Isotopic indicators of environmental change in a subtropical wetland

Abstract

The d 15 N and d 13 C signatures of major organic matter (OM) pools were measured across chemical and hydrologic gradients in a large (58,800 ha) subtropical wetland to evaluate whether stable isotopes were useful indicators of environmental change. Once a rainfalldriven wetland, the Loxahatchee National Wildlife Refuge in the Florida Everglades now receives agricultural and urban drainage that has increased phosphorus (P) and mineral loads around the wetland perimeter. Additionally, water impoundment at the southern end has produced a latitudinal hydrologic gradient, with extended hydroperiods in the south and overdrained conditions in the north. Detritus (� 4.8% to 8.6%), floc (� 1.4% to 3.6%), and metaphyton (� 6.6% to +7.4%) d 15 N declined southward with changes in hydrology as indicated by water depth. This pattern was attributed to higher mineralization rates under shorter hydroperiods. These signatures were also strongly correlated with increased nutrient and mineral loading. Rooted macrophyte d 15 N, by contrast, appeared more responsive to soil nutrient pools. Cattail (� 8.9% to +7.7%) was restricted to the wetland perimeter and had the widest d 15 N range, which was positively correlated with soil P. Sawgrass (� 5.3% to +7.7%) occurred across most of the wetland, but its d 15 N was not strongly correlated to any gradient. Patterns for d 13 C were more