Seagrass Monitoring in the Florida Keys National Marine Sanctuary

Data syntheses: Abstracts

Intra- and interannual variability in seagrass carbon and nitrogen  stable isotopes from south Florida, a preliminary study

W.T. Anderson^1,2 and J.W. Fourqurean^1,3

¹Southeast Environmental Research Center, Florida International University, Miami, FL 33199

²Earth Sciences Department, Florida International University, Miami, FL 33199

³Department of Biological Sciences, Florida International University, Miami, FL 33199

Abstract

δ13C and δ15N values were determined for Thalassia testudinum at four permanent seagrass monitoring stations in south Florida, USA, through a quarterly sampling program over three years (1996 to 1998). All sites are seagrass beds with water depths of less than 6 m. Two sites are located on the Florida Bay side of the Florida Keys, and the other two sites are on the Atlantic-side. The data analyzed over the three year study period display unique patterns associated with seasonal changes in primary productivity and potentially changes in the N and C pools. The mean carbon and nitrogen isotope values of T. testudinum from all 4 stations vary from –7.2 ‰ to –10.4‰ and 1.1‰ to 2.2‰, respectively. However, certain stations displayed anonymously depleted nitrogen isotope values (as low as –1.2‰). These values may indicate that biogeochmical processes like N fixation, ammonification and denitrification cause temporal changes in the isotopic composition of the source DIN. Both δ13C and δ15N values displayed seasonal enrichment-depletion patterns, with maximum enrichment occurring during the summer to early fall. The intra-annual variations of δ13C values from the different stations ranged from about 1‰ to 3.5‰; whereas variations in δ15N ranged from about 1‰ to 4.9‰. Certain sites showed a positive relationship between isotope values and productivity. These data indicate δ13C values display a high degree of seasonal variability as related to changes in productivity. δ15N values show clear intra-annual variations, but the observed changes do not necessarily follow a distinct seasonal cycle, indicating that changes in DIN will need further investigation.

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