Changes in Ecosystem Macronutrient Budgets, Microbial Characteristics, and Vegetation Patterns, along Phosphorus-enrichment Gradients in Everglades Wetlands.
Leonard J. Scinto, Daniel L. Childers, Evelyn E. Gaiser, Ronald D. Jones,
Michael Rugge, and Joel Trexler.
Florida International University (FIU), Miami, FL USA.
Robert F. Doren
Department of the Interior, Miami, FL USA.
Gregory B. Noe
United States Geologic Survey, Reston, VA USA.
William A. VanGelder
Southwest Florida Water Management District, Bartow, FL USA.
System changing perturbations, such as eutrophication processes, first affect those system components with short turnover times, rapid uptake and release kinetics, and short lifespan. Other system components with slower response kinetics subsequently show signs of change. Wetlands response to eutrophication tends to be more rapid and longer lasting in oligotrophic systems such as the Everglades. Thus ecosystem state change occurs as a cascade of response, first measurable in microbial and periphyton components, followed by sequential changes in soils, microinvertibrates, and finally in macroinvertibrates.
The original Everglades was an extensive (10 000 km2) oligotrophic wetland flowing from Lake Okeechobee south to the mangrove estuaries of Florida Bay. Approximately half the original Everglades have been developed for urban and agricultural use by filling and other hydrologic modifications including an extensive network of canals and water control structures. These hydrologic modifications have resulted in the compartmentalization of the once contiguous Everglades with water flows determined by placement and operation of water control structures rather than diffuse flow distributed across the landscape. Concurrent, to hydrologic modification was an increase in the nutrient load of the water entering the Everglades. For instance, canal water originating largely in the Everglades Agricultural Area (EAA) and discharging into northern Everglades marshes was shown to contain TP concentrations up to 30 times higher (100 – 300 g L-1) than typically found in unimpacted interior Everglades marsh water (<10 g L-1). These effects have been greatest in the northern Everglades and have led to a general north to south gradient in canal water nutrient concentrations. One characteristic response of wetlands to nutrient enrichment is an increase in soil nutrients and subsequent shifts in plant community composition. Well- documented gradients in soil TP and cattail (Typha domingensis) dominance have been observed in several areas of the Everglades.
In this project we sampled a total of 68 sites distributed among 5 transects throughout the Everglades. Transects were established perpendicular to inflow canals with sites located along suspected P-enrichment gradients. One transect was located in each of five areas: the Arthur R. Marshall Loxahatchee National Wildlife Refuge (a.k.a. WCA-1), WCA-2A, WCA-3A, and Shark River Slough (SRS) and Taylor Slough (TS) in Everglades National Park (ENP). At each site we examined patterns of P, N, and C, concentrations, standing stocks, and partitioning by sampling the surface water, periphyton, floc, soil, macrophytes and consumers. We positioned four of our transects (1999) in relation to transects that were originally sampled in 1989 to document changes in soil TP content and macrophyte species composition that has occurred over the past ten years.
Although water quality was shown to have improved throughout much of the Everglades in the 1990’s we found that water quality impacts worsened during this time in the northern Everglades, WCA-1 and WCA-2A. Zones of high soil P (>700 mg kg-1 dry wt. soil) increased to more than 1 km from the western margin canal into the WCA-1 marsh and more than 4 km from the northern boundary canal into WCA-2A. An asymptotic, exponential decrease equation generally modeled P-enrichment gradients, when present, in soil and floc and allowed evaluation of differences in TP concentrations in enriched versus unimpacted areas. The expansion of high soil TP zones paralleled an expansion of cattail-dominated marsh in both regions. Where P-enrichment leads to a dominance of cattails it causes a state change in the ecosystem to P-rich components and an increase in the relative importance of macrophytes to the ecosystem P budget. This state change also decouples P accumulation from N and C storage. The proportion of total ecosystem P in the soil and floc compartments was approximately 95% of the ecosystem P and did not vary across the landscape. The TP concentrations in soil, floc, periphyton, and macrophytes were all positively correlated across the Everglades. Macrophyte species richness declined in WCA-1 and WCA-2A but not in the less-impacted southern Everglades (WCA-3A, SRS, TS). There was not an apparent change in plant community patterns at WCA-3A or in SRS over the period 1989-1999. Periphyton exhibited a shift from mat-forming oligotrophic assemblages to filamentous green algal assemblages with P-enrichment. Measures of microbial activity (enzyme activities and CO2/CH4 evolution) were correlated locally to Soil TP concentration but not across the entire Everglades Landscape.
Leonard J. Scinto, Southeast Environmental Research Center, Florida International University, Miami, FL USA 33199
Phone: 305-348-1965, Fax: 305-348-4096, , Ecology