Question 3: Longitudinal Gradients
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Research QuestionQ3: What are the underlying mechanisms by which the freshwater-saltwater gradient drives ecosystem change along the longitudinal axis of an estuary? OverviewThe data collected to answer GCE question 1 (external forcing to the domain) and question 2 (patterns within the domain) can be used to describe the longitudinal salinity gradient of the estuary over time and space, and examine how well salinity correlates with observed patterns in ecosystem processes. To predict how future changes in salinity distributions might affect the ecosystem, it is necessary to understand the mechanisms that drive these patterns. In particular, we are interested in separating the effects of salt from that of sulfate on ecosystem processes, given that these factors are correlated across the estuarine gradient. Research Components
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Research Question BackgroundSalinity can be used as a first approximation to explain process differences among fresh, brackish, and marine tidal marshes. However, salt is not the only ecologically relevant component of seawater: saltwater has not only higher ionic strength compared to freshwater, but also about 280 times more sulfate. Differences in biogeochemical redox zonation and soil metabolism between freshwater and saltwater sediments result from differences in sulfate availability: in sulfate-poor freshwater sediments, terminal metabolism is dominated by methanogenesis and iron reduction, whereas in sulfate-rich marine sediments, it is dominated by sulfate reduction. Sulfides are toxic to both plants and animals, and increased sulfate availability may constrain the distributions of plants and animals lacking adaptations to high sulfide concentrations. Conversely, increased sulfate availability may facilitate the invasion of plants with a high metabolic requirement for sulfate. We hypothesize that variation in sulfur is as important as variation in salinity in producing variation in biogeochemical processes, soil structure and species distributions between tidal freshwater and marine marshes. Our approach to this question involves experimentally decoupling the importance of salinity and sulfate in order to improve our mechanistic understanding of seawater intrusion and ecosystem change along salinity gradients. Our objectives are to:
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Field surveyIn 2010 a field crew will survey soil bulk properties, geochemical speciation and redox zonation, vegetation light profiles and plant and invertebrate distributions and biomass at 20 stations spanning the full range of the Altamaha estuary from freshwater to fully marine areas. Sampling will be conducted twice during summer (June and August) and will focus on the mid-marsh zone where the experiment will be conducted, although some parallel measurements will also be made at the creekbank. The goal of the survey will be to quantify existing down-estuary patterns of important variables, identify an appropriate site for the experimental manipulation, and generate quantitative predictions for the experiment. Survey data will be analyzed with multivariate regression techniques, including path analysis, to identify relationships between salinity, sulfate, biogeochemical, soil and biotic variables. Field experimentIn summer 2010 we will set up a field experiment at a freshwater site along the Altamaha River about 30 km from the ocean (in the vicinity of GCE 7). The experiment will consist of four treatments (control, salinity-amended, sulfate-amended and salinity+sulfate-amended) in which freshwater sediments will be amended with increasing salinity (from 0 to 10 PSU) and/or sulfate (proportional increases, from 0 to 9 mM) in an orthogonal design over 12 months. We will increase pore water ionic strength and/or sulfate concentrations in 3 x 3 m plots (n=6/treatment, separated by > 3 m) by regular additions of NaCl or Na2SO4 to shallow (40-cm deep, 5-cm diameter) piezometers (multiple piezometers per plot) made of PVC with regular perforations 5 to 40 cm below the soil surface. Pore water salinity, sulfate and sulfide levels will be monitored weekly in the center of each plot, and additions will be adjusted as needed. The stabilized salinity and sulfate levels will then be maintained over the course of the project. Plots with different treatments will be interspersed within the site and will be maintained weekly. We will monitor changes in pore water and solid phase geochemistry and microbial activity, sediment CH4/CO2 fluxes, soil elevation, organic content and C, N and P pools, epibenthic and infaunal invertebrate abundance, and plant composition and productivity in experimental plots. Samples for determination of nutrients and dissolved gas concentrations will be collected using a piezometer in the center of the plot. Geochemical processes and gas fluxes from soil surfaces will be monitored quarterly while soil bulk properties, light profiles, and plant and invertebrate