Evaluating Patterns of Nitrogen Supply Using Macroalgal Tissue Content and Stable Isotopic Signatures in Tomales Bay, CA

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 To...