Issues Brief on Saltwater Intrusion in the Florida Everglades

Introduction

The Florida Everglades, one of the world’s largest subtropical wetlands, are home to incredible biodiversity, including a large number of endemic species. The ecosystem services provided by the Everglades benefit millions of people, both locally and globally. According to the National Park Foundation (n.d.), the 1.5-million-acre Everglades National Park has over one million visitors each year, many of whom visit from other parts of the world. The Everglades’ natural beauty and opportunities for recreation (such as fishing and boating) help to bolster the region’s economy by drawing tourists to the area. This wetland also supplies a great deal of freshwater to the inhabitants of Florida, from drinking water to the water used in agriculture and manufacturing. Additionally, fresh water from the Everglades replenishes the Biscayne Aquifer, an underground layer of porous rock that stores and transports freshwater. This aquifer is a critical water supplier to millions of residents in southern Florida (South Florida Water Management District, n.d.).

The Florida Everglades have long been under threat from human activities, such as development, poor water management practices, pollution, and the introduction of nonnative species. Now, a new threat is emerging in the form of saltwater intrusion.

Transition Zones and Saltwater Intrusion

Florida, with its flat land expanse, low elevation, and coastal location, is a place of dynamic interaction between freshwater and saltwater. Inland, freshwater from rainfall, rivers, and underground aquifers flows through the Everglades to the coast. Along the coast, saltwater from the ocean moves inland through estuaries and marshes. When these two forces meet, they create transition zones, or zones of dispersion, sites where freshwater and saltwater mix (Barlow, 2003). Under normal conditions, this phenomenon creates a delicate salinity gradient that is crucial for wetland ecosystems, from inland freshwater marshes to the more seaward brackish marshes and mangroves.

Saltwater intrusion occurs when saltwater moving inland from the sea overcomes the opposing force of freshwater and is able to advance into freshwater ecosystems and underground aquifers. The resulting hypersalinity constitutes a deviation from the conditions that normally support habitats in this region. Consequently, wetland ecosystems all along the southern coastal United States are in danger of being converted into grassy saltwater marshes and might eventually be overtaken by the ocean entirely (USDA, n.d.). Because the evolutionary mechanisms of flora and fauna are not equipped to match the rapid pace at which this change is expected to occur, this regions ecosystems could be negatively impacted.

Key Issues

Sea Level Rise

One of the many unique characteristics of the Florida Everglades is its connection to the ocean and the natural water exchange that occurs throughout the area. Under normal conditions, the dynamics between these two forces create a push and pull that is beneficial for wetland ecosystems. However, a deviation from normal conditions has occurred in the form of climate change, and sea levels are now projected to rise 77 cm (by conservative estimates) and 161 cm (by more dire estimates) in the Southern Florida region by the year 2100 (Park, Stabenau, & Kaoutun, 2017). This is anticipated to result in the inundation of important areas of the Everglades, such as Buttonwood Ridge, which delineates many of the area’s freshwater basins (Park, Stabenau, & Kaoutun, 2017). This sea level rise is also expected to cause increased expansion of Florida Bay into the Everglades freshwater ecosystems, resulting in higher wetland salinity.

Hydrological Alterations

Southern and central Florida have a long history of natural flooding that is necessary for distribution of water to surrounding areas. Increased rainfall during the summer and fall would cause lakes and rivers to overflow and result in flooding that moved across the land, infiltrating the Biscayne Aquifer until it reached Florida Bay (Gramling, 2018). This flooding made agriculture a difficult and risky endeavor for farmers. In response, a system of infrastructure was constructed in the mid-20th century to prevent flooding, including a series of dams and levees to control the flow of water and a number of large reservoirs intended to store water and release it during times of drought (Gramling, 2018). Although this development allowed for increased agricultural activity in the area, it created problems for ecosystems. Notably, the amount of freshwater that typically enters the wetlands (via both above-ground channels and below-ground aquifers) provides an important counterpressure to the saltwater that is also trying to expand inland. This counterpressure establishes the location of the freshwater — saltwater transition zone. The reduction of freshwater entering the Everglades has already caused this transition zone to move inland (Gramling, 2018).

