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

delta(13)C and delta(15)N values were determined for the seagrass Thalassia testudinum at four permanent seagrass monitoring stations in south Florida, USA, through a quarterly sampling program-over 3-years (1996-1998). 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 3 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 four stations vary from -7.2 to -10.4parts per thousand and 1.1 to 2.2parts per thousand, respectively. However, certain stations displayed anomalously depleted nitrogen isotope values (as low as -1.2parts per thousand). These values may indicate that biogeochemical processes like N fixation, ammonification and denitrification cause temporal changes in the isotopic composition of the source DIN. Both delta(13)C and delta(15)N values displayed seasonal enrichment-depletion patterns, with maximum enrichment occurring during the summer to early fall. The intra-annual variations of delta(13)C values from the different stations ranged from about 1 to 3.5parts per thousand; whereas variations in delta(15)N ranged from about 1 to 4.9parts per thousand. Certain-sites showed a positive relationship between isotope values and productivity. These data indicate delta(13)C values display a high degree of seasonal variability as related to changes in productivity. delta(15)N 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. (C) 2002 Elsevier Science Ltd. All rights reserved.