Predicting Saltwater Intrusion Effects on Tidal Freshwater Wetlands of the Lower Apalachicola River
Type of DegreeMaster's Thesis
Forestry and Wildlife Science
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Tidal freshwater forested wetlands are found between terrestrial and aquatic zones and are common near the outlets of the coastal rivers (Mitsch and Gosselink 2007). These wetlands are valuable resources due to the ecological services they provide. For instance, they are often part of the migration zones for birds during the winter time and a breeding area for both coastal fisheries and migratory birds (Costanza et al. 1998). Shallow water tidal habitats that include salt marshes to tidal freshwater wetlands also support the various prey species that are food for a large number of anadromous and marine fishes (Gunderson et al. 1990, Haley 1982, Simenstand et al. 1982). Anadromous species often occupy the tidal freshwater wetlands riparian zone because these areas provide suitable habitat (abundant insects communities, shade, refuge from predation) (Simenstand et al. 1982, Thorpe 1994) through their unique hydrologic characteristics. In eastern United States, tidal freshwater forested wetlands can be found in the Atlantic coast and Gulf of Mexico coastline that extends from Maryrland to Texas. (Odum 1988; Mitsch and Gosselink 2007). Conservative estimates by Field et al. (1991) indicate that there are approximately 200,000 ha of tidal freshwater swamps along the coast of the Southeastern United States (Field at al. 1991). Tidal freshwater includes both forests and marshes and are considered vulnerable to increased saltwater intrusion due to changes in relative sea level and reduced freshwater flows (Doyle et al. 2007). The normal fluctuation of tides and river discharge has been shown to vary seasonally and annually and as a result it is difficult to estimate the upriver extent of tidal forested wetland boundaries (Doyle et al. 2007). 2 Where the rivers meet the ocean they are influenced by tides and other oceanic forces. These rivers are named ‘tidal rivers’ however depending upon the magnitude of river discharge, these lower sections of the river may retain freshwater conditions (Hoitink and Jay 2016). The amount that tidal water moves upriver and salinity levels that occur depend on the elevation of the river mouth and its geomorphology (Doyle et al.2007). Tidal waters can normally reach further inland on larger rivers (Hoitink and Jay 2016). Tidal fluctuations and tidal asymmetry also play important roles (Hoitink and Jay 2016) Tides are highly predictable and fluctuate because of the combined forces of the sun and moon and the rotation of the Earth. In a given day, there may be two pairs of low and high tides. If these high tides and/or low tides are nearly the same height, the pattern is considered semidiurnal (Hoitink and Jay 2016;Figure 1. 1). There can also be extra-high and -low tides. These extra-high tides (spring tides) appear at the time of the new and/or full moon, and the sun, moon, and Earth must be in alignment. One week after a spring tide, the gravitation pull of the sun counters the moon’s gravitational force and creates extra-low (neap) tides (Sumich 1996). Tidal bore is another important circumstance that can affect the salinity level in tidal rivers. They are vertical walls at the surface of water; during a flood, tide bores travel upriver “tens of kilometers” in shallow estuaries (Hoitink and Jay 2016). In this situation, the wave movement can cause a solarity wave before the tidal wave energy is gone (Hoitink and Jay 2016). Salinity and flood regime along tidal wetlands have been impacted by sea level rise, and this change plays an important role on current and future vegetation communities (McKee and Mendelssohn 1989, Broome et al. 1995, Williams et al. 1999). However, there is no easy way of estimation for the direction and timing of these changes (Pereira et al. 2010, Bellard et al. 2012). Over the past few decades, studies on vegetation shifts along the coasts has increased. For example, based on a field survey in Southern New England, Field et al. (2016) detected in a tidal marsh zone that vegetation cover area decreased because of sea- level rise however this decrease was balanced by landward migration by the community. Their survey 3 results showed that the tidal marsh vegetation shifts were observed over a large area. Low mortality and high growth rates were also found at the forest boundary due to shifts that did not extend into that zone. In another study, Stagg et al. (2016) observed the tidal freshwater forested wetlands at the Georgetown South Carolina and they found that oligohaline marsh capacity to recover from sea-level rise over a five years period. Tidal marshes were more resilient than tidal freshwater forested wetlands. They found that elevation loss was observed in the study area, and tidal fresh water