ASSESSING THE EFFECTS OF LAND USE CHANGE ON SW FLORIDA ESTUARIES USING STABLE ISOTOPES

Many estuaries in southwestern Florida, including those within Rookery Bay National Estuarine Research Reserve (RBNERR), have different land use characteristics that have resulted in the fragmentation of natural landscapes and changes in estuarine ecosystem structure. Although water quality monitoring is in place within the reserve, the effects of agriculture and residential development on the estuarine ecosystems are unknown. To assess the influence of nutrient loading on primary productivity, we selected four estuaries within RBNERR experiencing different levels and types of disturbance. One of these estuaries is relatively undisturbed, thereby serving as our control site. Because land runoff is highly dependent on rainfall, samples were collected four times a year, twice during the wet season and twice during the dry season.

Estuarine productivity is assessed through the stable isotope composition of dissolved inorganic carbon (DIC), particulate organic matter (POM), and chlorophyll-a concentrations. Carbon isotope values for DIC samples taken along a salinity gradient in each estuary at high tide range from 0 to -14‰, and they cannot be explained in terms of a conservative mixing of freshwater and seawater end-members. These results then suggest a non-conservative effect of net organic carbon production and consumption (i.e. autotrophy and heterotrophy) in the studied estuaries, with heterotrophy occurring most often. Carbon isotope values for estuarine POM range from -12 to -36‰. The more negative values occur in the freshwater end members and indicate a terrestrial source of organic matter, whereas the more positive values indicate autotrophic sources of carbon from within the estuaries closer to the saltwater end member. This pattern is consistent across all four estuaries, including the control estuary. However, the distribution of estuarine DIC isotope values differ from that of the control site, indicating an altered carbon cycling possibly due to changes in the conditions of their watersheds.