Sediment Biogeochemistry in Coastal Environments

The impacts of global change on coastal marsh ecosystem dynamics

This work consists of two projects that explore the impacts of climate change (e.g., sea level rise, drought, climate variability, increased frequency of intense storms like hurricanes) on ecosystem services and dynamics of tidal marshes along the South Atlantic Coast. The projects are a collaborative effort between Dr. Joye and Drs. Christopher Craft (Indiana University), Mark Hester (U New Orleans), Irv Mendelssohn (Lousiana State U), and Steve Pennings (U Houston).

"Effects of Sea level Rise and Climate Variability on Ecosystem Services of Tidal Marshes, South Atlantic Coast"

We developed a conceptual model that describes how ecosystem services vary along the salinity gradient from tidal freshwater to salt marshes and how climate change (sea level rise, climate variability) will alter their delivery of ecosystem services. We hypothesize that ecosystem services of tidal freshwater provide higher levels of primary production, plant diversity, waste treatment (sedimentation, N&P accumulation, denitrification) and carbon sequestration relative to salt marshes. Salt marshes, however, provide higher levels of secondary production and biodiversity. Rising sea level is predicted to reduce the area of tidal marsh through submergence and conversion of tidal freshwater marsh to brackish and salt marsh habitat with resultant reduction in ecosystem services provided by salt and brackish marshes and near loss of services provided by tidal freshwater marshes. Greater inter-annual variability of climate will lead to greater frequency of drought, including marsh dieback & brown marsh, that reduces the delivery of ecosystem services and episodic freshwater pulsing that enhances the delivery of ecosystem services provided by tidal marshes.

We will employ a combination of mesocosm-, ecosystem- and landscape-scale measurements to test the effects of rising sea level and greater inter-annual variability of climate (precipitation) on alteration of area and ecosystem services of tidal marshes in three watersheds (Altamaha, Savannah, Satilla Rivers, Georgia) of the South Atlantic coast. Ecosystem services associated with biological productivity and diversity (macrophytes, microorganisms, invertebrates), nutrient cycling/waste treatment (sediment deposition, denitrification, N&P accumulation) and carbon cycling (decomposition, C sequestration, methanogenesis) will be measured in replicate (n=3) salt, brackish and tidal freshwater marshes of each watershed. A spatially explicit GIS model, developed using the Sea Level Rise Affects Marshes Model (SLAMM), will be employed to predict changes marsh area from marsh submergence and habitat conversion. Overlay of ecosystem-scale measurements will be used to predict how the cumulative delivery of ecosystem services in each watershed is altered in response to rising sea level. The GIS model also will be used to predict changes in commercial fisheries (shrimp) yields in response to marsh submergence and habitat conversion Mesocosms will be used to test the effects of increased salinity and flooding depth of selected ecosystem services.

The effects of climate variability will be evaluated by statistical and time series analysis of climate (rainfall, temperature, salinity) and ecosystem services data from permanent plots of ten marshes of the Georgia Coastal Ecosystems Long Term Ecological research (GCE LTER) study domain, collected since 2000. Data describing macrophyte and invertebrate productivity & diversity, marsh accretion, sediment deposition) were collected biannually through drought (2000-2001) and average rainfall years (2002-2003) and data collection continues today. The effect of long-term drought will be assessed by comparing ecosystem services of marsh dieback & brown marsh areas with (vegetated) salt and brackish marsh habitat.

The results of this work will (1) provide a basis for understanding how ecosystem services vary among tidal marshes along the salinity gradient, (2) predict how sea level rise will alter salt, brackish and tidal freshwater marsh area and their delivery of ecosystem services, (3) elucidate how variability in freshwater inputs from rainfall and river flooding affect temporal patterns of biological productivity, diversity, marsh accretion and sediment deposition and (4) evaluate the effects of drought and brown marsh on ecosystem services. A spatially explicit GIS model describing ecosystem services of river dominated estuaries, and the effects of rising sea level, will be developed that is applicable to other river dominated estuaries of the United States east coast and elsewhere.

Climate-linked alteration of ecosystem services in tidal salt marshes of Georgia and Louisiana

The proposed research will investigate the effect of climate change, expressed as drought-induced stressors, on key ecosystem processes within tidal salt marshes and the ecosystem services that are derived from these processes. We shall also determine how these effects change with time, vegetation density (both extent of surviving stem density and manipulated stem density), and hydrogeomorphic setting (e.g., macro- versus micro-tidal environments). Record droughts in Georgia and Louisiana 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.

This research will address the following questions:

> How are the key ecosystem processes that occur in tidal salt marshes affected by stressors associated with climate-change induced drought?

> How do these drought-induced changes in ecosystem processes influence the ability of tidal salt marshes to provide process-derived ecosystem services?

> How does the hydrogeomorphic setting of a tidal salt marsh modify the extent to which climate-change stressors affect ecosystem processes and resulting services?

> To what extent does post-drought vegetative condition (plant density) influence the degree of impact to ecological functions and ecosystem services?

This research will address four major ecosystem services that are linked to tidal salt marshes and that may be affected by drought as a result of climate-change. These services are eutrophication control, carbon sequestration, sustainable habitat, and faunal support. The impact of drought-induced stressors on these processes and functions will be assessed and their relationship to resulting ecosystem services identified. For example, carbon sequestration results from organic accumulation, which results from organic matter production, decomposition, and erosion. The consequences of drought on these ecological processes will be determined. In addition, the modulating effects of hydrogeomorphic setting and post-drought vegetative condition on these ecological processes will be determined.

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.