populations will be monitored annually. In addition to the field experiment, the effect of short term variations in substrate concentrations, ionic strength, pH, and H2S on potential rates of nitrification, denitrification, methane oxidation, methanogenesis and sulfate reduction will be evaluated in slurry experiments in the laboratory (Joye and Hollibaugh 1995; Rysgaard et al. 1999). To expand the number of species for which we can make inferences from this experiment, we will transplant selected plants (likely Aster tenuifolius, A. novae-angliae, Scirpus americanus, Juncus roemerianus, Polygonum sp.) and invertebrates (likely bivalves Polymesoda caroliniana and Geukensia demissa, gastropods Melampus bidentatus, Detracia floridana and Littoraria irrorata) into the experimental plots once salinity and sulfate conditions have stabilized in year 2 or 3 of the experiment (n=2/plot, to be treated as subsamples). Plants will be potted in sandy soil (to facilitate rapid equilibration with new abiotic conditions), acclimated in the lab for 2 weeks, and transplanted into experimental plots for 4 months. Invertebrates will be caged within experimental plots so that they are exposed to ambient sediment conditions for 6 months (Silliman and Bertness 2002). These experiments will test the hypothesis that sulfur is more important than salt in creating conditions inimical to species typical of freshwater marshes, and in creating conditions that favor the invasion of brackish marsh species. Biogeochemical modelingA numerical reaction-transport model (RTM) will be developed to assess bottom-up control of marsh biogeochemical processes. It will include descriptions of organic matter breakdown, solid phase formation, and reoxidation reactions and use kinetic formulations for microbial metabolic reactions which account for inhibition and competition for reactants (substrates) by competitive reaction pathways. We will use data on the effects of substrate concentrations, temperature, ionic strength, pH, and H2S on N and S cycling, as well as the role of temperature variations on breakdown of organic matter to parameterize the model. Model results will be calibrated by comparison to field data (concentration and rate profiles). The RTM will be used to systematically interpret the measured chemical and microbial gradients in terms of reaction pathways, transport rates and fluctuations in boundary conditions, with particular attention to how alteration of external forcings affects elemental budgets, benthic fluxes, redox zonation, pathway competition, microbial-geochemical couplings and nutrient regeneration. Although the model will be a general description of marsh biogeochemical processes, and hence widely-applicable to a range of problems, the short-term goal of the model will be to evaluate our understanding of marsh biogeochemistry by comparing model output with biogeochemical patterns observed in the salt-amended and sulfate-amended experimental plots. Additional studiesOther related studies include Altamaha River salinity modeling, Spartina species zonation along the Altamaha River estuary, estuarine plant transplant studies, assessment of ecosystem services in fresh, brackish and marine marshes and measurement of salinity intrusion in estuarine sediments. |
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Additional studiesAltamaha River salinity modeling (more info) Ecosystem services in fresh, brackish and marine marshes (more info) Salinity intrusion in estuarine sediments (more info) Spartina species zonation along the Altamaha River estuary (more info) |
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Journal ArticlesWeston, N.B., Dixon, R.E. and Joye, S.B. 2006. Microbial and geochemical ramifications of salinity intrusion into tidal freshwater sediments. Journal of Geophysical Research. 111:G01009. (DOI: 10.1029/2005JG000071) Sheldon, J.E. and Alber, M. 2002. A comparison of residence time calculations using simple compartment models of the Altamaha River estuary, Georgia. Estuaries. 25(6B):1304-1317. Theses and DissertationsConference Proceedings (Published Papers and Abstracts)Conference Posters and PresentationsComerford, C., Joye, S.B. and Weston, N.B. 2009. Presentation: Biogeochemical ramifications of salinity intrusion into freshwater wetlands. LTER Review Meeting 2009. Guo, H., Pennings, S.C. and Wieski, K. 2008. Poster: Physical stress, plant productivity, competition, and diversity in Georgia tidal marshes. Coastal Habitats. 93rd Annual Meeting of the Ecological Society of America, August 3-8, 2008, Milwaukee, Wisconsin. Wieski, K., Guo, H. and Pennings, S.C. 2008. Poster: Ecosystem functions of tidal fresh, brackish, and salt marshes. Estuarine, Coastal and Intertidal Systems. 