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Measuring stable isotope signatures to assess the biogeochemical impact of submarine groundwater discharge on coastal seagrasses in northwest Florida
Abstract

Measuring stable isotope signatures to assess the biogeochemical impact of submarine groundwater discharge on coastal seagrasses in northwest Florida

Haley Anne McQueen and M. C. Schwartz
American Geophysical Union Fall Meeting 2018, Vol.2018
American Geophysical Union fall meeting (Washington, D.C., 12/10/2018–12/14/2018)
12/2018

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Abstract

Submarine groundwater discharge (SGD) is a hydrological phenomenon of terrestrial groundwater entering coastal surface waters through porous sediments. While past research has analyzed SGD nutrient influxes, the distributions of seagrasses relative to SGD, and more, this research aims to connect SGD inputs to the ecological responses of Thalassia testudinum, a seagrass commonly known as turtle grass. More specifically, we are investigating the impact of SGD-transported nutrients into the local environment (including porewaters and bottom waters) in which Thalassia is growing by analyzing for variations in elemental abundance and stable isotope composition of seagrass in areas receiving SGD as compared to control areas in which no SGD is measured. We sampled eight site locations within Florida's Gulf Islands National Seashore's Naval Like Oaks (NALO) Preserve for bottom-water nitrate, nitrite, ammonium and phosphate; physical water quality parameters (dissolved oxygen, specific conductivity, temperature, pH, and turbidity); and dissolved radon. Additionally, Thalassia testudinum seagrass samples were collected from all stations for IRMS analysis for delta (super 15) N and delta (super 13) C to investigate SGD influences on C:N ratios and stable isotope composition in the local seagrasses, including spatial and tidal controls on any variability in water column chemistry and seagrass biogeochemistry. Preliminary stable isotope results from selected pilot sites showed a substantial difference in delta (super 15) N and delta (super 13) C between Thalassia rhizomes and leaves; however, they did not show stable isotope differences between SGD and non-SGD sites within the study area. We have added additional sampling sites to those pilot sites to further identify the relationship between SGD and seagrass biogeochemistry at the NALO site.

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