Storm Surges

Increased frequency and severity of coastal storms associated with climate change might result in higher and more frequent storm surges. Storm surges are caused primarily by the wind generated by a storm, which forces water inland (NOAA, n.d.). This results in higher coastal seawater levels than would be typical, which can periodically exacerbate saltwater intrusion into wetlands.

Areas of Impact

Habitat Changes and Biodiversity Loss

As saltwater continues to advance inland, freshwater habitats recede and make way for ecosystem features more suited to higher salinity levels. The anticipated movement of mangroves and brackish marshes inland would fundamentally alter the foundational aspects of the marsh ecosystems that define the Florida Everglades.

Wilson et al. (2018) found that during season dry-downs, pulses of increased salinity resulted in dramatic reductions in the root biomass of freshwater marsh plant species, including sawgrass. The negative implications of this finding are significant. As freshwater plant species die back, salttolerant species could proliferate, leading to a decrease in freshwater plant diversity and an increasingly homogenized ecosystem. Additionally, the loss of root biomass might also result in a loss of soil stability. Wilson et al. (2018) observed that as the freshwater root systems that provide wetland surfaces with structure and stability died, wetland surfaces sank. This phenomenon threatens to drown wetland ecosystems and convert them into areas of open water (Wilson et al., 2018).

Besides providing stability and structure to the above-water habitats of the Florida Everglades, the root systems of freshwater plants are also habitats in and of themselves. Field surveys in the Everglades found that as salinity increases, populations of freshwater prey fish decreased while saltwater prey fish populations increased (Romañach et al., 2019). This shift could have cascading effects throughout the food web, jeopardizing the populations of wading birds and other highertrophic- level species that depend on freshwater prey fish. While more research is needed to explore how saltwater intrusion contributes to habitat loss and declining biodiversity, current research suggests that increases in salinity may degrade freshwater habitats and might eventually lead to widespread reductions in biodiversity across the Everglades.

Peat Soil Collapse

Peat soil is an important feature of wetland ecosystems and provides rooting material and nutrients to the plants that live there. When dry peat soil is exposed to saltwater, it breaks down, resulting in the deterioration of the very foundation of these ecosystems, and the death of the plants that depend on it (Davis, n.d.). This soil collapse also puts wetlands at greater risk of saltwater intrusion by destroying some of the important physical barriers that separate freshwater from saltwater. The peat soil in wetlands also plays an important role in carbon sequestration, and as they break down, carbon is released from the soil back into the atmosphere (USDA, n.d.). In this way, peat collapse contributes to climate change.

Diminished Water Quality

Saltwater can intrude into groundwater and aquifers, contaminating freshwater supplies that humans rely on for agriculture and drinking water. This intrusion could lead to water insecurity and put the food supply from agricultural practices throughout Florida at risk.

Proposed Solutions

Climate Change Mitigation

The most effective way to combat sea-level rise is to prevent the melting of glacial ice in the first place. This necessitates turning our attention to the ongoing battle against climate change and the consumption of fossil fuels. It is crucial that we minimize our reliance on energy sources that produce carbon emissions in favor of more sustainable energy practices.

Infrastructure for Rising Seas

Because meaningful action toward climate change solutions is difficult to implement, and tends to be bogged down in politics, it may be beneficial explore for more readily available solutions to rising sea levels in the form of infrastructure construction. Seawalls might help to prevent seawater intrusion from swallowing up wetlands, both in the form of rising sea levels and storm surges. There are, however, two problems with this approach: seawalls large enough to encompass the Everglades would be extremely expensive (in funds, resources, labor, and time), and the presence of these seawalls might exacerbate flooding at other locations (Meadows, 2021).

Restoring Natural Flow Regimes

To restore balance to the forces within the saltwater—freshwater transition zone, more freshwater needs to be allowed to flow through wetlands. This could require statewide changes in water use and management, as well as modifications to the infrastructure that collects and stores freshwater.

Sydney Puett graduated with a bachelor's in Biology.