|Whole-ecosystem phosphorus budgets for freshwater Everglades wetlands
Gregory B. Noe
U.S. Geological Survey, Reston, VA
Daniel L. Childers
Florida International University, Miami, FL
We are currently developing synthetic, whole-ecosystem phosphorus (P) budgets for freshwater Everglades wetlands. The development of P budgets will aid attempts to understand the impact of P enrichment on oligotrophic Everglades wetland ecosystems and prioritize future research by identifying knowledge gaps. Preliminary P budgets were developed for oligotrophic wet prairie (slough; Figure 1), oligotrophic Cladium, mixed Cladium/Typha, and Typha marsh ecosystem types. Mean P standing stocks (g P m-2) for each component were quantified by reviewing published and unpublished literature on biomass and P concentrations in the Everglades. Fluxes (g P m-2 yr-1) were determined by a variety of methods, including estimates of C turnover (assuming fixed C:P), water velocity, 32P cycling, and long-term soil accretion rates. No reasonable estimate could be independently derived for many flux rates, in which case flux was determined by mass balance. As a first approximation, the P budgets assume steady-state conditions, 12-month hydroperiod, no mega-consumers, and no consumption of macrophyte detritus by consumers. In addition, nutrient turnover rates of periphyton, floc, and consumers in Cladium marsh were assumed to be equivalent to wet prairie marsh.
Mean total ecosystem P standing stocks ranged from 3.46 in wet prairie, 3.65 in Cladium, 7.29 in Cladium/Typha, to 10.44 g P m-2 in Typha marsh. In wet prairie, soils (0-10 cm) held the most P in the ecosystem (72%), followed by floc (22%), live macrophyte rhizomes and roots (2.3%), aquatic consumers (1.5%), periphyton (1.2%), live aboveground macrophytes (1.1%), dead aboveground macrophytes (0.4%), and the water column (0.2%). Phosphorus partitioning, the percentage of total ecosystem standing stock, differed in oligotrophic Cladium and P-enriched Cladium/Typha and Typha marshes. In Cladium marsh, P partitioning is 4x and 6x greater in live and dead aboveground macrophytes, respectively, and 5x less in periphyton compared to the same components in wet prairie. As Everglades wetlands receive additional P loading and shifts in macrophyte species occur, live and dead aboveground macrophytes store increasingly larger proportions of whole-ecosystem P standing stock. Live aboveground macrophyte tissues store 3x and 9x larger proportions of ecosystem P in Cladium/Typha and Typha marsh, respectively, than in oligotrophic wet prairie. Similarly, dead aboveground macrophyte tissues increase P partitioning 11x in Cladium/Typha and 7x in Typha marsh. Periphyton partitioning increases slightly in the P-enriched ecosystems. Finally, floc stores 2x smaller proportions of P in the ecosystem as it enriches with P.
Long-term, steady state P dynamics in oligotrophic Everglades marshes are limited by average inputs (atmospheric deposition, 0.03 g P m-2 yr-1) and outputs (soil burial, 0.09 g P m-2 yr-1). Very large quantities of P flow in and out of a given area of oligotrophic marsh (~ 800 g P m-2 yr-1), but net ecosystem uptake from the water column (0.06 g P m-2 yr-1) is constrained by atmospheric input and soil burial. In wet prairie marsh, slow turnover of macrophyte stems and low P standing stocks result in relatively low net annual P flux from macrophytes compared to periphyton, floc, and consumers. For example, periphyton net annual through-flux is estimated to be 1.33 g P m-2 yr-1, while live aboveground macrophyte tissues cycle 0.04 g P m-2 yr-1. The smaller periphyton and greater macrophyte biomass in Cladium marsh relative to wet prairie are paralleled in less periphyton (0.27 g P m-2 yr-1) and more macrophyte (0.23 g P m-2 yr-1) throughput. Phosphorus enrichment, and the invasion of Typha, greatly increases the importance of macrophytes (both P standing stocks and throughput) to the ecosystem P budget. Macrophyte senescence results in the annual net movement of 1.59 g P m-2 yr-1 in Typha marsh.
Although data on biomass and P concentrations, and hence P standing stocks, are plentiful, few measurements of P fluxes exist. Relative to fluxes involving soils and macrophytes, fluxes between the water column, floc, periphyton, and consumers are less well quantified. In addition, most P flux rates have been measured in wet prairie and few rate measurements exist for Cladium or Typha marsh. Furthermore, most P cycling occurs in the water column between water, periphyton, floc, and consumers, while soil stores the largest amount of P in the ecosystem. Finally, P enrichment in the Everglades results in an increased importance of macrophytes to P cycling.
As part of the Florida Coastal Everglades LTER, these nutrient budgets will be expanded in the future to include site-specific data and nitrogen and carbon dynamics. In addition, nutrient budgets can serve as an integrative and synthetic tool for comparing freshwater marsh, mangrove, and seagrass ecosystems, and the impacts of nutrient enrichment on ecosystems.
Figure 1. The P budget for oligotrophic wet-prairie marsh. Boxes represent the P standing stock (both organic and inorganic) of the different ecosystem components (g P m-2) and arrows indicate net annual P fluxes between components (g P m-2 yr-1). Underlined fluxes have high uncertainty.
Gregory B. Noe, U.S. Geological Survey, 430 National Center, Reston, VA, 20192, Phone: 703-648-5826, Fax: 703-648-5484, email@example.com, Water Quality