This 3-year study provides data on the spatial, seasonal and annual variability of hydrocarbons and total organic carbon present in marine sediments at three sites: Elkhorn Slough, Moss Landing Harbor and nearshore Monterey Bay in the vicinity of Moss Landing, California. The study provides baseline information that could be used to evaluate the potential impacts of future fuel oil releases occuring in the Moss Landing area. Groups of hydrocarbons were chosen to represent the hydrocarbon inputs into the Moss Landing area. These included the pesticide dichlorodiphenyltrichloroethane (DDT), polychlorinated biphenyl (PCB), phthalic acid ester (PAE), polycyclic aromatic hydrocarbon (PAH) and combustion PAHs (SigmaCOMBs). For SigmaDDTs, SigmaPCBs, SigmaPAEs, SigmaPAHs and SigmaCOMBs, the major sources of variability were between sites and random effects. Subsites within each site contributed little variability. No significant seasonal differences in any chemical contaminant group were found at any site. Significant seasonal differences in total organic carbon (TOC) and significant annual differences in SigmaPCBs, SigmaPAHs, SigmaCOMBs and SigmaPAEs were found at the nearshore Monterey Bay site. Significant annual differences in SigmaPAEs and TOC were found within Moss Landing Harbor, and significant annual differences in SigmaPAEs were found within the Elkhorn Slough site. Implications for future sampling designs in the Moss Landing area are that given the current baseline conditions (a stable, low rate of hydrocarbon input), a variability of 75-150 m(2) may not need to be heavily sampled. Spatial variability, not seasonal or annual variability, is the major source of hydrocarbon variability in Moss Landing sediments, although 3 years may not be long enough to establish long-term annual trends. Further research to determine the SigmaPAH spatial sampling scale for oil spills is needed.