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Title Multidecadal biomass declines and controlling variables for the keystone salt marsh species, Spartina alterniflora, in coastal Georgia
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We studied aboveground biomass dynamics and spatial patterns of Marsh Cordgrass, Spartina alterniflora, on the Central Georgia Coast. This keystone species accounts for 98% of the aerial extent of salt marshes in Georgia and comprise about 33% of all salt marsh on the U.S. Western Atlantic Coast. Geospatial techniques were used to scale up in situ biomass measurements within the NSF Georgia Coastal Ecosystems research domain on the Central Georgia Coast to landscape scale estimates using 290 Landsat 5 TM scenes from 1984 to 2011. Climate and hydrological variables were then used to explain variations in aboveground production for each of the three height classes of S. alterniflora. River discharge, total precipitation, minimum temperature, and mean sea level had positive relationships with and best explained biomass variation for all dates. Over the 28-year study period we documented biomass declines of 31.6 %, 33.4 %, and 38.7% for tall, medium, and short S. alterniflora height classes. These biomass declines were linked to increased drought severity and frequency over the last half of our study period. We then applied a larger, synoptic scale approach to riverine and tidal watersheds containing 620 square kilometers of S. alterniflora marshlands on the Georgia Coast (Savannah River to St Simon's Sound) and found similar inter and intra-annual biomass patterns. Importantly, S. alterniflora biomass production was greatest in areas closest to larger inputs of freshwater and high precipitation. We infer that these areas of high production, especially in the Lower Altamaha River Watershed, were better buffered against drought stress, including soil salinization, and also experienced greater nitrogen loading rates. Overall, this much larger study area experienced a 20.6% average decline, representing a reduction of about 108,000 MT in aboveground live carbon biomass. This loss in marsh production presumably affects valuable ecosystem services, including wetland soil carbon sequestration and organic matter export (both particulate and dissolved). Thus, declines in marsh production could significantly reduce nutritional support to food webs and carbon biogeochemical cycling, as well as commercial fish and shellfish production, in Georgia's estuaries and coastal ocean ecosystems. Finally, we are carrying our analyses forward using newer Landsat 8 OLI imagery. In addition, Landsat 7 ETM imagery is serving as a "bridge" to cross-calibrate between the Landsat 5 and Landsat 8 sensors and provide "gap coverage" between 2011 and 2014.

Contributors John F. Schalles, John O'Donnell, Nicholas Nealy, Trek Mizoguchi and Christine M. Hladik

Schalles, J.F., O'Donnell, J., Nealy, N., Mizoguchi, T. and Hladik, C.M. 2018. Presentation: Multidecadal biomass declines and controlling variables for the keystone salt marsh species, Spartina alterniflora, in coastal Georgia . SS86: Connecting the Dots - Signals of Global Change Effects in Freshwater and Marine Ecosystems. ASLO 2018 Summer Meeting, June 15, 2018, Victoria, British Columbia, BC.

Key Words climate drivers, plant biomass, remote sensing, river hydrology, SINERR Publication, Spartina alterniflora, Student Publication
File Date 2018
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This material is based upon work supported by the National Science Foundation under grants OCE-9982133, OCE-0620959, OCE-1237140 and OCE-1832178. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.