Bioavailability and export of dissolved organic matter from a tropical river during base‐ and stormflow conditions

Abstract

Concentrations, bioavailability, and export of dissolved organic matter (DOM), particulate organic matter (POM), and nutrients from the Wailuku River, Hawai'i, U.S.A., were examined under base‐ and stormflow conditions. During storms, DOM and POM concentrations increased approximately by factors of 2 and 11, respectively, whereas NO 3 − concentrations decreased by a third. The organic carbon (OC) pool was dominated by dissolved OC (DOC) during baseflow conditions, whereas DOC and particulate OC contributed equally during storms. The nitrogen (N) pool was dominated by NO 3 − during baseflow conditions and by particulate organic N (PON) during storms. Dissolved organic N (DON) comprised a similar percentage of the N pool under base‐ and stormflow conditions. DOC and DON bioavailability both decreased by half during storms in the Wailuku River. Differences in DOM concentrations and bioavailability between base‐ and stormflow conditions suggest that DOM flowpaths to the river changed from deep to shallow, and that DOM sources switched from autochthonous to allochthonous. Although DOM bioavailability decreased during storms, calculations suggest that storms contributed >90% to the annual OC and N yields and >80% to the bioavailable DOM yield from the Wailuku River to coastal waters. Overall, our results suggest that storms are important for delivering bioavailable DOM to coastal waters, where they may stimulate primary and secondary production.