Nitrogen & Phosphorus Dynamics below Glen Canyon Dam

Erin F.E. Lennon

Abstract: Nitrogen (N) and phosphorus (P) dynamics in the Colorado River below Glen Canyon Dam have important implications for primary productivity, aquatic food webs, and overall water quality. Typical phytoplankton N:P is 16 (Redfield, 1958), but this value varies with algal adaptations and riverine features. My literature and data syntheses work to understand historical and current factors influencing Colorado River N:P. USGS data from 1979-2013 reveals years of very high peak N:P events at Lees Ferry and Diamond Creek: 1990-1991 (500 and 200), 1999 (127 and 72), 2006 (no data and 180), and 2012 (197 and no data). These high N:P events are likely due to dam operations influencing sediment-bound phosphate. I suggest that P influences have been particularly under-studied in this system. As climate data and dam management point toward altered flows, N:P ratios need to be more actively included in water quality evaluations.