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Gulf of Mexico Origin, Waters, and Biota
Volume 4, Ecosystem-Based Management
By John W. Day, Alejandro Yáñez
Texas A&M University PressCopyright © 2013 Texas A&M University Press
All rights reserved.
System Functioning and Implications for Coastal Zone Management
John W. Day, Fred Sklar, Jaye E. Cable, Daniel L. Childers, Carlos Coronado-Molina, Steve E. Davis, Steve Kelly, Christopher J. Madden, Brian Perez, Enrique Reyes, David T. Rudnick, and Martha A. Sutula
Like many ecosystems around the world, the Florida Everglades is threatened by global change. One of the world's largest wetlands, it was described at one time as a vast, free-flowing "river of grass" extending from the Kissimmee chain of lakes to the Gulf of Mexico and Florida Bay (Douglas 1988). These subtropical wetlands support a rich diversity of plants, fish, and animals, including prolific populations of alligators, deer, panthers, manatees, wading and migratory birds, and mosquitoes. The historical Everglades encompassed a broad area of "ridge and slough" landscape (freshwater sloughs with periphyton mats, sawgrass ridges, and tree islands), marl-forming prairies on adjacent higher ground, and to the south, mangrove forests and saline tidal flats of Florida Bay (Fig. 1.1). Additionally, the very health and nature of the Everglades is closely tied to the volume, periodicity, and distribution of water entering the wetlands from Lake Okeechobee overflow. Over the past 100 years, however, the hydrology, chemistry, and biology of this ecosystem were altered dramatically to accommodate rapidly growing urban populations and industrial agriculture in south Florida. Two major effects on the ecosystem include large-scale diversions of freshwater from the Everglades to the Atlantic Ocean and Gulf of Mexico and the conversion of large areas of wetlands for agricultural and urban uses. Now this dynamic relationship between the upstream freshwater lake system and the downstream "river of grass" is fundamentally altered by encroaching anthropogenic pressures.
Coupled to this freshwater lake–wetland connection is the downstream marine end member of Florida Bay, a large, shallow subtropical marine system. At least 22 commercially or recreationally important aquatic nektonic species use Florida Bay as a nursery ground including pink shrimp (Farfantepenaeus duorarum), which is the basis of a multimillion dollar fishery in the Tortugas. As anthropogenic pressures on the Everglades increase, they also affect this downstream marine bay. Over only a few months in the summer of 1987, approximately 400 km2 of seagrass, primarily Thalassia testudinum, began to die at an alarming rate in western Florida Bay (Sklar et al. 2005). During the drought years of 1988 and 1989, summer salinity in central Florida Bay exceeded 60 and water temperature exceeded 40°C (Sklar et al. 2005). This massive die-off event was followed by phytoplankton blooms and high sponge mortality. Conditions in Florida Bay continued to decline, including increased turbidity, increased algal blooms, more seagrass loss, and reduced fish and shellfish populations. Wading birds, forage fish, and juveniles of game fish species also were affected. The link between the Everglades and Florida Bay is critical to the long-term health and survival of this important freshwater wetland–estuary–Gulf of Mexico continuum.
Everglades restoration plans aim to restore flows of high-quality water to the southern Everglades and to Florida Bay (Sklar et al. 2001; Sklar et al. 2005). However, concern exists that increased water and nutrient flows to Florida Bay will lead to further environmental deterioration. Over the past 4 decades, considerable research has taken place on material fluxes into and from coastal ecosystems (Nixon 1980; Odum 1984; Childers et al. 2000. From 1995 until 2001, a series of studies were carried out to characterize the ecology of the salinity transition zone from the southern border of freshwater marshes, through mangroves, to northern Florida Bay to understand the effects of increased freshwater flow on the functioning of this regional ecosystem (e.g., Davis et al. 2001a, 2001b; Sutula et al. 2001; Coronado-Molina et al. 2003; Davis, Childers et al. 2003; Davis, Coronado-Molina et al. 2003; Sutula et al. 2003; Davis et al. 2004). In this chapter, the management perspective that pollutant (i.e., nutrient) load reduction and hydrologic alterations are the critical issues considered. Biogeochemical and hydrologic relationships among the Everglades mangrove wetlands, the Taylor Creek estuary, and Florida Bay are provided as a backdrop for the critical importance of wetland preservation and coastal zone management. The goals of this chapter are (1) to determine how water management and climate affect the exchange of water and nutrients between Florida Bay and the transitional, mangrove-dominated wetlands at the southern boundary of the Everglades, (2) to determine the structural and functional response of the transition zone to the quantity and quality of freshwater flows in the southern Everglades watershed, and (3) to determine this region's sensitivity to water management practices.
Historical Everglades: Era of Human Influence
Developmental activities have resulted in significant change to the Everglades, including a reduction by half in the areal extent of the wetlands. Surficial aquifer water levels have dropped as municipal and agricultural demand for freshwater has increased. Wetland inundation frequency and duration have been altered as a result of diversions, impoundment structures, and the increased demand for water. As a result, wildlife has been reduced, water quality has been degraded, and habitats have been invaded by nonindigenous plants. These effects are the direct or indirect result of alterations in hydrology and landscape. The following description provides a brief historical account of the events leading to modern wetland conditions.
