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Ecosystem scale nitrification and watershed support of tidal creek productivity revealed using whole system isotope tracer labeling

Tidal creeks are common coastal features of low‐slope continental margins that connect nitrogen (N)‐replete watersheds to N‐limited coastal oceans. We conducted a 3‐week whole tidal creek 15 NO3− tracer labeling experiment to quantify creek‐scale NO3− inputs and transformations and to evaluate ecosystem dependence on watershed NO3− delivery. Isotopic 15N enrichments of creek NO3− were hundreds to thousands of per mil depending on tidal stage and watershed discharge. We detected tracer movement into microalgae, macrophytes, zooplankton, epifauna, water column ammonium, and bulk sediments throughout the tidal prism. Tracer enrichment of dissolved N2 was detected along the study reach and permitted estimation of denitrification at the whole‐system scale. All data were used to build a NO3− isotope budget to quantify whole system nitrification and to calibrate a first order biota tracer model that yielded species‐specific turnover times and estimates of how much N demand was ultimately derived from the labeled creek 15 NO3− . Combining results from the NO3− isotope budget and the biota tracer model supported the following conclusions: (1) Under base flow conditions, the watershed flux of NO3− was small compared to the amount of NO3− produced within the creek via nitrification; (2) Approximately half of all NO3− inputs to the creek were attenuated within the creek principally by biological assimilation prior to downstream transport; (3) Biological uptake reduced watershed derived NO3− loads by ∼ 50%, but the majority of their N requirement was generated within the creek.