Spatial Variability of Nutrient Concentrations and Stream Temperatures within the McKenzie River Basin: Abiotic and Biotic Controls

Ecosystem controls of nutrient dynamics and of stream temperature are complex, especially as we scale from headwaters to larger systems. Water temperature is influenced by biotic, climatic, hydrologic and geomorphic factors, yet the interactions of these factors contributing to spatial variation in temperature is not well understood. Similarly, how well can we predict nutrient dynamics with increasing drainage area of streams? Can nutrient concentrations be interpolated or extrapolated from known points within the stream network? Independent measures of stream temperature and stream nutrient concentrations within the McKenzie River Basin reveal the influences of flow path and hydraulic retention times as dominant factors for stream temperature and phosphorous. Stream flows originating from subsurface aquifers have distinct phosphorous and temperature signals compared to those from near surface sources. In this region, P concentrations are highly influenced by volcanic bedrock and streams with the highest P also have cold summer temperatures, suggesting that their source of water is deep aquifers, with long residence times of water. Geomorphic controls (substrate type, hyporheic flow and groundwater inputs) can have as large an impact on diurnal stream temperature dynamics as the removal of riparian vegetation. Conversely, nitrogen dynamics and dominance of DON versus DIN are less predictable. Previous experiments showed rapid transformation of ammonia to nitrate, leading to a hypothesis of increased nitrate concentrations with distance downstream at low flow. However, synoptic nutrient sampling from first- through eighth- order streams and rivers found the highest nitrate concentrations and greatest variability among first- and second-order streams. Larger streams within the H.J. Andrews Experimental Forest and downstream to the confluence of the McKenzie River with the Willamette showed less variability. Surprisingly, downstream sites had lower nitrate concentrations than upstream, even with point and non-point anthropogenic inputs. These results confirm that abiotic processes are controlling availability of P in streams, while species of N are more tightly coupled to biotic mechanisms and transformations. Synoptic nutrient sampling along the longitudinal gradient of rivers coupled with mechanistic studies at headwater sites will increase our understanding of biotic and abiotic interactions which influence nutrient retention and release from basins.