Identifying nitrogen sources to thermal tide pools in Kapoho, Hawai'i, U.S.A, using a multi-stable isotope approach

Nitrogen (N) enrichment often results in coastal eutrophication, even in remote areas like Hawai'i. Therefore, determining N sources to coastal waters is important for their management. This study identified N sources to tide pools in Kapoho, Hawai'i, and determined their relative importance using three stable isotopes (δ¹⁵N, δ¹⁸O, δ¹¹B). Surface waters and macroalgal tissues were collected along 100-m onshore-offshore transects in areas of high groundwater input for three months at low tide. Water samples from possible N sources were also collected. Mixing model output, along with macroalgal δ¹⁵N values, indicated that agriculture soil (34%) was the largest anthropogenic N source followed by sewage (27%). These findings suggest that more effective fertilizer application techniques and upgrading sewage treatment systems can minimize N leaching into groundwater. Overall, our multi-stable isotope approach for identifying N sources was successful and may be useful in other coastal waters. - Highlights: • Multi-stable isotope approach successfully identified N sources to tide pools. • NO3-isotopes identified agriculture and groundwater as key N sources, not sewage. • δ¹⁵N indicated tide pool algae used N from agricultural soils. • Mixing model apportioned contributions of N sources to tide pools. • Agricultural soils were largest anthropogenic N source to tide pools.