Linking measurements made at different spatial scales is an ongoing challenge in making ecological research findings relevant for natural resource managers and decision-makers. In streams, many empirical studies focus on either small patch-scale or whole-stream measurements of ecosystem functions, but explicit comparison of measured rates and pools of potential drivers between these scales are generally lacking. Our study sought to directly compare measurements made at different spatial scales across a stream network. We quantified ecosystem functions (ecosystem respiration [ER], gross primary production [GPP], ammonium uptake [NH4]) and pool sizes of structural components (coarse and fine benthic organic matter, chlorophyll-a) as potential explanatory variables using both chamber and reach-scale measurements in nine first- to fourth-order streams throughout the Coweeta Hydrologic Laboratory (Macon County, North Carolina). Whole-stream rates were estimated using single-station oxygen diel oxygen change (ER, GPP) or the Tracer Additions for Spiraling Curve Characterization (TASCC) method (NH4) while chamber rates were estimated using linear regression of dissolved oxygen change within a recirculating chamber (ER, GPP) or log-linear regression of ammonium concentration over time (NH4).
Results/Conclusions
The degree of match/mismatch between rate estimates varied by stream and ecosystem function, but rates tended to be more similar in smaller streams. Modeled whole-stream ER was higher than chamber estimates in 1st and 2nd order streams, but chamber rates exceeded whole-stream ER 6-58x in the 5 largest streams, where modeled estimates of whole-reach ER were very low (0.01-0.15 gO2m-2d-1). Whole-stream estimates of GPP were 2-14x higher than those made in chambers throughout the stream network, and the degree of mismatch was positively correlated with stream discharge. Differences in rate estimates between measurement scales resulted in stark differences in estimated productivity to respiration (P:R) ratios. In chambers, all streams were net heterotrophic (P:R<1), with chamber-scale NH4 uptake inversely related to P:R (F1,28=4.40, p=0.045). In contrast, whole-stream estimates of ER and GPP resulted in P:R ratios that were autotrophic (P:R>1) in nearly half of daily measurements. TASCC estimates of NH4 uptake also exceeded chamber rates in larger streams. Examining potential drivers of functions at each scale, including organic matter, temperature, and light, may help elucidate causes of discrepancies to allow reasonable comparison of rates from different scales to facilitate estimation of network-scale functions, which will ultimately help guide watershed management decision-making.