In calm bays and inlets where there is little to no wave or current energy, fine sediments settle and form mudflats.  Often such areas have seasonal freshwater inflow from streams, but salinities are not regularly below 30 ppt. Such habitats support a rich infauna and are used extensively by birds as foraging areas; these habitats and their estuarine counterparts support the highest densities (seasonally) of marine birds in the region. These include great blue herons, gulls, terns, turnstones, dunlin, sandpipers, grebes, and ducks. Areas with eelgrass are used by black brant and wigeon, and to a lesser extent by Canada geese, pintails, and greater scaup.  Seals sometimes haul out on undisturbed mudflats.  Drift algae and seagrass may accumulate.

Diagnostic Species

Zostera marina, the clam Macoma nasuta (especially in areas with freshwater inflow), the polychaete Polydora (Pseudopolydora) kempi japonica, the mud shrimp Upogebia pugettensis.

Common Associates

The clams Mya arenaria, Transennella tantilla, Macoma inquinata, M. balthica, and Clinocardium; the polychaetes Abarenicola pacifica, Thelepus crispus, Lumbrineris zonata, Pygospio elegans, Notomastus tenuis, Capitella capitata, Chaetozone sp., Streblospio benedicti, Eteone longa, and Glycinde picta; the crabs Cancer magister, C.gracilis, and C. oregonensis; and the opisthobranchs Melanochlamys diomedea and Haminoea spp. Starry flounder, juvenile English sole, tube-snout, shiner perch, bay pipefish, saddleback gunnel, bay goby, and sculpins (Pacific staghorn, silverspotted, sharpnose, tidepool, padded) are characteristic species that use mud flats for feeding and as nursery areas. Herring often spawn on Zostera.

Sites Surveyed

Westcott Bay, Garrison Bay, and other sites in the San Juan Islands, and Jamestown/ Port Williams.

Sources

Shelford et al., 1935; Thorson, 1957; Nyblade, 1979a; Long, 1983.