The carbon and nitrogen isotopic values of particulate organic material from the Florida Keys: a temporal and spatial study

The δ¹⁵N and δ¹³C values of particulate organic material (POM) were analyzed from 35 sites in the Florida Keys over the time interval 2000 to 2002. The sites within the study area were delineated into nine transects stretching from Key West to Key Largo. Each transect consisted of three to five sites extending from close to the Keys to the edge of the reef tract. The POM had mean δ¹⁵N and δ¹³C values of +3.6‰ (σ = ±3.2‰) and −19.9‰ (σ = ±0.6‰) respectively. Over the study period there were no statistically significant changes in δ¹⁵N, δ¹³C, or C:N. For the majority of the sampling dates, the δ¹³C values showed a distinct inshore (δ¹³C = −18.3‰, σ = ±1.0‰) to offshore gradient (δ¹³C = −21.4, σ = ±0.9‰). In contrast, the δ¹⁵N values showed no consistent patterns related to the distance from land. The more positive δ¹³C values of the nearshore samples suggest that the source of the carbon and the nitrogen in the POM in the nearshore was mainly derived from the degradation of seagrass detritus and not from the input of anthropogenically derived material from the Florida Keys. In contrast, the POM on the outer reef was dominated by marine plankton. As mineralization and nitrification of the organic nitrogen pool are major contributors to the dissolved inorganic nitrogen in the water column, it is unlikely that variations in the δ¹⁵N of the algae and other benthic organisms reported in the Florida Keys are related to the input of sewage.