Revisiting phosphorus in the Everglades
New models suggest that the ecosystem can handle more of the nutrient than regulations currently allow.
Long-term studies in Florida's "river of grass" have provided a window into the unique ecosystem of the Everglades over the past several decades. Regulators typically have taken data from these studies into account when setting limits for phosphorus. New research published in ES&T (DOI: 10.1021/es062624w) applies a nontraditional statistical approach to assess how much phosphorus the complex and slow-moving ecosystem can tolerate. The results suggest that the Everglades—and wetlands in general—may be able to assimilate more phosphorus than current federal regulations allow.
The research could impact the ongoing debate about how to regulate agricultural runoff from sugarcane fields and other nonpoint sources of nutrients in the region. Multi-million-dollar efforts (PDF: 3.8 MB) are ongoing to restore the Everglades, parts of which contain 50 µg/L or more of phosphorus despite efforts to control nutrient inputs and remediate historical levels.
The new work, conducted by Curtis Richardson of the Duke University Wetlands Center and colleagues, follows up on a previous paper published in ES&T (1999, 33, 1545–1551). In 1999, Richardson and Song Qian of Portland State University proposed that a wetlands ecosystem might assimilate phosphorus in the long term without swinging wildly out of balance. Partial funding for the past and current research came from the Everglades Agricultural Area Environmental Protection District, which was set up to support research through agricultural industry monies, among other sources. The statistical approach taken in the latest work could enable scientists and managers to predict windows of tolerance in an ecosystem on the basis of how and when key species respond, Richardson says.
Plants pick up phosphorus sorbed to soils, draw it up through their roots, and as they die and decay, release it back to the soils, where some of it is reburied. Some of the phosphorus is also taken up by microbes that feed on the detritus, as well as by insects and worms. Starting in a relatively pristine portion of an Everglades water conservation area, which is not impacted by phosphorus from outside inputs, Richardson's team built 10-meter-long wooden flumes that stretched north–south, generally parallel to the flow of water in the wetlands.
One flume served as a baseline control, with background levels of soluble reactive phosphorus at 5 µg/L. The researchers dosed the northern, upstream ends of four other flumes with successively higher amounts of Na2HPO4, to reach 22, 39, 57, and 126 µg/L of available phosphorus. From 1992 to 1998, Richardson and co-workers monitored changes in several species downstream, including Utricularia purpurea (a perennial bladderwort), algae (diatoms and mats), and macroinvertebrates (such as worms)—organisms with intermediate, fast, and slow responses, respectively, to shifts in phosphate levels.
The team fed more than 50 ecological indicators for these species, such as population densities and the numbers of species present, into a Bayesian statistical model that calculated weighted tipping points of change over time. The team concluded that the ecosystem's threshold tolerance for phosphorus is 12–15 µg/L.
This window is higher than the acceptable level of 10 µg/L set by the state and U.S. EPA. The lower limit, according to researchers who have spent years studying the Everglades, came about because of observed changes in the species central to Richardson and colleagues' modeling. But it also relies on a precautionary approach and does not address whether a system can assimilate phosphorus inputs and recalibrate itself, experts add. "Our perspective is that you should be looking at the very beginning of changes" to the structure and function of the Everglades ecosystem, says Nick Aumen of the U.S. National Park Service, who coleads the federal Everglades Program Team responsible for monitoring and management.
Applying a Bayesian approach is a "different way of analyzing data" for most ecologists, notes Joel Trexler of Florida International University, who recently coauthored reports on "ecosystem state change" in the Everglades as a result of additions of above-ambient amounts of phosphorus (J. Environ. Qual. 2005, 34, 717–723) (PDF: 265 KB). Although the new method may be useful, Trexler wonders whether it misses longer-term adjustments to phosphorus by averaging the plant and animal responses in the flumes over the 6 years of the study. He also questions whether the systems studied were truly at equilibrium.
Bill Orem, who has worked in the Everglades for more than a decade as a biogeochemist with the U.S. Geological Survey, says that the Bayesian modeling quantifies some previously observed qualitative threshold effects for various species. However, he points out that the team did not take into account synergistic effects such as simultaneous sulfate loading, which enhances phosphorus's effects, or dessication and flooding, which can release sequestered phosphorus.
Scientific proof for any ecosystem response can be elusive, and this shifts management and restoration to values-based decision making, comments Penny Johnes, director of the Aquatic Environments Research Centre at the University of Reading (U.K.). The Bayesian approach taken by Richardson and co-workers seems to be "a really useful contribution to the debate over ecological thresholds," Johnes comments, that has implications for the restoration of wetlands in terms of both structure and function, in the Everglades and elsewhere.
