Joye Research Lab

We have several projects that explore the impacts of climate change (e.g., sea level rise, drought, climate variability, increased frequency of intense storms like hurricanes) on Georgia coastal ecosystem dynamics. We collaborate with Dr. Christof Meile (UGA) on some of these projects.

We are investigating drought-induced stress on key sediment processes in tidal salt marshes. Record droughts in Georgia have induced major alterations in coastal ecosystems (acute marsh dieback) in these locations, providing the opportunity to conduct a natural experiment examining the consequences of drought-induced plant mortality on a suite of ecosystem services commonly associated with tidal salt marshes. We are using laboratory bioreactor experiments to simulate drought and monitoring the resulting impacts on sediment processes. Post doc Laura Palomo is conducting this work.

Post doc Laura Palomo: lpalomo (at) uga (dot) edu

This research will address the following questions:

What are the consequences of drought on sediment biogeochemical signatures?
What is the time-scale of drought-induced geochemical alteration and how does it take for sediments to recover from drought?
How does drought or variability in rainfall alter sediment processes and change sediment-water fluxes in coastal environments?
How fast do sediment microbial communities recover from drought-induced stress?

Temperature Driven Decoupling of Carbon Cycling in Freshwater Sediments and the Relative Production and Flux of Methane Versus Carbon Dioxide {Learn More}

Methane (CH4) is a potent atmospheric greenhouse gas that has important biological sources and is involved intimately in the global carbon cycle. Significant quantities of CH4 are produced in and subsequently released from freshwater wetlands and these habitats account for 20-40% of the CH4 flux to the atmosphere each year. However, critical aspects of the factors regulating CH4 production and release from freshwater wetlands remain poorly understood. To better understand freshwater sediment CH4 dynamics, we are studying benthic carbon cycling in three coastal freshwater habitats in Florida, Georgia and Maine. We are explicitly examining coupling between the hydrolysis and fermentation of organic matter and terminal metabolism to CH4 and carbon dioxide (CO2). Further, we are evaluating how temperature, organic carbon availability and microbial populations interact to regulate mineralization of complex organic matter and production of terminal metabolites, CH4 and CO2. Field measurements are used to characterize the environmental settings and provide integrative measures of whole sediment-responses while laboratory experiments provide insight into the temperature dependency of individual processes, and on how these processes interact with each other. These data will be incorporated into a model to achieve a comprehensive quantitative description of anaerobic sediment metabolism under different temperature regimes.

This research program emphasizes in-situ sampling and laboratory experimentation and is organized around the working hypothesis that changes in temperature influence the rate and efficiency of organic carbon mineralization and the type (CH4 vs. CO2) of terminal products produced in freshwater sediments. This project will address a series of major research questions, including:

Do temperature-induced changes in coupling between hydrolysis/fermentation and terminal metabolism alter the efficiency of organic carbon mineralization?
Do variations in microbial populations contribute to changes in organic carbon mineralization rates, the efficiency of mineralization, and/or the CH4 to CO2 production ratio?
Do temperature-driven differences in process rates significantly affect benthic exchange fluxes?
Does thermodynamic homeostasis regulate the habitat geochemical signature and alter pathway(s) of carbon cycling?
Are measured activity patterns amendable to a unified, temperature-dependent view of anaerobic carbon metabolism that is valid across sites and seasons?

This project describes novel study of benthic carbon dynamics in freshwater environments. We will explicitly quantify rates of hydrolysis and fermentation and terminal metabolism and examine how both of these suites of processes are influenced by changes in temperature. The relative importance of carbon availability (and lability) will be evaluated in the lab by amending sediments with cellulose or acetate. Variations in microbial populations will be documented by quantifying the abundance of microbial biomarkers. This project will fill a critical data gap by improving our understanding of CH4 cycling in freshwater habitats and by discerning how environmental (temperature, carbon availability) and microbial factors impact the production of CH4 and its flux to the atmosphere.

Salinization of Coastal Freshwater Wetlands: Assessing the long-term impacts of Salt versus Sulfate addition on ecosystem function {Learn More}

Saltwater intrusion into fresh water habitats, which may have serious repercussions on ecosystem-level dynamics. Saltwater intrusion derives from human interventions (damming rivers; freshwater diversion; etc.) and natural variability (droughts or increases in sea level). Increased salinity leads to a cascade of events that impact coastal ecosystems at a variety of scales ranging from small (cm to m) scale changes in sediment biogeochemistry and materials processing to large (m to km) scale alterations in the distribution of flora and fauna. Despite the global scope of this impact, many of the effects of saltwater intrusion on coastal ecology and biogeochemistry remain poorly understood or undocumented.

The key impacts of saltwater intrusion likely stem from the increases in chloride (salt) and sulfate concentration. Small changes in chloride concentration can physiologically stress microorganisms, animals and plants and alter metabolic pathways, rates of activity and abundance. Sulfate is a terminal electron acceptor for anaerobic respiration that is present in limited amounts in freshwater but that is abundant in sea water. Changes in sulfate availability may result in dramatic changes in sediment biogeochemistry and we predict that such changes in turn result in ecosystem-level alterations of biogeochemical cycles that impact plant and animal distributions.

Sediments are integral components of coastal systems that contribute to ecosystem metabolism and provide habitat for macrofauna and plants. Sediments are complex adaptive systems that are comprised of multiple components that interact and mutually affect each other. Variability in reaction and transport, coupled to biological and chemical variability generate complex reaction networks in coastal sediments. Despite this complexity, interactions between geochemical and biological components generate globally recognized patterns of emergent behavior, referred to as biogeochemical redox zonation.

Redox zonation is observed in sediments at a variety of spatial scales, from the mm to cm scales in coastal sediments to m scales in the deep sea. Redox zonation is also observed in stratified water columns, e.g., in lakes and fjords, in groundwater aquifers and in deep ocean brines. The differences in scale illustrate that the emergent behavior depicted by redox zonation is sensitive to boundary and/or initial conditions, a characteristic of complex adaptive systems. The resulting patterns are robust, however, suggesting that similar underlying mechanisms operate in different environments. The widespread occurrence of redox zonation illustrates the generality of this concept and underscores the need to understand how basic patterns are generated over a variety of spatial and temporal scales. Elucidating the biological, chemical and physical mechanisms that contribute to the emergent behavior known as biogeochemical redox zonation is a primary goal of the proposed work.

We are examining the response of coastal freshwater marshes to changes in salinity and sulfate levels. The centerpiece of our work is an integrated effort to quantify the interplay between geochemical factors, microbial community composition and activity, and plants and infauna in freshwater, brackish, and marine sediments. Field and laboratory geochemical, microbiological, molecular biological and ecological data will be used to unravel the network of biogeochemical reactions governing the cycles of carbon, oxygen, sulfur, nutrients and trace metals in freshwater, brackish, and marine sediments. Field and laboratory data will be used to develop reaction-networks describing aqueous and solid phase speciation as well as the kinetics of microbial processes and abiotic processes. These data will be used as input to a reaction-transport model which will permit us to identify the factors influencing and driving patterns of sediment biogeochemcal redox zonation.

We thank the Environmental Protection Agency Climate Change program.

**Disclaimer** The content of this page is based in part on work supported by the EPA. Any opinions, findings, and conclusions or recommendations expressed here are those of the author (Mandy Joye) and do not necessarily reflect the views of the EPA.