The first major efforts to drain the Everglades began in 1880, and by 1940, surficial aquifer water tables decreased as much as 3 m, large areas of organic soils decomposed and subsided, and topographic changes of 0.5 to 1.5 m due to subsidence actually reversed the direction of surface water flow (Davis 1943). As a result, Lake Okeechobee surface water levels were lowered and overflows from the lake into the Everglades were reduced. From 1906 to the 1920s, drainage resulted in the diversion of approximately 2 billion cubic meters of water to the Atlantic Ocean and the drainage of over 600,000 ha of wetlands (Parker 1955; SFWMD 1998). Hydrologic isolation of surface water in the Everglades from Lake Okeechobee, completion of the 4 major canals, and construction of the Tamiami Trail all strongly affected Everglades hydrology. For example, water levels measured adjacent to the New River Canal in 1915 and 1939 had decreased from about 30 cm below ground (1915) to more than 1.5 m below ground (1939)—a decrease of more than 120 cm over the intervening 24 years. As a result of these changes, a major reduction of water flow to the southern Everglades occurred.
As water levels decreased in the Everglades, more area of organic soil (i.e., peat) was exposed to aerobic conditions for progressively longer periods each year, and soils began to subside, partly due to physical compaction and burning, but mostly due to oxidation (Clayton 1936) (Table 1.1). The average decline in soil elevation was 2.5 cm per year (Clayton 1936; Clayton et al. 1942; Stephens and Johnson 1951; Shih, Mishoe et al. 1979; Shih, Stewart et al. 1979). Between 1912 and 1940, as much as 2 m of soil elevation was lost in the Lake Okeechobee area (Stephens and Johnson 1951).
By the 1940s, recognition that overdrainage had resulted in widespread soil loss led to the construction of levees to impound undeveloped lands (Allison 1943; Advisory Committee 1944). The Central and South Florida Project, authorized by Congress in 1948, resulted in the construction of the Water Conservation Areas and the partitioning off of 283,000 ha for the Everglades Agricultural Area by the 1960s. From 1965 to 1973, the Central and South Florida Project was modified to satisfy the Everglades minimum monthly water requirements as defined for Shark River Slough, Taylor Slough, and the eastern panhandle of the Everglades National Park on the basis of 1940s and 1950s guidelines.
Southern Everglades and Florida Bay
Everglades National Park (ENP) encompasses 5700 km2 of former "ridge and slough" landscape, marl-forming prairies, mangrove forests, and tidal flats, and is considered one of the nation's 10 most endangered national parks. As far back as the late 1930s, decline in ENP biological resources has been linked primarily to changes in Lake Okeechobee overflows, freshwater diversions, canals, and disruption of the wetland water path by the Tamiami Trail (Beard 1938) and later, Interstate 75. The reduction of water into Shark River Slough due to minimum water flows and a shift of water flow to the west created shortened hydroperiods and decreased water depths, and virtually eliminated water flow into northeast Shark River Slough. As a result, areas in northeast Shark River Slough dominated by sawgrass (Cladium jamaicense) stands have increased in size while aquatic slough communities have declined (Alexander and Crook 1975; Davis and Ogden 1994; Olmsted and Armentano 1997).
The construction of the South Dade Conveyance System along the southeastern boundary of ENP fostered agricultural and urban development in the eastern Everglades, decreased the extent of marl-forming prairies, and interfered with freshwater flows through ENP's second-most important floodway, Taylor Slough (Van Lent and Johnson 1993). Much of the freshwater flowing naturally through Taylor Slough and Shark River Slough to the coast was diverted by canals to other coastal areas (Light and Dineen 1994). The effect of this wetland water diversion is a noticeable decline in Taylor Slough water levels during the spring and fall (Table 1.2).
An important goal of Everglades restoration is reestablishment of the hydrologic conditions and ecological characteristics of the coastal zone. Florida Bay restoration is of particular concern because it has been subject to drastic ecological changes during the past century (Boesch et al. 1993; Fourqurean and Robblee 1999). These changes include increased seagrass mortality and algal blooms and decreased water clarity. Decreased freshwater inflow from the Everglades and resultant increases in salinity are implicated as contributing to these ecological changes (Sklar et al. 2001; Sklar et al. 2005). Restoration of the southern Everglades and Florida Bay to alleviate some of the deleterious effects of the hydrologic alterations is currently under way. Structural and operational changes made during the past 10 years include (1) government acquisition of the Frog Pond agricultural area adjacent to the eastern boundary of ENP, and (2) removal of the south levee on the lower C-111 canal. Right of way through Frog Pond allows increased water deliveries through Taylor Slough and into Florida Bay, while more flow from the C-111 canal increases water flow through the southeastern Everglades (ENP panhandle) into the northeast corner of Florida Bay (Figs. 1.1 and 1.2). Calculation of the Florida Bay water budget indicates from 1970 to 1995, runoff from the Taylor Slough/C-111 Basin represented less than 10% of input from direct rainfall into Florida Bay (Nuttle et al. 2000).
