Coastal Estuaries and Lagoons: The Balance Between Fresh and Salt Water

By Paul A. Conrads, Kirk D. Rodgers, Davina L. Passeri, Scott T. Prinos, Christopher Smith, Christopher M. Swarzenski and Beth A. Middleton, U.S. Geological Survey

A ghost forest — trees killed by rising salinity on the U.S. Gulf Coast. USGS.

Coastal estuaries and lagoons are defined by the mixing of fresh and salt water. Coastal communities increasingly worry about that balance, because sudden shifts can harm critical ecosystems and infrastructure — and any change in the delivery of water from either side can affect human health, wildlife and coastal structures.

The balance

Where plants and animals live in an estuary depends on where fresh and salt water meet — set largely by how much river water reaches the coast.

Salinity in an estuary is a balance between freshwater delivery and marine water levels. USGS.

From the rivers: droughts, floods, groundwater seepage, rain, evaporation and transpiration, dams and regulation, and water taken for farms, industry and public supply all change how much fresh water arrives. At any point in an estuary, salinity is lower when streamflow is medium or high. In low flow and drought, salt water pushes in and the boundary moves upstream.

On the Savannah River, Georgia: when flow at Clyo tops 6,000 cubic feet per second during the high spring tides of the new moon, specific conductance (a measure of salinity) stays low at Port Wentworth (Conrads and others, 2013). USGS.

From the sea: water levels rise and fall with daily tides; winds raise or lower them; tropical cyclones drive high salt water ashore into fresh areas. Over decades to centuries, sea-level rise plus storms can push more salt water into low-lying estuaries.

How ecosystems respond

Vegetation

Estuary plants sort themselves by salinity tolerance. Hurricanes, droughts and floods can push salinity to extremes. If such events become more frequent and intense (Knutson and others, 2010), plant communities may change dramatically — tidal freshwater marsh giving way to more salt-tolerant plants, or to open water.

• Short-term changes can be weathered.
• Long-term, plant zones may move inland as salinity limits are exceeded.
• Wetlands may turn to salt-tolerant species, or die off.
• In the Mississippi River Delta, a multiyear drought with higher salinity turned a freshwater marsh into an intermediate marsh.

Monthly salinity in intermediate, brackish and saline waters of Barataria Bay Basin, Louisiana, 2000–2008, through droughts and back-to-back hurricanes. Plants shift landward or seaward with their salinity tolerance. USGS.

Fisheries

More fresh water can help or hurt:

• Blue crab catches in the Southeast track the fresh–salt balance (Childress, 2012). In coastal drought, crabs move upstream — fewer alligators, but more deadly parasites.
• On the Suwannee River, more fresh water expanded habitat for juvenile Gulf sturgeon (Randall and Sulak, 2007).
• In Apalachicola Bay, Florida, less fresh water during drought cut oyster production (Havens and others, 2013).

What people stand to lose

Drinking water and health

• Water intakes: many municipal systems on the southeastern coast are vulnerable to saltwater intrusion and climate change (Furlow and others, 2002). Intakes often sit near the fresh–salt boundary of tidal rivers, so managers need estimates of how often, long and severe future intrusions could be — with sea-level rise or more upstream withdrawals. Groundwater can be affected too (Prinos and others, 2014).
• Toxic algae: a saltier estuary can favor different algae, some of which make toxins — bringing low oxygen, fish kills, shellfish poisoning in people, and lost recreation.
• Metals: mercury, arsenic and lead become more available when salinity is low. As salinity rises, metals can detach from clay or turn into more toxic forms, such as methylmercury, and bioaccumulate up the food chain — dangerous if people eat contaminated fish.

Safety and security

Saltwater intrusion near critical infrastructure can threaten public safety and national security. Coastal communities can run short of fresh water if there's no alternative. Industrial intakes in tidal freshwater have very low salt tolerance because seawater corrodes machinery, especially boilers — and the same risk can disrupt military bases and nuclear power plants.

Storm protection

Coastal vegetation absorbs destructive wave energy. U.S. wetlands provide about $23 billion a year in storm-surge protection — protection lost if they are damaged (Costanza and others, 2008).

Navigation

Deepening shipping channels for bigger cargo ships can push the fresh–salt boundary upstream, exposing water intakes to salt and worsening intrusion events.

Tourism and recreation

Estuaries draw hikers, anglers, bird watchers, canoeists and hunters. They cover just 12.6 percent of the continental United States, but account for 49 percent of the nation's economic output (Pendleton, 2011). In 2011, more than 46 million recreational anglers generated over $48 billion in retail sales (Southwick Associates, 2013).

What the USGS offers

The USGS helps decision makers understand salinity and other forces on coastal habitats with monitoring data, biological data, scientific reports, data releases, scientific support and analysis tools for estuaries and lagoons.

Sources

Based on Coastal Estuaries and Lagoons: The Delicate Balance at the Edge of the Sea, by Paul A. Conrads, Kirk D. Rodgers, Davina L. Passeri, Scott T. Prinos, Christopher Smith, Christopher M. Swarzenski and Beth A. Middleton, USGS Fact Sheet 2018–3022, U.S. Geological Survey; a work of the United States government in the public domain. The four figures are taken from the fact sheet's PDF, which the import had left out.