This dataset is a compilation of multiple studies over different periods and in many environments as described below. These data were collected to measure temporal fluctuations in plant isotopic composition associated with seasonal cycles of growth and environmental variability. In 1990, 1992, and 1998-2000, a total of 868 samples were collected and analyzed to determine carbon and nitrogen mass and 13C:12C and 15N:14N ratios by mass spectrometry. Intensive sampling was done in 1999 when species of vascular plants common to wetlands of northern California were sampled biweekly to monthly for one year along a gradient of habitats (salt-marsh creek, freshwater brackish marsh, seasonal wetland) within the San Francisco Bay National Wildlife Refuge. Additional spatial variability was measured from seasonal sampling in the freshwater-brackish marshes, river channels, tidal lakes, and riparian corridors of the Sacramento-San Joaquin River Delta and from salt marshes and mudflats of the other marine embayments of San Francisco Bay (see attached map for sample locations). Target plant communities included terrestrial woody perennials of riparian habitat, emergent vascular plants of freshwater and brackish marshes, floating and submerged vascular plants, multicellular freshwater algae, benthic diatoms from intertidal mudflats, and C3 and C4 salt-marsh plants. Seston samples were collected in 1990, 1992 and 1999-2000 during algal blooms in river/lake habitats and San Francisco Bay to provide surrogate measures of the isotopic signatures of freshwater and estuarine-marine phytoplankton. The taxonomic identifications are based on 1994 naming conventions. Cloern, J. E., Canuel, E. A., & Harris, D. (2002). Stable carbon and nitrogen isotope composition of aquatic and terrestrial plants of the San Francisco Bay estuarine system. Limnology and oceanography, 47(3), 713-729.