Professional Paper 1758
Prepared in cooperation with
Pinellas County
Southwest Florida Water Management District
Tampa Bay Water
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CONTENTS Abstract Introduction Purpose and Scope Description of Study Area Rainfall Patterns and Regional Ground-Water Withdrawals Description of Study Design Acknowledgments Wetland Hydrogeologic Setting Regional Hydrogeology Hydrogeologic Methods Basin Stratigraphy Sub-Wetland Stratigraphy Radium-226 Evidence of Karst Features in Wetland Basins Ground-Water Flow Patterns in Wetland Basins Overview of Wetland Hydrogeologic Settings Wetland Water Budgets Methods of Computation Rainfall and Evapotranspiration Wetland Stage, Volume, and Area Wetland Leakage Effect of Downward Head Differences on Wetland Leakage Effect of Hydraulic Conductivity on Wetland Leakage Case Studies of Wetland Leakage Duck Pond Augmented Marsh S-63 Augmented Cypress W-5 Augmented Cypress Runoff to Wetlands Overview of Wetland Water Budgets Wetland Water Quality and Geochemistry of Wetland Basins Water-Quality and Geochemical Methods Water-Quality Constituents Field Properties and Major Ions Nutrients and Dissolved Organic Carbon Stable Isotopes Basin Geochemistry Field Properties and Major Ions Nutrients and Dissolved Organic Carbon Stable Isotopes Overview of Water Quality and Geochemistry Wetland Flooding Characteristics Methods of Flooded Area Determination Changes in Extent of Flooded Area Marshes Cypress Wetlands Comparison of Recent and Historical Flooded Area Duration Distributions Natural Wetlands Augmented Wetlands Impaired Wetlands Seasonal Average Flooding Patterns Overview of Flooding Characteristics Wetland Ecology Methods of Ecological Data Collection and Interpretation Periphyton Wetland Vegetation Macroinvertebrates Periphyton Biomass and Chlorophyll-a Community Composition Wetland Vegetation Comparison of Vegetation Communities Species Richness Relative Abundance of Wetland Plants by Indicator Category Plant Biomass in Marshes Tree Density and Size in Cypress Wetlands Effects of Environmental Stressors on Wetland Plant Communities Macroinvertebrates Marsh Macroinvertebrate Communities Cypress Macroinvertebrate Communities Functional Feeding Groups Macroinvertebrates as Ecological Indicators in Wetlands Overview of Wetland Ecology Summary and Conclusions References Cited Glossary
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Comparing altered wetlands to natural wetlands in the same region improves the ability to interpret the gradual and cumulative effects of human development on freshwater wetlands. Hydrologic differences require explicit attention because they affect nearly all wetland functions and are an overriding influence on other comparisons involving wetland water quality and ecology. This study adopts several new approaches to quantify wetland hydrologic characteristics and then describes and compares the hydrology, water quality, and ecology of 10 isolated freshwater marsh and cypress wetlands in the mantled karst landscape of central Florida. Four of the wetlands are natural, and the other six have water levels indirectly lowered by ground-water withdrawals on municipally owned well fields. For several decades, the water levels in four of these altered wetlands have been raised by adding ground water in a mitigation process called augmentation. The two wetlands left unaugmented were impaired because their water levels were lowered. Multifaceted comparisons between the altered and natural wetlands are used to examine differences between marshes and cypress wetlands and to describe the effects of augmentation practices on the wetland ecosystems.
In the karstic geologic setting, both natural and altered wetlands predominantly lost water to the surficial aquifer. Water leaking out of the wetlands created water-table mounds below the wetlands. The smallest mounds radiated only slightly beyond the vegetated area of the wetlands. The largest and steepest mounds occurred below two of the augmented wetlands. There, rapid leakage rates regenerated a largely absent surficial aquifer and mounds encompassed areas 7-8 times as large as the wetlands.
Wetland leakage rates, estimated using a daily water-budget analysis applied over multiple years and normalized as inches per day, varied thirtyfold from the slowest leaking natural wetland to the fastest leaking augmented wetland. Leakage rates increased as the size of the flooded area decreased and as the downward head difference between the wetland and the underlying Upper Floridan aquifer increased. Allowing one of the augmented wetlands to dry up for about 2.5 months in the spring of 2004, and then refilling it, generated a net savings of augmentation water despite the amount of water required to recreate the water-table mound beneath the wetland. Runoff from the surrounding uplands was an important component of the water budget in all of the unaugmented wetlands and two of the augmented wetlands. At a minimum, runoff contributed from half (45 percent) to twice (182 percent) as much water as direct rainfall at individual wetlands.
