Saltmarshes support large populations of larval and adult invertebrates, which are consumed by the shorebirds and waterfowl that live in or visit the estuary. The Coos estuary contains less than 10% of its original saltmarsh habitat, due to fill, dredging, and other development-related disturbances. Significant portions of the saltmarshes remaining in the Coos estuary are found in South Slough. Marshes occur sporadically in the northern end of the Slough, and become larger and more frequent towards its southern reaches. All South Slough saltmarshes may experience less tidal influence than saltmarshes in the main Coos estuary waterbody, but the biological differences between the two areas remain to be studied.
South Slough contains high and low saltmarshes which have experienced varying degrees of disturbance and recovery. Plant communities of South Slough saltmarshes are typical of the region, and include tufted hairgrass (Deschampsia caespitosa), saltgrass (Distichlis spicata), pickleweed (Salicornia virginica), Baltic rush ( Juncus balticus), jaumea
( Jaumea carnosa), and Lyngby’s sedge (Carex lyngbyei). A few small marshes in the South Slough support the less common salt marsh bird’s beak (Cordylanthus maritimus) and sea lavender (Limonium californicum).
The South Slough estuary contains several low islands of a few hundred square yards, created by deposits of dredge spoils during the past hundred years. These islands appear to provide habitat for invertebrates and feeding areas for some birds, but the biota of these islands, like the biota of the more tidally-flushed Coos estuary spoil islands, remain to be studied.
Mudflats and Channels
The sediments of the Coos estuary are primarily soft and unconsolidated. They create large mudflats which are exposed for extended periods during low tides. Most of the tidelands of the South Slough estuary are composed of mud or muddy sand. The mudflats are often partially covered by mats of Vaucheria (a green alga), and typically harbor cockles (Clinocardium nuttalli), gaper (Tresus capax), bentnose, and littleneck (Protothaca staminea) clams. Ghost shrimp (Callianassa californica), mud shrimp (Upogebia pugettensis), juvenile Dungeness crab (Cancer magister) also inhabit mudflats in the estuary. The mudflats are a rich feeding ground for a number of small shorebirds, and for great blue herons, egrets, and kingfishers.
Beds of eelgrass (both native Zostera marina, and the introduced Zostera japonica) cover an estimated 100 acres of the Coos estuary bottom. In late summer, up to 160 acres of eelgrass may be found in the Reserve, with additional beds of unknown size in the northern part of South Slough. These eelgrass beds, together with deeper tidal channels in the estuary, shelter a large number of fish and invertebrates. The blades of these plants create attachment area for algae, planktonic larvae, and snails. Eelgrass also provides cover for juvenile crab, juvenile ling cod, salmonids, starry flounder, English sole, and other fish and invertebrates. The plants’ height allows development of several vertical layers of structurally diverse and productive habitat.
South Slough does not contain rocky or cobble-bottomed areas of significant size. The single known exception is at the northern end of the watershed, at the Coos estuary mouth, on intertidal land below a small cliff. This small rocky patch is occupied by a diverse community of hard-bottom and
boring invertebrates. These include barnacles, mussels, sea anemones, snails, and clams.
Sandy bottom and sand flats are very uncommon in the South Slough. Within the Reserve itself, one sand flat area occurs off the northwest corner of Valino Island, and a second narrow strip runs along part of Long Island Point’s west shore. A few coves north of the Reserve contain the only other sandy areas in the slough.