Effects of experimental nutrient loading on phosphorus uptake by biofilms: evidence for nutrient saturation in mid-Atlantic streams

Biofilms generally account for the majority of P assimilation in streams. However, relatively few direct measurements have been made that link nutrient loadings to biofilm assimilation. We measured P uptake rates for natural benthic biofilms sampled from 8 streams situated along a productivity gradient in the mid-Atlantic region, where point-source nutrient loadings were applied with an in situ enrichment system (ISES). ISES consisted of vials spiked with increasing concentrations of P with and without N. Short-term radiotracer ((H3PO4)-P-33) experiments were conducted to measure P uptake by biofilms growing on the surface of the vials collected from the streams. The relationship between P uptake and P loading in each stream was explained using simple linear regression. In all streams, P uptake rates declined with increased experimental P loading and as a function of stream productivity. Biofilms had significantly higher uptake rates overall in low-vs high-productivity streams (1.29 and 0.84 log(10)[nmol P mu g(-1) chlorophyll d(-1)] respectively) (F-1,F-30 = 6.21, p = 0.018). These results indicated that assemblages in low-productivity streams were physiologically adapted to use new P additions, whereas biofilms in productive streams showed signs of P saturation such that these assemblages had reduced demand for new P loadings. N loading synergistically affected biofilm P uptake ability in high-productivity streams, where N appeared to have been a secondarily limiting nutrient. Our work emphasizes that P uptake was heterogeneous across landscapes. P-saturation was common in high-productivity streams (embedded in agriculturally dominated watersheds) and was mediated by N-enrichment and nutrient stoichiometry.