## 4 Discussion
### Metabolism of Turkey Creek  
According to the River Continuum Concept by [PERSON] _et al._[18], forested headwater streams should have rates of respiration higher than their rates of primary production. Other studies have found this to be true. For instance, in a study of stream metabolism in eastern Tennessee, [PERSON] _et al._[6] found the system to be strongly heterotrophic, with average _GPP_ rates of 1.34 g O\({}_{2}\) m\({}^{-2}\) day\({}^{-1}\) in 2004 and 1.42 g O\({}_{2}\) m\({}^{-2}\) day\({}^{-1}\) in 2005, and _CR_ rates of 4.51 g O\({}_{2}\) m\({}^{-2}\) day\({}^{-1}\) in 2004 and 3.54 g O\({}_{2}\) m\({}^{-2}\) day\({}^{-1}\) in 2005. In a study comparing stream metabolism across regions and under differing land use, [PERSON] _et al._[10] found that streams in forested areas had lower mean rates of _GPP_ than un-forested streams, and that CR increased with increasing organic matter. Elevated levels of organic matter are typical in forested headwaters, as shown by [PERSON] _et al._[49] in their study of Piedmont streams in North America which found that coarse particulate organic matter and large woody coarse particulate organic matter were both significantly higher in forested than in deforested streams. Results from our study indicate an ecosystem similar to other small, forested headwaters, although the extrapolation of these results from sites N1 and N2 to the entire Turkey Creek would necessitate assumptions including relatively even dispersal of in-stream flora, detritus (and heterotrophic consumers of this detritus), and hydrological characteristics. For most of the duration of this four-year study the _GPP/CR_ ratio was below one (_GPP/CR_ was less than one for 82.3% of the time at N1, and 81.6% of the time at N2, and _GPP/CR_ medians at N1 and N2 were 0.57 and 0.59, respectively), indicating a heterotrophic system that releases more carbon than it assimilates. The _GPP/CR_ averages were higher than the medians (and at N2, higher than one for both pre- and post-harvest days) due to a relatively small number of days with high _GPP/CR_, which seemed to occur mainly when water temperature was lower, though no correlation was found between _GPP/CR_ and water temperature. The dominance of heterotrophy in forested headwaters is typical, due to plentiful allochthonous input and little direct sunlight to drive photosynthesis from nearly complete canopy cover [49]. In another study comparing stream metabolism among four forest and desert stream systems, [PERSON] _et al._[20] reported similar findings of a predominance of heterotrophy and the lowest rates of _GPP_ occurring in forested headwaters.  
A number of factors can affect primary productivity and community respiration rates in streams. These include, among others, the availability of sunlight, concentration of nutrients such as nitrogen and phosphorus, and temperature [29, 50, 51]. Limited sunlight due to riparian vegetation and the low concentrations of both nitrogen and phosphorus generally present in forested streams [52] typically mean lower _GPP_ rates for forested headwaters. In our study, average _GPP_ rates at the upstream reference site and the downstream site were low throughout the four-year study period. These sites were well shaded before timber harvest and stream conditions were well maintained during and after timber harvest with the BMP implementation of SMZs.