Nutrient profiles in the everglades: examination along the eutrophication gradient

We examined the concentration profiles of nutrients in the surface water, soil and pore water along the eutrophication gradient of the Water Conservation Area-2A (WCA-2A) in the northern Everglades. Phosphorus levels in the surface waters contributed by the agricultural runoff showed an exponential decrease downstream of the inflow structures attaining background values of 7–12, 7–9 and 5–6 μg l−1 of TP, TDP and PO4P, respectively, at distances of 8–10 km. The pore water PO4P concentration in the oligotrophic areas ranged between 5 and 10 μg l−1. Molar ratios of dissolved inorganic N and P suggest a possible switch in nutrient limitation in the surface water from P in the oligotrophic areas to N in the eutrophic areas (DIN:DIP ∼ 5). External nutrient loading has also contributed to a three- to four-fold increase in soil TP concentration and enhanced pore water PO4P in the northern marshes. Unlike P, C and N concentration in the soils remained fairly uniform along the eutrophication gradient. 210Pb dating of soil cores suggests that the increase in soil P concentration (from < 500 to 1500 μg g−1) and P accumulation rate (from 0.06 to 0.46 g P m−2 per year) at the eutrophic site correlates with the installation of inflow structures in 1960–1963 through which agricultural drainage from the Hillsboro canal enters the marshes. Organic P makes up 70–90% of the total P in the soils as uptake by algae and macrophytes is the primary mechanism of P removal in these wetlands. Calcium supply from the underlying bedrock suggested from the surface and pore water chemical profiles has important consequences for P-cycling in the Everglades as Ca-bound P is the major form of inorganic P storage in the soils.