Correspondence Comment on “Estimating Ecological Thresholds for Phosphorus in the Everglades” Everglades wetlands are naturally depleted in phosphorus (P), and water deliveries of above-ambient concentrations of P have caused ecological effects cascading through the food web that persist kilometers downstream of these inputs (1-3). A comprehensive evaluation of decades of scientific effort to determine the concentration of P that elicits ecosystem change concluded that any persistent increase of P above ambient background concentrations causes a biological imbalance (4). This science supported a 10 µg L-1 marsh criterion for water column total P (TP) that was accepted by the Environmental Regulation Commission of the State of Florida in 2003 and approved by the U.S. Environmental Protection Agency in 2005 (5). A higher threshold TP concentration (12-15 µg L-1) has been proposed in a recent article by Richardson and colleagues (6). Their study was based on 7 years of semicontinuous P enrichment in flumes with inflow concentrations ranging from approximately 22 to 126 µg L-1. Using Bayesian changepoint analysis, they determined the geometric mean TP threshold concentration eliciting change in algal, macrophyte and macroinvertebrate community structure, and found changepoints to range from 8.2 to 23.5 µg L-1 (6). Here we examine possible causes and implications of the higher changepoint (12-15 µg L-1) derived by this study (6) than that approved by EPA (10 µg L-1) to protect Everglades wetlands. The Richardson et al. (6) approach to threshold detection involves averaging changepoints across time and space and among ecosystem attributes. By calculating the changepoint annually for each attribute and averaging among years, Richardson et al. (6) treated the enrichment effect, which has been shown to increase over time (3, 7), as static. The result was to inflate the derived threshold range. The hysteretic effects of sustained P loading and downstream spiraling is the source of P-effect “fronts” that continue to move to the interior of Everglades wetlands even when P loading is reduced (2, 3, 8). If the P-limited system shows a dynamic long-term response, chronic exposure to added P will stimulate transitions in community composition and productivity that would lead to a new biological starting point each period of exposure, as has been shown in P dosing studies in the southern Everglades (9). Higher P supplies would be required to shift the new community to a new state each time, and if the system is already biologically saturated in P (as may be expected in the very high treatment levels employed here), TP would remain in the water, increasing the probability of finding an increasingly higher water TP threshold over time. Richardson et al. (6) also suggest that averaging changepoints among ecosystem attributes provides a robust measure of the TP threshold. This use of averaging to derive a “protective” water-quality standardsone that protects all ecological characteristics deemed important to the integrity of the ecosystemsis a particularly disturbing aspect of their statistical approach. The Everglades phosphorus standard, as codified in Florida State law (Everglades Forever Act, Fl Statute Chapter 373.4592; Florida Administrative Code 62-302.540), is required to result in no imbalance in native flora or fauna (our italics). By averaging responses across metrics with varying sensitivities to phosphorus enrichment, 6770
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 17, 2008
Richardson et al.’s (6) approach accepts that imbalances will occur in certain metrics. Since different ecological metrics represent a succession of response unfolding over time (9), averaging the P changepoint for these metrics ensures that the changepoint is exceeded for some. This inescapable characteristic was one of several reasons why their approach was rejected by state and federal regulatory agencies as a valid means of deriving a “no imbalance” threshold for phosphorus in the Everglades as required by State law. The importance of a temporal and cascading response to aboveambient P exposure was why the State primarily used water quality and associated ecological data from established nutrient gradients to derive the TP criterion, using the experimental data as supportive information. We assert that Richardson et al. (6) recommend an environmentally risky approach to water quality assessment and management that decreases the chance of detecting P-elicited changes before the ecosystem has transitioned to an irreversible stable state (10). In the Everglades, this is a particular source of concern, as no practical means of restoring oligotrophic habitat is currently known and for all practical purposes, P-enriched wetlands will remain degraded for decades (10).
Literature Cited (1) McCormick, P. V.; Newman, S.; Maio, S.; Gawlik, D. E.; Marley, D.; Reddy, K. R.; Fontaine, T. D. Effects of Anthropogenic Phosphorus Inputs on the Everglades. In The Everglades, Florida Bay, and Coral Reefs of the Florida Keys: An Ecosystem Sourcebook; Porter J. W., Porter, K. G., Eds.; CRC Press: Boca Raton, FL, 2002; pp 83-126. (2) Childers, D. L.; Doren, R. F.; Noe, G. B.; Rugge, M.; Scinto, L. J. Decadal change in vegetation and soil phosphorus patterns across the Everglades landscape. J. Environ. Qual. 2003, 32, 344–362. (3) Gaiser, E. E.; Richards, J. H.; Trexler, J. C.; Jones, R. D.; Childers, D. L. Periphyton responses to eutrophication in the Florida Everglades: Cross-system patterns of structural and compositional change. Limnol. Oceanogr. 2006, 51, 617–630. (4) Payne, G. G.; Weaver, K. C.; Xue, S. K. Chapter 2C: Status of phosphorus and nitrogen in the Everglades protection area. South Florida Environmental Report; South Florida Water Management District: West Palm Beach, 2004. (5) Payne, G. G.; Weaver, K. C.; Xue, S. K. Chapter 2C: Status of phosphorus and nitrogen in the Everglades protection area. South Florida Environmental Report; South Florida Water Management District: West Palm Beach, 2006. (6) Richardson, C. J.; King, R. S.; Qian, S. S.; Vaithiyanathan, P.; Qualls, R. G.; Stow, C. A. Estimating ecological thresholds for phosphorus in the Everglades. Environ. Sci. Technol. 2007, 41, 8084–8091. (7) Sklar, F. H.; Chimney, M.; Newman, S.; McCormick, P.; Gawlik, D.; Miao, S.; McVoy, C.; Said, W.; Newman, J.; Coronado, C.; Crozier, G.; Korvela, M.; Rutchey, K. The ecological-societal underpinnings of Everglades restoration. Front. Ecol. Environ. 2005, 3, 161–169. (8) DeBusk, W. F.; Newman, S.; Reddy, K. R. Spatio-temporal patterns of soil phosphorus enrichment in Everglades Water Conservation Area 2A. J. Environ. Qual. 2001, 30, 1438–1446. (9) Gaiser, E. E.; Trexler, J. C.; Richards, J. H.; Childers, D. L.; Lee, D.; Edwards, A. L.; Scinto, L. J.; Jayachandran, K.; Noe, G. B.; Jones, R. D. Cascading ecological effects of low-level phosphorus enrichment in the Florida Everglades. J. Environ. Qual. 2005, 34, 717–723. (10) Kadlec, R. H. The limits of phosphorus removal in wetlands. Wetlands Ecol. Manage. 1999, 7, 165–175. 10.1021/es800347t CCC: $40.75
2008 American Chemical Society
Published on Web 08/15/2008
Evelyn E. Gaiser
Robert F. Doren
Department of Biological Sciences, and Southeast Environmental Research Center, Florida International University, Miami, Florida 33199
South Florida Ecosystem Restoration Task Force, Florida International University, Miami, Florida 33199
Paul V. McCormick and Susan Newman Jennifer H. Richards and Joel C. Trexler Department of Biological Sciences, Florida International University, Miami, Florida 33199
South Florida Water Management District, 3301 Gun Club Rd, West Palm Beach, Florida 33406 ES800347T
VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6771
