We've just heard from NSF that our proposal to study the wetlands of Big Cypress National Preserve will be funded! The Big Cypress landscape is a mosaic of isolated wetlands, grasslands, and pine forests. The core observation that motivates our proposal is that the Cypress wetlands appear to be regularly spaced. This sort of regular pattern occurs in dryland vegetation, in peatlands (including the nearby Everglades) and elsewhere, and is thought to arise from feedbacks that are spatially-dependent. Basically, organisms improve the environment in their immediate vicinity, but that has the effect of making more distant locations unsuitable. In Big Cypress, we think that cypress trees essentially capture water from the surrounding landscape by dissolving the limestone bedrock and creating wetland depressions. Pretty smart! Testing this core hypothesis, and all of its pieces, requires an interdisciplinary team. My colleagues at UF, who are going to do most of the field work, include ecohydrologists, soil scientists, and organic and inorganic geochemists. Brad Murray and I are in charge of developing a model of this landscape. It's gonna be fun.
Meredith Steele's paper (with a long list of co-authors) on the surface water characteristics of US cities has been accepted at Ecosystems! Congratulations Meredith!
Abstract: Earth’s surface is rapidly urbanizing, resulting in dramatic changes in the abundance, distribution and character of surface water features in urban landscapes. However, the scope and consequences of surface water redistribution at broad spatial scales are not well understood. We hypothesized that urbanization will lead to convergent surface water abundance and distribution: in other words, cities will gain or lose water such that they become more similar to each other than are their surrounding natural landscapes. Using a database of more than 1 million water bodies and 1 million km of streams, we compared the surface water of 100 US cities with their surrounding undeveloped land. We evaluated differences in areal (AWB) and numeric densities (NWB) of water bodies (lakes, wetlands, etc.), the morphological characteristics of water bodies (size), and the density (DC ) of surface flow channels (i.e. streams and rivers). The variance of urban AWB, NWB, and DC across the 100 MSAs decreased, by 89%, 25%, and 71% respectively, compared to undeveloped land. These data show that many cities are surface-water poor relative to undeveloped land; however, in drier landscapes urbanization increases the occurrence of surface water. This convergence pattern strengthened with development intensity, such that high intensity urban development had an areal water body density 98% less than undeveloped lands. Urbanization appears to drive the convergence of hydrological features across the US, such that surface water distributions of cities are more similar to each other than to their surrounding landscapes.
An unformatted version of Meredith Steele's paper on the morphological characteristics of urban lakes is now available online.
Understanding and managing urban aquatic ecosystems has to begin with good information about their distribution and about characteristics like size and shape (hydrography). We've just received word that our paper on this topic, led by post-doc Meredith Steele, has been accepted for publication in Ecological Applications! One simple thing that makes this paper significant is the spatial extent of Meredith's analysis (100 cities across the whole US). A second simple thing about this paper is what the data show: urban water bodies (lakes, ponds, wetlands) are of moderate size. That is, in comparison to undeveloped land outside cities, there are fewer very large and fewer very small water bodies. The more complicated thing we want to know is how these differences arise. One possibility is that cities are located near particular types of water bodies. A second possibility is that, during urban development, we remove small (and large?) water bodies, and build medium-sized ones. We use some subtler patterns in our data to distinguish among these processes, and ask how their importance might change over time. This paper is the first of several papers Meredith has in review or in prep that will deal with other scales and other aspects of urban hydrographic change. Very exciting! And congratulations Meredith!
Congratulations are due to Ewan Isherwood, who successfully defended his MS thesis this morning at FIU! Ewan's research addresses the distinctness of vegetation communities (ridges and sloughs) in the Florida Everglades, with a particular focus on how these these distinct communities become more blended as a result of hydrologic modification and microtopographic flattening. To do this, Ewan developed statistical proxies of distinctness using multivariate techniques, and applied them across our Everglades-wide sampling regime.
A few weeks ago, Dr. Todd Kana delivered a new instrument to the River Center. The Membrane Inlet Mass Spectrometer, which Todd designs and builds, measures dissolved gases directly from water samples. We had one of these instruments in our lab at FIU, and used it to study denitrification in the Floridan Aquifer and in spring-fed and blackwater rivers.
Yesterday, Todd returned to us to train about 8 people from the River Center and other labs on the use of the new instrument, which is a slightly different model than the old MIMS. We also had lunch with about a dozen people where we discussed the different applications of the MIMS, including the different gases it can measure and the different kinds of samples it can measure (air, water; field samples, in line experiments, etc.). We are very excited to have this new capability in the lab, and look forward to all of the neat things we will learn with it!
Megan Fork's paper on DOC and denitrification, published in Ecosystems, is now available online. This work, which comes from Megan's MS thesis, uses a natural gradient in dissolved organic matter (DOC) concentration to understand how terrestrially-derived DOC influences aquatic denitrification. We found that this DOC does not directly stimulate or inhibit denitrification, but that denitrification becomes more limited by DOC as DOC concentrations increase. The explanation for this counter-intuitive finding is that DOC reduces light, and therefore the release of labile DOC by macrophytes and algae. This paper provides additional evidence for the importance of in-stream primary production for denitrification in larger rivers, and has implications for how aquatic ecosystems may respond to future changes in DOC. Read the whole thing here, and congratulations Megan!
A belated congratulations to soon-to-be alumni/ae Tim Covino and Meredith Steele! Tim has accepted a position as an assistant professor in the Department of Ecosystem Science and Sustainability at Colorado State University. Meredith has accepted a position as an assistant professor in the Department of Crop and Soil Environmental Sciences at Virginia Tech. Both begin their new gigs in January 2014. Hooray!
A big welcome to Chelsea Clifford, the newest PhD student in the Heffernan lab. Chelsea is supported by a Graduate Research Fellowship from the National Science Foundation. She received her BA in Biology from Carleton College in 2010, and has worked for the MacAurther Agro-ecology Research Station and the Chesapeake Bay Foundation. Chelsea's interests are primarily in the ecology and biogeochemistry of designed, altered, and restored ecosystems.
New paper on hydrologic feedbacks in the Everglades has been published in PLoS One. In this study, we develop a mathematical model of interactions between peat accumulation, vegetation productivity, soil elevation, and water flow. We show that the resulting feedbacks can cause spontaneous divergence of ridges and sloughs, and that these feedbacks act differentially with direction. The model provides a range of predictions that we are hoping to test with data from our Everglades monitoring project. You can download the paper here.
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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