Wetland flooded areas, derived using wetland water levels and bathymetric data and presented as a percentage of total wetland area, were used to compare and contrast hydrologic conditions among the 10 wetlands. The percentages of the natural wetland areas that flooded during the study were comparable, despite differences in the sizes of the wetlands. The percent flooded area in each wetland was calculated daily over the study period and monthly for up to 16 years using historical water-level data. Historical flooding in the natural wetlands spanned a greater range in area and had more pronounced seasonality than historical flooding at either the impaired or augmented wetlands. Flooding in the impaired and natural wetlands was similar, however, during 2 years of the study with substantially reduced well-field pumping and above average rainfall.
Comparisons indicated several hydrologic differences between the marsh and cypress wetlands in this study. The natural and impaired marshes leaked at about half the rate of the natural and impaired cypress wetlands, and the marshes collectively were underlain by geologic material with lower vertical leakance values than the cypress wetlands. The natural marshes had higher evaporation rates compared to cypress wetlands, and their more isotopically-enriched surface waters indicated longer water residence times than the cypress wetlands. Over the same 8-year period, marshes spent from 16 to 30 percent more time (or about 15 to 29 months more) than cypress wetlands with greater than half of their total areas flooded. Cypress wetlands were nearly dry a greater percentage of time than marshes; however, more than 80 percent of their area was flooded a greater percentage of time than marshes. The water quality of natural marsh and cypress wetlands was similar, with a low pH, low conductivity, minimal alkalinity, and low concentrations of major ions; therefore, periphyton communities in natural marsh and cypress wetlands also were similar. Vegetation is inherently different between marsh and cypress wetlands, and among wetland sites of the same type there was a large variety and small overlap of vegetation species. Macroinvertebrate taxa richness and density were generally greater in natural marshes than in natural cypress wetlands.
The hydrology and water quality of augmented wetlands differed substantially from natural wetlands, but ecological differences were less apparent. Augmentation preserved between 40 and 80 percent of the original surface areas of four wetlands. The water levels in augmented wetlands, however, fluctuated less than in natural wetlands and augmented wetlands dried out far less frequently, accelerating sediment accumulation. Year-round augmentation of the deepest and fastest leaking wetland, Duck Pond Augmented Marsh, required a volume equivalent to a 60-foot column of water over an area of about 3 acres. The bottom sediments in augmented wetlands did not show enrichment of radium-226, as has been reported in augmented lakes in the area. Augmentation shifted wetland water quality from an acidic, dilute, and sodium-chloride dominated chemistry to a calcium-carbonate rich water with much higher alkalinity, specific conductance, and pH. The abundance of periphyton species known to prefer higher pH, conductivity, and nutrient concentrations was greater in augmented wetlands. Plant species richness and biomass were higher in the augmented wetlands than in unaugmented wetlands, most likely in response to more prolonged flooding and greater availability of nutrients released by accumulated decaying plant material. The natural variability of macroinvertebrate communities in marsh and cypress wetlands in this study exceeded the differences attributable to augmentation, although the presence of gastropods at augmented wetlands of both types was due to inherent water-quality differences. The comparisons of macroinvertebrate communities between natural and augmented wetlands would be more useful if a larger population of wetlands was available for study.
Quantifying wetland hydrology along with water quality and ecological indicators makes the results from the comparative analyses of these 10 wetlands generic. The approaches used in this study can be applied to future studies and those results can be compared to this initial study population, allowing the comparative analyses to describe an increasing number of wetlands.
Lee, T.M., Haag, K.H., Metz, P.A., Sacks, L.A., 2009, Comparative Hydrology, Water Quality, and Ecology of Selected Natural and Augmented Freshwater Wetlands in West-Central Florida: U.S. Geological Survey Professional Paper 1758, 152 p.
U.S. Geological Survey
Florida Integrated Science Center
Suite 215
10500 University Center Dr.
Tampa, FL 33612-6427
813-975-8620
Terrie M. Lee at tmlee@usgs.gov
or
Kim H. Haag at khhaag@usgs.gov
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