forested wetlands were affected by subsurface process (root zone expansion and/or compaction). Another parameter affecting the salinity level in the tidal freshwater forested wetlands is river discharge. Coupled with tidal stage, these factors create a unique salinity range in tidal freshwater forested wetlands. Because of this, tidal freshwater forested wetlands are highly sensitive to river basin management, which can affect the amount and frequency of freshwater delivery. Although waters in these wetlands are usually fresh, their proximity to oceanic waters means that occasionally they are exposed to mixed saline waters. However, hydrology of the system is changed by river management practices (Ward 1998). Under normal conditions there is a negative correlation between salinity and river flow, and if the river flow from the upstream is reduced, the salinity level may increase which can damage biological communities (Copeland, 1966). Tidal forested wetlands must receive an adequate flow of freshwater in order to keep surface water salinities at or below 0.5 parts per thousand (ppt) (Cowardin et al. 1979). Changing river flow regimes related to the operation of dams within a basin may reduce flow and cause greater salinities in these freshwater tidal zones. In the Mid-Atlantic States, nearly 90 hydroelectric dams were built along the coastal plain rivers (Schneider et al. 1989). These dams are an important reason for changing hydrologic conditions and changes in the amount and frequency of peak river discharge (Livingston 2008). Tidal freshwater forested wetlands along the Apalachicola River are the focus of this study. The Apalachicola River (Figure 1. 2) is the biggest alluvial rivers in Florida (Anderson and 4 Lockaby 2012). The river is part of the Apalachicola-Chattahoochee-Flint (ACF) River basin (50 688km2; Anderson and Lockaby 2012). The Apalachicola Bay is generally shallow and its depth is normally around 2 m (Freeman et al. 2012). Jim Woodruff Lock and Dam was built in 1952 at the confluence of the Chattahoochee and Flint Rivers to create the Seminole Lake reservoir. It empties into Apalachicola River which drains to the Gulf of Mexico. The operation of the Jim Woodruff Lock and Dam plays a critical role in providing freshwater to the Apalachicola River. River management here and elsewhere in the ACF basin is a critical component of water supply for Georgia, Alabama, and Florida, as this system provides drinking water for millions of people. Discharge, as moderated by JW dam, along with tide levels govern the extent of salt water intrusion along the Apalachicola River- Bay system. Tides in Apalachicola Bay are semi-diurnals and range around 1-m in height (micro-tidal) (Doyle et al. 2007). Vegetation along the lower tidal sections of the Apalachicola River consists of a variety of marsh and forested wetland communities. In the tidal freshwater forests, several species are common. Anderson and Lockaby (2011) observed swamp tupelo (N. biflora), bald cypress (T. distichum), water tupelo (N. aquatic), cabbage palm (S. palmetto), Carolina ash (F. carolina), overcup oak (Q. nigra), and Ogeechee tupelo (N. ogeechee) along the Apalchicola River (Anderson and Lockaby 2011). These are similar to species seen in other freshwater tidal swamps. Along a similar zone on the Savannah River in Georgia, USA, Duberstein and Kitchens (2007) noted water tupelo (N. aquatic), swamp tupelo (N. biflora), water oak (Q. nigra), ash (Fr. ssp), sweetgum (Liquidambar styraciflua), red maple (Acer rubrum), bald cypress (Taxodium distichum), American hornbeam (Carpinus caroliniana). Some of these species are considered ecologically and economically significant. For instance, Ogeechee tupelo is economically valuable because Tupelo honey, one of the world's most renowned honeys, is produced from the Ogechee tupelo along the Apalachicola River and its tributaries Northwest Florida (Watson 2016). 5 The Apalachicola system is an extremely clean and productive body of water, a type of system that is uncommon in the United States (Livingston 2008). It contributes to the productivity of Apalachicola Bay which supports 90% of Florida’s and 10% of nationwide oyster harvest, along with being highly important to shrimp harvests (Huang et al. 2001). The Apalachicola River-Bay system promotes high phytoplankton productivity (Boynton et al. 1982) because the amount of freshwater is relatively high compared to others (Myres and Inverson 1981). Phytoplankton productivity is a major contributor to estuarine food webs along the Gulf coast (Livingston 2008). If the salinity of the bay and the along the Apalachicola distributaries change in the near future due to sea level rise and/or increased/reduced freshwater input all these species might become stressed. Therefore, it is vital to study the potential changes in the salinity levels in this sensitive system in response to various environmental variables.