SEASONAL PATTERNS IN STREAMWATER NUTRIENT AND DISSOLVED ORGANIC-CARBON CONCENTRATIONS - SEPARATING CATCHMENT FLOW PATH AND IN-STREAM EFFECTS

Distinct seasonal patterns in streamwater nutrient and dissolved organ ic carbon (DOC) concentrations are evident in the data record from 7 y ears of weekly sampling in the West Fork of Walker Branch (WB) and 4 y ears of weekly sampling in a nearby stream, upper White Oak Creek (WOC ), both first-order streams in the Ridge and Valley Province of easter n Tennessee. Concentrations of NO3 and soluble reactive phosphorus (SR P) in both streams showed a repeated pattern of annual maxima in summe r and biannual minima in autumn and spring. Concentrations of DOC in W B exhibited distinct autumn maxima. To determine whether temporal vari ations in catchment hydrological processes could explain the seasonal nutrient and DOC variations in WB, we used an end-member mixing analys is involving Ca and SO4 concentrations to separate stream discharge in to three catchment how paths of differing nutrient concentrations. Str eam NO3, SRP, and DOC concentrations were predicted solely on the basi s of temporal variation in the importance of these how paths and measu rements of nutrient concentrations for the different flow paths. Ratio s of observed/predicted concentrations in stream water near 1.0 sugges ted that catchment effects alone explained streamwater concentrations, whereas ratios substantially different from 1.0 suggested that in-str eam processes were important determinants of streamwater concentration s. Observed/predicted NO3 and SRP concentration ratios showed repeated annual patterns with values closer to 1.0 during winter and summer (g enerally 0.8-1.2) and minima (<0.6) in spring and autumn, suggesting s ubstantial in-stream net uptake at these times. Observed/predicted DOC concentration ratios were more variable and generally greater than or equal to 1 but did show consistent autumn maxima (>2.5), indicating s ubstantial in-stream DOC generation at this time. Observed/predicted r atios for all nutrients were generally less variable and were closer t o 1.0 at high flow compared to low flow, suggesting that in-stream con trols on streamwater chemistry are less important at high discharge th an at low discharge. Our results indicate two general modes of control of stream nutrient concentrations: (1) catchment control via seasonal variation in the dominant hydrologic pathway (greater proportion of d eep groundwater in summer), which produces lower winter and higher sum mer concentrations, and (2) in-stream control via high rates of net nu trient uptake during the spring (primarily by autotrophs) and autumn ( primarily by heterotrophs).