Ground truthing national multiscale landscape indices with nitrogen-stable isotopes for low-gradient coastal stream...

Low-gradient coastal stream systems support important landscape level ecological functions by connecting uplands and marshes directly to large tidal rivers, estuaries, and coastal waterbodies. In this study, we develop and evaluate innovative monitoring and assessment methods to support biological indicator development for waterbody types and taxa that lack nationally consistent and reliable approaches. Low-gradient tidal and non-tidal coastal stream systems are infrequently included in national and regional monitoring programs and may require different methods than freshwater streams (US EPA National Rivers and Streams Assessment) or large tidal rivers (US EPA National Coastal Condition Assessment). Results from this study demonstrate that stable isotopes of nitrogen (δ¹⁵N) bioindicators from a variety of biotic trophic levels can be used as an efficient and effective rapid monitoring tool for screening level assessment of biotic condition in low-gradient coastal systems. These stable isotope indicators also provide empirical field measurements for ground-truthing national indices which are derived from remotely sensed national landscape data layers. The δ¹⁵N biotic responses were consistently significant across all trophic levels and negatively correlated with landscape level stressor exposure conditions reflected in the national and regional scale indices. Verifying national multiscale indices with site-scale field measured bioindicators (δ¹⁵N) provides coastal managers, states, tribal, regional, and local watershed organizations confidence in using these national indices at local watershed and catchment scales for identifying and prioritizing protection of healthy coastal stream networks and watersheds, as well as targeting critical functional elements of watersheds for restoration efforts.