The benthos of a portion of the Sacramento River (San Francisco Bay Estuary) during a dry year

Early in 1976 benthic studies were initiated in a 20 kilometer long portion of the Western Sacramento-San Joaquin River Estuary. Water quality determinations indicated little vertical or horizontal differences in pH, temperature, or dissolved oxygen concentration within the study area. Low river outflows allowed the encroachment of seawater into the study area, an area normally exposed to fresh or slightly brackish water. The sediment composition changed dramatically at most stations during the year, being dominated by sands early in the year but by silts and clays in late summer. The shift in sediment composition was accompanied by an increase in grease and oilland metals content. The benthic community of the study area was generally dominated by the Asiatic clam (Corbicula manilensis), Macoma balthica, oligochaetes, the amphipods Corophium stimpsoni and C. spinicorne, nematodes, and a spionid polychaete, Boccardia ligerica. These taxa comprised 98% on average of the total benthic macroinvertebrates collected at each study site. The benthic assemblages of each of the stations were generally very similar to one another. Faunal similarities and changes in benthos composition were related to substrate composition and salinity incursion. In general, the upstream-channel stations had higher abundance of benthos than the other stations in the study area. Total benthic abundance was lowest at the downstream end of the study area. Total standing crop peaked in June and was lowest in November. Our studies indicate that the most important factors controlling the size and species composition of the benthos of the study area are salinity and sediment composition.