93rd Annual Meeting of the Ecological Society of America, August 3-8, 2008, Milwaukee, Wisconsin. Weston, N.B., Vile, M.A., Velinksy, D.J., Joye, S.B. and Neubauer, S.C. 2007. Presentation: Rising sea levels and salinity intrusion into tidal freshwater marshes: Shifting microbial communities and pathways of organic matter mineralization. ASLO Aquatic Sciences Meeting, Santa Fe NM, February 2007. Weston, N.B., Vile, M.A., Velinksy, D.J., Joye, S.B. and Neubauer, S.C. 2007. Presentation: Shifting pathways and magnitude of organic matter mineralization in tidal freshwater marshes following sea-level rise. Estuarine Research Federation 2007 Annual Meeting, 4-8 November 2007, Providence, Rhode Island. Alber, M. and Sheldon, J.E. 2006. Calculating estuary turnover times during non-steady-state conditions using freshwater fraction techniques. Southeastern Estuarine Research Society meeting, Ponte Vedra Beach, Florida. Alber, M. and Sheldon, J.E. 2006. Presentation: Simple tools for assessing coastal systems: can we get there from here? Coastal Observing Systems Workshop, LTER All Scientists Meeting, September 20-24, 2006, Estes Park Colorado. Pennings, S.C. 2006. Presentation: Sea-level rise and ecosystem services of tidal marshes. Sea-level rise, hurricanes, and the future of our coasts. Sigma Xi Meeting,Texas A&M University, March 30, 2006. Sheldon, J.E. and Alber, M. 2005. Presentation: Beyond whole-estuary flushing times: Using transport times through salinity zones to explain chlorophyll patterns in the Altamaha River estuary (Georgia, USA). Estuarine Interactions: biological-physical feedbacks and adaptations. 2005 Estuarine Research Federation Meeting. October 16-20, 2005, Norfolk, Virginia. White, S.N. and Alber, M. 2005. Presentation: The response of Spartina species to prolonged drought in the Altamaha River Estuary, Georgia. 2005 Estuarine Research Federation Meeting. October 16-20, 2005, Norfolk, Virginia. Edmonds, J.W., Weston, N.B., Joye, S.B. and Moran, M.A. 2004. Presentation: Changes in Microbial Community Structure and Activity in Response to Fluctuations in Organic Carbon Pools in Salt Marsh Sediments. Annual Meeting of the American Society of Microbiology. American Society of Microbiology, June, 2004, Atlanta, Georgia. Sheldon, J.E. and Alber, M. 2004. Presentation: SqueezeBox: Flow-scaled 1-D box models for estuary residence time estimates. NOS Workshop on Residence/Flushing Times in Bays and Estuaries. National Oceanic and Atmospheric Administration, June 8-9, 2004, Silver Spring, Maryland. Sheldon, J.E. and Alber, M. 2004. Presentation: SqueezeBox: Flow-scaled 1-D box models for estuary residence time estimates. Spring 2004 meeting. Southeastern Estuarine Research Society (SEERS), October 14-16, 2004, Wilmington, North Carolina. Sheldon, J.E. and Alber, M. 2003. Presentation: The equivalence of estuarine turnover times calculated using fraction of freshwater and tidal prism model. 2003 Estuarine Research Federation meeting. Sept. 14-18, 2003, Seattle, WA. White, S. and Alber, M. 2003. Poster: Controls of Spartina zonation patterns along the Altamaha River Estuary, GA: Abiotic and biotic mechanisms. 2003 Estuarine Research Federation meeting. Sept. 14-18, 2003, Seattle, WA. White, S. and Alber, M. 2003. Presentation: Salinity, Sulfate, and Competition: Exploring interactions and impacts on Spartina alterniflora and S. cynosuroides growth. Southeastern Estuarine Research Society meeting. March 2003, Atlantic Beach, NC. White, S.N. and Alber, M. 2003. Presentation: Will the marsh paradigm hold? Spartina distributions along the length of the Altamaha River Estuary, GA. 2003 Ecological Society of America meeting, Aug. 2003, Savannah, GA. White, S. and Alber, M. 2001. Presentation: Distribution of two salt marsh bank species along a salinity gradient in the Altamaha River Estuary: A fraction of the sum. Southeastern Estuarine Research Society Meeting. Southeastern Estuarine Research Society, Mar 01, 2001, Charleston, South Carolina. White, S.N. and Alber, M. 2001. Presentation: Distribution of S. alterniflora and S. cynosuroides along the Altamaha River: Potential impacts of competition, salinity and sulfate availability. ERF 2001: An Estuarine Odyssey. Estuarine Research Federation, Nov. 4-8, 2001, St. Pete Beach, Florida. Alber, M. and Sheldon, J.E. 2000. Presentation: Residence times in the Altamaha River Estuary: a progress report. Southeastern Estuarine Research Society Meeting. Southeastern Estuarine Research Society, Oct 01, 2000, Tampa, Florida. |
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Research Project Principal InvestigatorsMerryl Alber, University of Georgia Christopher B. Craft, Indiana University at Bloomington Samantha B. Joye, University of Georgia Christof Meile, University of Georgia Steven C. Pennings, University of Houston Other Associated PersonnelMerryl Alber, University of Georgia Christopher Comerford, Department of Marine Sciences Christopher B. Craft, Indiana University at Bloomington Ray E. Dixon, University of Georgia Hongyu Guo, University of Houston Samantha B. Joye, University of Georgia Steven C. Pennings, University of Houston Daniel F. Saucedo, University of Georgia Marine Institute Joan E. Sheldon, University of Georgia Nathaniel B. Weston, University of Georgia Susan N. White, University of Georgia Kazimierz Wieski, University of Houston |
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