nicholas School of the Environment at Duke University. My research is focused on the causes and consequences of major changes in ecosystem structure, mostly in streams and wetlands.
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