The distribution and function of biogeochemical interfaces, gradients, and fluxes in a coastal spring-fed riverine and estuarine system

Transparent freshwater rivers emerge from first-magnitude springs (> 2.8m³/s) along the karst northeastern shore of the Gulf of Mexico, gradually draining into the estuarine meanders that comprise Florida’s coastal salt marsh. The resulting gradients in the physical, chemical, and biological components of water and benthic substrate provide an intriguing environment for the study of biogeochemical cycles. This unique environment serves as crucial habitat for the endangered West Indian Manatee and migratory bird species, supports productive fisheries, and attracts recreational tourists.  These natural assets also serve as the foundation of a significant portion of local and regional economies. Population growth, specifically residential and golf course fertilization have resulted in a 50-fold increase in nitrate concentrations as well as elevated concentrations of soluble reactive phosphorus at spring boils. A decade of water quality monitoring has demonstrated that inorganic nutrients are rapidly utilized once discharged into the riverine and estuarine system, however many questions remain as to the factors controlling nutrient utilization, as well as their associated uptake thresholds. This research focused on quantifying controls on organic and inorganic N and P concentrations along this freshwater-estuarine gradient. The spatial distribution, physical and chemical properties, and biogeochemical function of distinct benthic and water column environments were examined using a combination of benthic and water column nutrient flux chamber experiments, microbial enzyme assays, isotopic analyses of C and N, and measures of physical and chemical parameters of soil and water. All fieldwork was based in the Chassahowitzka River and Estuary, parts of which constitute the Chassahowitzka National Wildlife Refuge.