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Through bioassay techniques and field collections of red macroalgae (Gracilariopsis sp.) and eelgrass (Zostera marina), we evaluated growth, tissue %Nitrogen (N) and N stable isotopic signatures as bioindicators of potential changes in N supply to Tomales Bay, CA (USA). Gracilariopsis sp. collected, cultured, and outplanted across a spatial gradient in Tomales Bay showed pronounced changes in N patterns from past studies in this system, and was superior to field collections in detecting spatial N signals. Rather than a single peak in N concentration near the oceanic source found in previous work, we detected two N peaks, one near the bay head and one near the bay mouth. This spatial pattern suggests two sources account for these discrete regions of increased N supply. The temporal N patterns showed marked seasonality, with greater tissue N concentration during the wet season and reduced N concentrations during the dry season. The spatial patterns presented here suggest shifting nutrient dynamics within Tomales Bay, with increased N supply detected near the major watershed inflow. Nitrogen isotopic values suggest an enriched wastewater source, but additional work is needed to confirm the source of this newly reported N signal. [ABSTRACT FROM AUTHOR]