Uptake of nutrients and organic C in streams in New York City drinking-water-supply watersheds

Uptake of nutrients and organic C was measured once annually between 2000 and 2002 in each of 10 streams within the water-supply source areas for New York City. Nutrients (PO43– and NH4+) and organic C (glucose and arabinose) were injected into the streams for 1 to 2 h, and uptake lengths were estimated from the longitudinal declines in downstream concentration relative to that of a conservative tracer. Uptake lengths increased with stream size and were converted to uptake velocities, Vf, to remove scaling effects. Vf s of PO43– and NH4+ varied inversely with the ambient concentration of total dissolved P (TDP) and total dissolved N (TDN), respectively, and were described by a model based on Michaelis–Menten kinetics. However, Vf s of glucose and arabinose were unrelated to the concentrations of any solute. Vf s of PO43–, NH4+, arabinose, and (with less certainty) glucose varied positively with measures of ecosystem metabolism (24-h community respiration and gross primary productivity). Uptake flux (U) of NH4+ also varied positively with ecosystem metabolism, but Us of PO43–, glucose, and arabinose did not. The Vf s of PO43– and NH4+ were positively related to invertebrate species richness and % forest cover, and negatively related to molecular tracer concentrations (polyaromatic hydrocarbons and fecal steroids [PO43–-Vf], fragrance materials [NH4+-Vf]) and population density. Spiraling, as a measure of ecosystem function, was sensitive to human impacts, most clearly through responses to nutrient loadings, but very probably through responses to other impacts as well.