From Source to Sink: An Isotopic Perspective on the Biogeochemical Relationship between the Everglades and Florida Bay

Increasing human populations and activities in coastal areas have led to high nutrient loading and estuarine ecosystem decline. Natural hydrological patterns in South Florida have been drastically altered by changes in water management and land use practices. As a result Florida Bay has experienced a series of negative ecosystem effects including hypersalinity events, degradation of water quality, and harmful algal blooms and declines in upper trophic level populations. To remediate ecosystem decline in Florida\u27s coastal ecosystems, the Comprehensive Everglades Restoration Plan proposes to restore a more natural hydrologic flow in the Everglades. It is expected hydrologic restoration efforts will change the amount, sources and ratios of dissolved nutrients (organic and inorganic) delivered to the bay potentially inducing an ecosystem response of changing structure and function in both planktic and benthic habitats. Identifying biogeochemical linkages between external nutrient inputs from the Everglades and internal cycling processes of Florida Bay is critical to understanding the effects of hydrological restoration and changing nutrient regimes on Florida Bay. A nitrogen (δ15N) and carbon (δ13C ) stable isotopic approach affords an effective means of assessing the fate of varying nutrient sources and delineating the dominant biogeochemical processes governing nutrient cycling in the bay. This study\u27s main goals were to use stable isotopic analyses of C and N in dissolved and particulate materials to determine spatial and seasonal relationships between Everglades nutrient sources and their biological sinks in Florida Bay, examine the biogeochemical relationships among inorganic and organic components of the water column and benthos in Florida Bay, and assess future ecological response to changing nutrient inputs resulting from restoration efforts. A large east to west gradient from more enriched to more depleted δ15N values was noted in both dissolved nitrogen pools and organic components of the bay. This trend indicates that there are differing nutrient sources and biogeochemical processes influencing the various regions of the bay. Isotopic similarity of the dissolved nitrogen pools from the Everglades and particulate organic matter in the bay points to a strong relationship between both ecosystems. Everglades nutrient inputs delivered to the bay in the wet season directly influence ecological responses in the bay, in some cases increases in algal biomass. Seasonality also influences nitrogen transformations in the dissolved nitrogen pools and the sediments. During dry periods when there is little or no hydrologic flow from the Everglades into the bay, denitrification is a major process affecting nitrogen cycling in the eastern and central regions of the bay. During the wet periods, denitrification becomes suppressed and dissimilatory nitrate reduction (DNRA) is favored. Increased hydrologic flow brings fresh organic matter that fuels DNRA. There was a consistent spatial pattern from more depleted to more enriched δ13C values, onshore to offshore relative to the mainland which indicates strong terrestrial influence on Florida Bay sites along the mangrove boundary with the Everglades. Particulate organic matter exhibited a shift to more enriched δ13C values during the wet season which reflects an increase in algal biomass. A shift to more depleted δ13C values of DOM indicated increased terrestrial influence from the Everglades during the wet season. The approach undertaken in this study identifies a strong linkage between nutrient inputs from the Everglades and biogeochemical processes in the bay. These findings underscore the need to consider the impact of both allochtonous nutrient inputs and the dominant processes governing cycling in the bay when making management decisions that continue to refine hydrologic restoration plans