West Cook Inlet's Salt Marshes: Where Tides, Bears and a Raised Coast Meet

Saltwater meets freshwater as the tides carry nutrients and sediments into the streams and salt marshes. Photo: NPS/D. Liles.

Salt marshes occupy less than one percent of the total land area of Lake Clark National Park and Preserve, yet the National Park Service calls them vital to the park's ecosystem. They are a big part of why the coast between Chinitna and Tuxedni Bays is such productive habitat, and why coastal brown bears are dense there.

Tides do the work

Cook Inlet is an extremely dynamic estuary dominated by the tidal cycle. Normal tidal heights range from 18 feet (5.5 m) at Kachemak Bay to 29 feet (8.8 m) at Anchorage, with extreme tides over 36 feet (11 m), some of the largest tidal ranges in the world. NPS compares each tide to a basin filling and draining, carrying and depositing nutrients. The strong currents bring organisms and sediments into the inlet, and circulate nutrients and oxygen. They also carry waste from human activities such as logging, prospecting and mining, and road building. The Dena’ina name for Cook Inlet is Tikahtnu (in the Outer Inlet dialect), meaning “Ocean River.”

As tides flood the land, streams rise and tidal channels form. When the tide turns, nutrients settle among the plants. The mix of marine and land nutrients creates highly productive, adaptable salt marshes, which support coastal food webs.

The mixing of saltwater and freshwater carries nutrients to meadows that support many animals. Photo: NPS/A. Jones.

A biological supermarket

Marshes sit between fresh and salt water, so the water is brackish and rises and falls with the tide. Salt-tolerant sedges and grasses grow there, supporting intertidal invertebrates, fish, migratory birds and mammals. Salt marshes are among the most productive ecosystems in the world, serving as food sources, nurseries for fish, shelter from predators and nesting and breeding grounds. They absorb nutrients before they reach the sea and give back great volumes of food: dead leaves and stems break down into small particles that feed small aquatic insects, shellfish and small fish, which feed larger fish, birds and mammals.

Bears depend on this in spring. During hibernation their protein reserves run down and they lose a significant amount of fat. The coast gives brown bears protein-rich sedges such as Carex ramenskii and other salt-tolerant plants, which they need to survive and feed young cubs, until the salmon arrive.

Wetlands give bears protein until fish arrive. Photo: NPS/A. Jones.

What marshes do for the coast

• They limit erosion, by buffering waves and trapping sediment against wind, water and ice.
• They filter water. Marsh grasses and peat, a spongy mass of live roots, decaying material and soil, filter pollutants such as heavy metals and waste, sediments and nutrients.
• They store carbon. Coastal wetlands keep carbon in their plants and soil rather than releasing it as carbon dioxide, which helps moderate global climate at an early stage of the carbon cycle.

An uncertain future

Coastal wetlands can move inland as the sea rises, or be submerged and lost. In the U.S., sea level is rising a few inches to a foot or more per century, outpacing marsh growth. Cook Inlet's west side is a special case: the 1964 Alaska earthquake raised its coastline 3 feet (1 m) relative to sea level, which brought rapid changes, including trees and shrubs moving in.

Aerial images showing increased vegetation growth in response to the coastal uplift after the 1964 earthquake. Images: NPS/SWAN.

Sources

• National Park Service, Lake Clark National Park and Preserve, West Cook Inlet Ecology; rewritten in hubnx's own words.