Macroalgal tissue nutrients coupled with bacterial assays suggest persistent highly derived nitrogen inputs in an...

Two common metrics for assessing water quality in coastal systems are the abundances of nutrients, such as NH₄⁺ and NO₃^-, and bacteria. Elevated levels of these contaminants can cause ecological harm, including harmful algal blooms and waterborne illnesses, but may be highly variable. This study utilized monthly field collections from September 2023 through September 2024 of the macroalgae Ulva spp. and Gracilaria tikvahiae from three field sites in Charleston Harbor, SC, USA, to assess tissue nutrients and bacterial dynamics in comparison to discrete in situ water samples. Macroalgal tissue δ¹⁵N ranged from 6.1‰ to 17.4‰, and tissue %N ranged 2.2% to 4.9%. These values, coupled with coliform bacteria averaging 3346 CFUs/100 mL, suggested the persistent introduction of highly derived nitrogen. Additionally, presumptive Vibrio spp. from macroalgal surfaces averaged 12,183 CFUs/100 mL, exceeding the average in water samples by 627%. These data suggest persistent highly derived nitrogen enrichment and presumptive bacterial contamination in Charleston Harbor, although direct measurements of wastewater tracers, human-specific fecal markers, pathogenic Vibrio genes, or seafood/water exposure risk are necessary to determine the nitrogen sources and to better assess any health risks associated with the Vibrio spp. population in Charleston Harbor. Bacterial assays coupled with macroalgal tissue nutrient and stable isotope analyses provide a useful multi-level approach to establish a baseline of the general abundance of dissolved N in a system (via tissue %N) and the potential trophic origin of the N (via tissue δ¹⁵N and coliform bacteria abundance) in an urbanized estuary.