Goals and Objectives
From 1996 to 2001, we undertook a large study to gain an understanding of the potential effect of increased freshwater flow on the salinity transition zone and northeastern Florida Bay. This chapter represents a synthesis of the research conducted over this period where we investigated material fluxes at several interfaces, including sediment–water, groundwater–surface water, wetlands–creek, upstream–downstream, and creek–bay (e.g., Davis et al. 2001a, 2001b; Sutula et al. 2001; Coronado-Molina et al. 2003; Davis, Childers et al. 2003; Davis, Coronado-Molina et al. 2003; Sutula et al. 2003; Davis et al. 2004). In addition, we studied the structure and productivity of the plant community and its relationship to biogeochemical sources and sinks. The goal of this 2-phase, 7-year project was to determine how water management and climate affect the exchange of water and nutrients between Florida Bay and the transitional, mangrove-dominated wetlands at the southern boundary of the Everglades. Our objectives were (1) to determine the seasonal and inter-annual variability of phosphorus (P) and nitrogen (N) inputs to northeastern Florida Bay from creeks that contribute most of the channelized flow of freshwater to the bay; (2) to determine the relationship between rates of P and N inputs to Florida Bay and rates of freshwater flow; (3) to determine the importance of the freshwater sawgrass marshes of Taylor Slough as a source of nutrients for the transition zone and Florida Bay; (4) to determine the importance of transition zone mangroves, ponds, and creeks as a source or sink of nutrients that can cross the wetlands–bay boundary; (5) to determine the importance of groundwater flow to nutrient movement through the transition zone; (6) to determine the relationship between plant productivity rates, hydrological conditions, salinity levels, and nutrient availability in the transition zone; and (7) to synthesize this information to predict the effects of changing freshwater flow and water levels on sources, fate, and transport of nutrients and on productivity across the southern Everglades transition zone. Our ultimate goal was to understand processes and source and sink relationships for future management of the ecosystem.
This work was designed to address 2 overarching questions: (1) what is the ecological role of the salinity transition zone (STZ) in mediating freshwater and nutrient inputs to Florida Bay, and (2) how are ecological patterns and processes of the salinity transition zone controlled by changes in key environmental drivers (especially freshwater flow). In the first question, our hypothesis was that the STZ filtered nutrient inputs from the freshwater Everglades, and thus would reduce nutrient loading to Florida Bay proportional to increases in freshwater flow. In addressing this hypothesis, we further hypothesized that (1) Florida Bay is the main phosphorus source to the STZ, while the upper Everglades is the main N source; (2) the STZ acts as a sink for both P and N; (3) the STZ is a net source of dissolved organic material to Florida Bay; and (4) Florida Bay is the principal source of inorganic particulates to the STZ, while organic particulates are derived from STZ internal processes. In question 2, our hypothesis was that increasing freshwater and decreasing salinity would enhance the productivity of the STZ and the ability of mangroves to adjust to sea-level rise and sedimentation flux. Additionally, the ecological filtering capability of the STZ is regulated by pulsing of environmental drivers at several scales, primarily wet–dry seasonal effects and episodic storm events. We investigated tides, wet and dry seasons, interannual climate variability, cold-front storms, cyclonic storms, and anthropological and cultural effects on the ecosystem of the Everglades (mangrove) STZ to address multiple pulsing scales in controlling biogeochemistry and productivity.
Sampling was carried out in the STZ of the southeastern Everglades. The major freshwater input to the area is the Taylor Slough/C-111 watershed, a broad shallow depression draining into northeastern Florida Bay via several creeks (Fig. 1.2). The area is characterized by 3 climatic seasons. A dry season occurs from February to May with mild temperatures and low precipitation. During this period, the creeks are characterized by high salinities and low discharge. Next, a rainy season typically exists from June to October when precipitation is produced primarily by convective thunderstorms and less frequently by tropical storms and hurricanes (Chen and Gerber 1990). Wet season precipitation is, on average, 77% of total annual precipitation. The creeks are fresh during this period, and high freshwater discharge occurs to northeastern Florida Bay. Lastly, a winter frontal season characterized by sporadic rains, low temperatures, and variable discharge (primarily a function of wind direction and intensity) predominates the period between the wet and dry seasons (Chen and Gerber 1990). Cold fronts are characterized by southerly winds during the prefrontal stage, intense squalls with rainfall as the front passes, and strong, cold, northerly winds in the post-frontal stage (Moeller et al. 1993).
Excerpted from Gulf of Mexico Origin, Waters, and Biota by John W. Day, Alejandro Yáñez. Copyright © 2013 Texas A&M University Press. Excerpted by permission of Texas A&M University Press.
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