The short term temporal and spatial variability of nitrogen and phosphorus in two Oregon Coast Range streams

Graduation date: 1996High intensity sampling was undertaken to characterize the temporal and spatial\ud variability of oxidized nitrogen (NO3-N + NO2-N), ammonia-nitrogen (NH3-N), total\ud dissolved phosphorus, total unfiltered phosphorus and orthophosphorus (PO4-P) from two\ud adjacent small streams in Western Oregon's Coast Range, Deer Creek (303 ha) and Flynn\ud Creek (203 ha). Deer Creek has been 39 % clearcut from 1966 to 1987 while, Flynn\ud Creek has never been logged and remains a "control" watershed for various research\ud projects. A sequential wet-deposition precipitation sampler was also used to determine\ud the temporal variability of oxidized-nitrogen, ammonia-nitrogen, total dissolved\ud phosphorus and orthophosphorus entering a watershed during two storm events.\ud Samples collected every one hour over a 25 to 26-h period during summer low\ud flows indicated that oxidized nitrogen, ammonia-nitrogen, total unfiltered phosphorus,\ud and orthophosphorus remained relatively constant. Total dissolved phosphorus\ud concentrations were the most variable but did not have a discernible diel pattern.\ud On a spatial scale, total unfiltered phosphorus and total dissolved phosphorus\ud remained relatively constant or showed no discernable patterns when sampled over five\ud 250-m intervals on each creek during summer low flow. Oxidized nitrogen and\ud orthophosphorus concentrations increased on Deer Creek and decreased on Flynn Creek\ud in an downstream direction. Ammonia-nitrogen concentrations decreased in a\ud downstream direction on Deer Creek and remained constant on Flynn Creek.\ud Nutrient constituent concentrations observed during high intensity sampling of\ud three storm events had a variable response with stream discharge. Oxidized nitrogen\ud concentration levels collected during the first fall storm appeared to be consistent with\ud other research that has shown a flush of oxidized nitrogen out of the forest soil profile\ud during the first fall storm. Sample concentrations from the first sampled storm had a 36 %\ud decrease in concentration with the falling limb of the first sampled storm on both Deer\ud Creek and Flynn Creek. Whereas, oxidized nitrogen concentrations had a 9 to 25 %\ud decrease with an increase in discharge on the two studied creeks and returned to\ud pre-storm levels with a decrease in discharge during two winter storms. Total unfiltered\ud phosphorus concentrations had a 90 to 1150 % (0 to 10 fold) increase with a rise in storm\ud discharge and decreased with the fall in storm discharge depending on the storm event and\ud creek sampled. Ammonia-nitrogen, total dissolved phosphorus and orthophosphorus\ud concentrations were not related to changes in discharge.\ud Precipitation concentrations of oxidized nitrogen and ammonia-nitrogen either had\ud a variable response or became diluted with an increase in rainfall amounts. The different\ud responses appear to be related to storm intensity, with greater dilution in higher intensity\ud storms.\ud Results from this study indicate that the input and output of nitrogen and\ud phosphorus into forested streams can be quite variable on both small temporal and spatial\ud scales depending on the particular nutrient sampled, the particular creek sampled, stream\ud flow conditions and season. It appears that sampling schemes designed for monitoring\ud water chemistry or nutrient flux should initially presume significant short interval (2 to\ud 20-h) variation until intensive sampling is able to prove otherwise