Enviro-Days 1997
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A. Christy Tyler
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Enviro-Days 1997
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A. Christy Tyler
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Porewater salinity interferes with Spartina alterniflora growth to the extent that the interannual trends in productivity may be the result of salinity fluctuations caused by changes in mean sea level. Salinity is a function of landscape and position in the marsh. In concert with the ceiling placed on plant growth by nutrient limitations and redox potential, salinity adds a spatial aspect to the window of conditions for optimum Spartina production. In older, highly organic marsh sediments, anoxia may be a more important limiting factor; however, in young, sandy marshes, such as some back-barrier marshes, the redox potential is most likely sufficiently high that salinity is a more significant factor in Spartina growth. Salts are removed from the sediment by flushing out of the creekbank, or by dilution from fresh groundwater input. In narrow fringing marshes, the inland portion of the drainage area is much closer to the marsh edge than in other types of marshes, so that the type of area that is drained is more important.
This study examines the interaction between the variation in drainage area types (upland (creek Y1) and salt flat (creek Y2)), the hydrologic properties, the salinity and the resultant Spartina productivity in two young creeks of similar ages and elevations on Hog Island, VA. The vertical and horizontal hydraulic head gradients were measured along a transect at each creek using piezometers fitted with a HOBO pressure transducer over 50 tidal cycles during the summer/fall of 1996. At the site with upland drainage the average height of the water table was significantly (p<0.0001) higher relative to the salt flat site over the course of a tidal cycle for all positions along the transect. Groundwater discharge through the system was estimated for each site at slack low tide, when the water table is static. Assuming constant discharge over a tidal cycle, the estimated groundwater discharge is 331.8 l/day and 35.9 l/day from Y1 and Y2, respectively. The mean salinity over the 1995 and 1996 growing seasons is 26.8 ppt for Y1, and is 36.4 ppt for Y2. There were no significant differences in nutrient or redox conditions between the sites. The two year average Spartina alterniflora biomass is 574.7g/m2 and 156.5g/m2, for Y1 and Y2 respectively. It is evident that greater groundwater discharge corresponds with lower porewater salinities and greater Spartina biomass.