Phosphorus and nitrogen inputs to Florida Bay: The importance of the Everglades watershed

A large environmental restoration project designed to improve the hydrological conditions of the Florida Everglades and increase freshwater flow to Florida Bay is underway. Here we explore how changing freshwater inflow to the southern Everglades is likely to change the input of nutrients to Florida Bay. We calculated annual inputs of water, total phosphorus (TP), total nitrogen (TN), and dissolved inorganic nitrogen (DIN) to Everglades National Park (ENP) since the early 1980s. We also examined changes in these nutrient concentrations along transects through the wetland to Florida Bay and the Gulf of Mexico. We found that the interannual variability of the water discharge into ENP greatly exceeded the interannual variability of flow-weighted mean nutrient concentrations in this water. Nutrient inputs to ENP were largely determined by discharge volume. These inputs were high in TN and low in TP; for two ENP watersheds TN averaged 1.5 mg 1^-1(0.11 mM) and 0.9 mg 1^-1 (0.06 mM) and TP averaged 15 μg 1^-1 (0.47 μM) and 9 μg 1^-1 (0.28 μM). Both TP and DIN that flowed into ENP wetlands were rapidly removed from the water. Over a 3-km section of Taylor Slough, TP decreased from a flow-weighted mean of 11.6 μg 1^-1 (0.37 μM) to 6.1 μg 1^-1 (0.20 μM) and DIN decreased from 240 μg 1^-1 (17 μM) to 36 μg 1^-1 (2.6 μM). In contrast, TN, which was generally- 95% organic N, changed little as it passed through the wetland. This resulted in molar TN: TP ratios exceeding 400 in the wetland. Decreases in TN concentrations only occurred in areas with relatively high P availability, such as the wetlands to the north of ENP and in the mangrove streams of western ENP. Increasing freshwater flow to Florida Bay in an effort to restore the Everglades and Florida Bay ecosystems is thus not likely to increase P inputs from the freshwater Everglades but is likely to increase TN inputs. Based on a nutrient budget of Florida Bay, both N and P inputs from the Gulf of Mexico greatly exceed inputs from the Everglades, as well as inputs from the atmosphere and the Florida Keys. We estimate that the freshwater Everglades contribute < 3% of all P inputs and < 12% of all N inputs to the bay. Evaluating the effect of ecosystem restoration efforts on Florida Bay requires greater understanding of the interactions of the bay with the Gulf of Mexico and adjacent mangrove ecosystems.