Tidal Mudflats

Characterized by twice daily exposure to sun and tides, minimal vegetation
As the tide empties Willapa Bay, large expanses of muddy flats are exposed/Photo Courtesy of Curt Stephens

Visiting Willapa Bay at low tide is an extreme site. It appears that most of the area is nothing but a vast expanse of glistening, stinky mud. Don’t be fooled by appearances – these tidal mudflats are rich in life and support dozens of species (many that are of commercial importance, such as Dungeness crab, oysters and clams).

What Makes a Mud Flat? 

Intertidal flats are those areas of mud or sandy mud that are affected by the rising and falling of the tides. These flats are often submerged, but are gradually exposed as the tide lowers. At low tide much of Willapa Bay is drained, exposing extensive mudflats. More than 50 percent of the total high tide surface area is exposed at low tide, about 84 square miles! These mudflats tend to be very soft in many locations due to the deposition of fine sediments combined with organic matter, water saturation, and bacterial influence. The substrate characteristics vary from being sandy in the northern region of the bay to silty clay in the southern region. Typically, vegetation is scarce, but beds of eelgrass are present within Willapa Bay.

An Abundance of Life 

Intertidal flats support an abundance of invertebrates including oysters, clams, mussels, amphipods, polychaete and oligochaete worms, insect larvae, and nematodes. Foraging shorebirds follow the receding tide across the flats, and fish and waterbirds frequent the flats when they are flooded. Organisms of intertidal flats must cope with the stress of currents, varied wave action, and tides. Intertidal life is also be affected by light level, temperature change, amounts of oxygen, salinity, and exposure to air and wind.

Meadows of Eelgrass, An Important Estuary Habitat 

Native eelgrass (Zostera marina) is a seed-producing marine plant that provides food and habitat for a variety of organisms. Vast beds of eelgrass occur at the lower levels of the intertidal zone and are a staple food for brant, a sea goose that migrates through or winters in Willapa Bay. American wigeon, mallard, northern pintail, and canvasback also feed on eelgrass. Roots and stems of eelgrass assist in stabilization of mudflats. Blades of eelgrass are grazed and also support the growth of diatoms and small invertebrates that accumulate on the blades. Eelgrass also supplies detritus, dead material, which contributes to the food cycle. Eelgrass provides habitat for numerous species of mollusks and crustaceans, and serves as a nursery ground for juvenile, resident, and migrating fish. It is used for refuge and feeding by salmon species and Pacific herring. The upper edges of the intertidal flats are ringed by salt-tolerant plants called halopytes which serve as sediment traps and add much organic matter to the estuarine system.

A Major Threat 

A recent major ecological concern involved the substantial loss of high intertidal flats and native salt marsh vegetation due to invasion by Spartina alterniflora (smooth cordgrass). Spartina, a non-native cordgrass that was accidently introduced to the Willapa Bay ecosystem from the East Coast in the late 1800s, formerly covered a large portion (>14,000 acres) of Willapa Bay’s intertidal mudflats.  Spartina forms dense monotypic stands, traps sediment, and alters natural hydrologic processes. The loss of most of Willapa Bay’s intertidal mudflats and native salt marsh communities was imminent. Spartina had, and would have continued to have, a devastating effect on use of the bay by shorebirds, brant and other waterfowl species, anadromous fish, and Willapa Bay’s oyster and hardshell clam aquaculture industry. However, due to eradication efforts by Federal, State, and county agencies as well as the efforts of the oyster industry and private landowners, and additional support by Washington State University, the University of Washington, and nongovernmental organizations, including The Nature Concservancy, Spartina is now nearly absent from Willapa Bay. The major portion of the intensive eradication effort took place from 2003 through 2008. Use of areas within the bay by shorebirds and waterfowl dramatically increased after removal of Spartina from tidal mudflats. Learn more about the spartina eradication effort…

Facts About Tidal Mudflats

Smell like rotten eggs

Are packed with invertebrates and bacteria

More than 50 percent of the total high tide surface area is exposed at low tide