The south Florida wetlands ecosystem (the Everglades and Florida Bay) has historically been an oligotrophic environment. Anthropogenic activities (especially agriculture) during the past century, however, have delivered excess phosphorus (P) to some areas of the northern Everglades, contributing to changes in water quality and ecology. The fate of this excess P and its long-term effects on biological resources are unclear. South of the Everglades, changes in nutrient loads to Florida Bay are hypothesized to be linked to recent events of seagrass die-off and microalgal blooms. Restoration of the Everglades and Florida Bay to a more natural nutrient balance will require crucial baseline information on the ecosystem's recent nutrient history, the sources of nutrients to the ecosystem, and the biogeochemical cycling of nutrients within the ecosystem. This project ³ has focused on establishing: (1) the source(s) of nutrients to contaminated areas of the northern Everglades and to Florida Bay, (2) the spatial and temporal variation of nutrients within the ecosystem, and (3) the rates of accumulation and recycling of nutrients in sediments (including the flux of nutrients from sediment porewater to surface water).

Our results show that concentrations of P at pristine sites in the freshwater Everglades range from 1-20 µg/L in surface water, 10-100 µg/L in sediment porewater, and 300-500 µg/g (dry wt) in sediments. At contaminated marsh sites, however, P concentrations often exceed 100 (g/L in surface water, 3,000 µg/L in porewater, and 2,000 µg/g in surface sediment. Profiles of total P in dated (²¹⁰Pb and ¹³⁷Cs) sediment cores from Water Conservation Area (WCA) 2A show that greatly increased loads of P were delivered to sediments near the Hillsboro Canal from the 1920's to the present. The area of P contamination is currently confined to a band in WCA-2A adjacent to the Hillsboro Canal and other marsh sites adjacent to canals in the WCA. Cattails (eutrophic macrophytes) have colonized the P-contaminated areas, displacing the oligotrophic sawgrass. Accumulation rates of P in sediment at contaminated sites are typically 100 times higher compared to pristine areas. Porewater at contaminated sites, however, also has 30-300 times higher concentrations of P than pristine sites. This suggests that the rate of microbial recycling of P and the flux of P from porewater to surface water is also much higher at contaminated sites. This may reflect both nutrient stimulation of microbial activity and the chemical lability of organic matter in the eutrophic cattails present in P-contaminated areas. Hence P, while rapidly accumulated in contaminated areas, is also rapidly recycled. The balance between P accumulation and recycling in marsh areas dominated by eutrophic macrophytes is critical for estimating the long-term effectiveness of constructed wetlands (which will likely contain mostly eutrophic macrophytes) designed as nutrient buffer areas to protect the Everglades.

We have used the concentration and alpha activity ratio (²³⁴U/²³⁸U) of readily exchangeable uranium (U) as a geochemical tracer to examine the source(s) of P to the northern Everglades. This approach is predicated on the high concentration of U in phosphate rock used to make phosphate fertilizer (correlated with P concentration), and its distinctive activity ratio (AR) of 1.00 ± 0.05. The concentration and activity ratio of U was determined in fertilizer used in the Everglades Agricultural Area (EAA), in sediment cores, and in surface water from: the Kissimmee River and creeks entering Lake Okeechobee, Lake Okeechobee, canals and fields in the EAA, and marsh sites in the WCA. Sediments from P-contaminated sites in WCA-2A near the Hillsboro Canal had U concentrations in excess of 1 µg/g, and uranium AR ranging from 0.97-1.03, values permissive of a fertilizer source. In contrast, a pristine site in the center of WCA-1A had U concentrations of < 0.2 µg/g and AR values of 1.10-1.22, inconsistent with a fertilizer source. Concentrations of U in canal water from the EAA average about 0.3 µg/L and have AR values of around 1.00, permissive of a fertilizer source. These results suggest that the source of U (and by analogy P) in contaminated areas of WCA-2A is P fertilizer used in the EAA, and affirm previous assumptions about P contamination in the Everglades.

In preliminary work, we have determined that tree islands represent another zone of P enrichment in the Everglades. Studies of sediments from the heads of two tree islands in WCA-3B showed concentrations of up to 3,000 µg/g on one island and up to 1,500 µg/g on the second. These concentrations are far in excess of those observed in the surrounding pristine marsh (values of around 200-300 µg/g), and approach or exceed P concentrations in contaminated areas of WCA-2A. The source of the high levels of P on tree islands is uncertain, but we hypothesize that it originates from the guano of nesting birds. The P concentrations of sediments in the tail and edge of the tree island are also in excess of concentrations in the surrounding marsh, but less than values found on the head. Runoff of P from the head may explain the distribution of P in the tail, and P-enrichment may play a role in the development of tree island tails.

In the southern Everglades, we have conducted extensive sediment and water sampling in Taylor Slough and the C-111 basin. In addition, water sampling and sediment coring have been undertaken at sites in Florida Bay and the mangrove fringe zone surrounding Florida Bay. The objective of this work was to examine source(s) of nutrients and nutrient cycling in the southeastern Everglades and northeastern Florida Bay. Analysis of samples and data from this study is still underway; some preliminary conclusions, however, have been developed. Concentrations of total P in sediments at the head of Taylor Slough approach values of 1,000 µg/g, in excess of concentrations for pristine areas of the Everglades. This suggests P contamination at the head of Taylor Slough from canals discharging at the head of the Slough. Total P concentrations in sediments diminish to the south to nearly background levels (< 500 µg/g) by the center of the Slough. Accumulation rates for P range from 1.4 gP/m²-yr at the head of the slough to only 0.04 gP/m²-yr at midslough. Although concentrations of total P in Florida Bay sediments range from only 100-300 µg/g, P accumulation rates in Florida Bay (0.2-1.4 gP/m²-yr) are higher than those in pristine areas of Taylor Slough. Surface water and sediment data show that Taylor Slough is not a major source of P to northeastern Florida Bay under current hydrologic flow conditions. Preliminary results suggest that Florida Bay may actually serve as a source of nutrients to the mangrove fringe zone between Taylor Slough and Florida Bay. Downcore profiles at several sites in eastern and central Florida Bay show surficial anomalies of P and N concentrations that suggest recent increases in nutrient load to the sediments, beginning in the early 1980's. It is unclear if these anomalies represent anthropogenic inputs of nutrients to Florida Bay. It is noteworthy, however, that downcore profiles at sites of relatively slow sedimentation in central Florida Bay show significant anomalies in total nitrogen and organic carbon on a decadal timescale, with extrapolated dates (²¹⁰Pb) in the mid-1700's. This may have been a period of increased rainfall in south Florida and concomitant increased hydrologic flow to Florida Bay, with the effect of increasing nitrogen input to the bay, and triggering an increase in productivity. Thus, periods of increased nutrient input to the bay can follow natural, climate-driven cycles, as well as being influenced by anthropogenic input.

³This work was conducted in close collaboration with scientists from the U.S. Geological Survey, the South Florida Water Management District, the Florida Game and Fresh Water Fish Commission, the U.S. Department of Agriculture, and the University of Florida.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)

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