Fluvial nitrogen and phosphorus in Hawaii: Storm runoff, land use, and impacts on coastal waters

In Hawaii, small, steep watersheds and highly variable rainfall result in episodic, but intense, runoff events. Stream discharges consist of a small but steady baseflow component, and a transient, sometimes large, storm runoff component. Baseflow and storm runoff discharges vary between sites, and storm runoff can vary widely from year to year. The timing, magnitude, and form (dissolved vs. particulate, speciation) of fluvial nutrient (N and P) fluxes depend on the magnitudes of baseflow and storm runoff discharges, and on the characteristic nutrient forms and concentrations in each flow component. Samples from Oahu study sites showed that baseflow and storm runoff quality differ at many sites, and that land use (conservation, urban, and agricultural) affects dissolved nutrient concentrations. Particulate inorganic phosphorus was elevated strongly in storm runoff from urban sites, and particulate organic nitrogen and carbon were reduced at urban and agricultural sites. Annual baseflow fluxes of dissolved nutrients were comparable to or greater than storm runoff fluxes, while particulate fluxes were associated almost exclusively with storm runoff. Comparison to global river and stream data indicate that Hawaiian watersheds export water, sediment, and nutrients at rates similar to, or lower than, global averages based on watershed size and runoff.; The impacts of fluvial nutrients were evaluated in southern Kaneohe Bay on Oahu. Nutrient budgets demonstrate that streams contribute roughly half of the reactive nitrogen and the majority of the reactive phosphorus to the Bay, mostly via storm fluxes of particulate organic N and P. Present-day dissolved inorganic nutrient fluxes are elevated compared to pristine conditions, but analysis suggests that ecosystem-scale impacts probably are minor. Obvious runoff impacts were restricted to the region around stream mouths due to the unusually low runoff characterizing the study period, but historical data show that storm events can have significant impacts on water quality and biota in the Bay. Extrapolation of study results to other sites in Hawaii indicates that impacts normally should be minor, but subtle impacts on ecosystem structure may be present, and fluvial sediments probably are an important source of N and P to some coastal ecosystems. Nutrient loading analysis in coastal ecosystems should consider both dissolved inputs and recycling of particulate nutrients delivered with terrestrial sediment.