Spatial segregation of periphyton communities in a desert stream: causes and consequences for N cycling

The goal of this research was to understand the underlying causes of patterns of distribution and abundance of N-fixing periphyton and non N-fixing periphyton in a desert stream, and to evaluate potential consequences of these patterns for stream N cycling. Sampling of periphyton communities at sandbar edges in Sycamore Creek, Arizona, revealed community differences related to direction of hydrologic exchange with the subsurface. N-fixing cyanobacteria were abundant at sandbar inwelling edges. Non N-fixing taxa predominated at outwelling edges. We evaluated several predictions of the hypothesis that this difference was attributable to differences in N availability at sandbar edges. NO3-N at outwelling edges was 3.5 times higher than at inwelling edges, consistent with past work showing microbial nitrification to characterize sandbars. Non N-fixing and N-fixing periphyton biovolume were related to NO3-N concentration at edges. Reciprocal transplants of periphyton from original edges to edges with the opposite direction of hydrologic exchange revealed significant declines in periphyton relative to controls. Experiments using NO3-N diffusing substrates revealed non N-fixing taxa to be more abundant where NO3-N was elevated. N-fixing periphyton was, in contrast, more abundant at naturally low NO3-N or unenriched sites. These results support the hypothesis that differences in N availability between inwelling and outwelling edges caused the observed pattern of periphyton distribution at sandbar edges. Last, 80% of outwelling NO3-N was removed from outflowing water by periphyton at sandbar edges. This temporary retention of available N results in a surface stream depleted of inorganic N and accentuates spatial heterogeneity of periphyton communities in this ecosystem.