These habitats occur in backwaters or on deltas away from large distributary channels. They consist of mixed sand and mud with small amounts of gravel or with some clay and peat. The substratum is generally stable, firm, and organic-rich. Productivity is high due to eelgrass, micro- and macro-algae, and salt marsh vegetation. Drift algae and seagrass may be abundant seasonally. Areas with gravel are characterized by an abundance of hard-shelled clams. Detritivores in the sediment are very dense, and are preyed upon by other invertebrates as well as by numerous birds and fishes. The amphipod Corophium provides a major food resource for numerous fish and shorebirds. Because of the presence of eelgrass (which reaches its highest densities in muddy sand) and marsh grasses, these habitats are used by a variety of birds: great blue herons, mergansers, western grebes, and brant. Areas without eelgrass are much less diverse, although crows, gulls, killdeer, great blue herons, mallards, and pintails forage in muddy sand. Raccoons, deer, skunks, and weasels forage on these shores
There are four marsh communities which occur in the habitat: (1) Carex lyngbyei-Distichlis spicata co-dominated community; (2) Distichlis spicata-Salicornia virginica-Triglochin maritimum co-dominated community; (3) Jaumea carnosa-Salicornia virginica-Triglochin maritimum co-dominated community; and (4) Salicornia virginica-Triglochin maritimum co-dominated community. In order these four communities occur from higher to lower in the low marsh zone with the fourth colonizing tide flats.
Diagnostic benthic species in lower zones are Zostera marina, Protothaca staminea, Saxidomus giganteus, and Callianassa californica.
Common Associates: Marsh
These communities tend to be species rich for being low marsh. Glaux maritima, Stellaria humifusa, Puccinellia spp., Spergularia spp. Carex lyngbyei, Triglochin concinnum, Plantago maritima, and Atriplex patula are common vascular plant associates.
Common Associates: Benthos
Leafy green ulvoid algae are characteristic on the surface in spring and summer, and a variety of small algae (Smithora, Ceramium, Chondria) are epiphytic on Zostera. The introduced species Zostera japonica is found in some high intertidal areas. Infauna include the clams Mya arenaria, Macoma secta, Clinocardium nuttallii, Protothaca tenerrima, Tresus spp., and Cryptomya californica; the introduced oyster Crassostrea gigas; the crabs Cancer magister, Hemigrapsus oregonensis, and Pugettia producta; the moon snail Polinices lewisii and the sea slug Haminoea virescens; the polychaetes Abarenicola pacifica, Manayunkia aestuarina, Pygospio elegans, Polydora kempi japonica, Hobsonia florida, Euzonus williamsi, Nereis limnicola,
Nephtys spp., Eteone longa; the crustaceans Corophium salmonis, Eogammarus confervicolus, Leptochelia savignyi, and Upogebia pugettensis; and ophiuroids. Coastal estuaries also have high densities of Streblospio benedicti and may have mats of the algae Vaucheria longicaulis, Rhizoclonium riparium, and Polysiphonia hendryi. Some areas of the uncommon enteropneust Saccoglossus sp. are found in Willapa Bay. Numerous, diverse fish are found in these habitats, especially at high tide: Pacific herring, Pacific sand lance, tube-snout, juvenile English sole, starry flounder, sculpins (tidepool, rosylip, sharpnose, Pacific staghorn, great), three-spined stickleback, bay pipefish, snake prickleback, saddleback gunnel, crescent gunnel, surf smelt, shiner perch, and juvenile chum and chinook salmon. It has been said that the eelgrass fish fauna is the “richest, most abundant pelagic fish fauna of any habitat sampled” in the Puget Sound region. In addition, coonstripe and broken-back shrimp can be found in channels and pools.
MARSHES — Dosewallips River Delta, Skokomish River Delta, North Bay, Oakland Bay,
Chapman Cover, Nisqually River Delta, Kennedy Creek, Skookum Inlet, Tokeland marsh, Long Island, Seal Slough, Niawiakum River, Bone River, Elk River Estuary, Westport marsh, Markham Island, North Bay.
BENTHOS — Kiket Island, Skagit Flats, Ellisport (Vashon), Anderson Cove and other sites in Hood Canal, Bellingham Bay, Drayton Harbor, Padilla Bay, inner Dungeness Spit, Grays Harbor and Willapa Bay.
MARSHES — Kunze, 1984; Kunze and Cornelius, 1982; Frenkel, Boss and Schuller, 1978; Jefferson, 1975.
BENTHOS — Houghton, 1973; Thom et al., 1984; Eckman, 1979; Gallagher et al., 1983; Chew, 1970; Webber, 1978; Schneider and Dube, 1969; J.E. Smith, 1980; Thom, 1981; Albright, 1982; Hedgpeth and Obrebski, 1981; Albright and Borithilette, 1982; Thom et al., 1988; Thom, 1988; Harman and Serwold, 1978; Mumford and Shaffer unpubl. data; C. Simenstad unpubl. data.