## 4 Discussion  
Ecosystem metabolism of streams draining agricultural land is known to be elevated due to the impact of cultivation related stressors, such as increased light and nutrient inputs [45]. The average metabolic rates of the 12 streams in our study exhibited a range of 0.88-35.93 g O\({}_{2}\)/m\({}^{2}\)/day for daily GPP and 0.73-26.78 g O\({}_{2}\)/m\({}^{2}\)/day for daily ER. Metabolism values from the studied agricultural catchments (_i.e._, 78%-97% cover) are comparable to values reported by previous studies in agricultural streams in Central USA (e.g., [PERSON] _et al._[45]: GPP = 0.1-44.2 g O\({}_{2}\)/m\({}^{2}\)/day, ER = 6.2-41.6 g O\({}_{2}\)/m\({}^{2}\)/day; [PERSON] _et al._, 2010 [46]: GPP = 0.1-16.2 g O\({}_{2}\)/m\({}^{2}\)/day, ER = 0.9-15.7 g O\({}_{2}\)/m\({}^{2}\)/day; [PERSON] _et al._[47]: GPP = 0-8.4 g O\({}_{2}\)/m\({}^{2}\)/day, ER = 0.03-36 g O\({}_{2}\)/m\({}^{2}\)/day). Although the maximum rates of GPP and ER from our study sites are almost two-fold higher than rates from [PERSON] _et al._[46] and [PERSON] _et al._[47], this difference may be attributed to the shallow (mean = 0.14 m) and relatively slow moving (mean = 0.05 m/s) streams assessed in our study. Shallow depths are associated with increased light availability and associated primary production at the streambed in all areas of the stream [48]. Likewise, low flow rates can allow for the accumulation of nutrients and organic matter in the sediments and water column, facilitating uptake by biota and increasing instream production and respiration [49]. Our study also examined metabolism over a two-week period in summer. Temporal differences associated with seasonality have been seen to alter the magnitude of stream metabolism, with greatest rates generally occurring from spring through autumn [50]. However, the greater light intensity and duration, and resultant increase in water temperature in the summer months can promote a greater activity of both photosynthetic and heterotrophic organisms facilitating higher metabolic rates. Concentrations of major nutrients sampled in our study were variable among catchments, and frequently exceeded both national and provincial water quality guidelines. The average TP concentration of all samples was approximately 9-fold greater than the provincial threshold for the protection of aquatic life (0.03 mg/L) [51] and reflective of a hyper-entropic status (>0.1 mg/L) [52]. Likewise, the average NH\({}_{4}\) concentration was approximately 10-fold greater than the proposed national guideline (0.019 mg/L) [53]. However, the average nutrient concentrations of \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Metabolism Parameter** & **Mean** & **Min.** & **Max.** & **Median** & **St. Dev.** & **CV** \\ \hline \(k\) (day\({}^{-1}\)) & 21.99 & 6.41 & 44.84 & 17.77 & 11.75 & 0.53 \\ GPP (gO\({}_{2}\)/m\({}^{2}\)/day) & 11.75 & 0.88 & 35.93 & 8.45 & 10.67 & 0.91 \\ ER (gO\({}_{2}\)/m\({}^{2}\)/day) & 11.11 & 0.73 & 26.78 & 9.02 & 8.95 & 0.81 \\ NEM (gO\({}_{2}\)/m\({}^{2}\)/day) & 0.64 & \(-\)5.82 & 9.16 & 1.22 & 4.20 & 6.53 \\ P/R & 1.48 & 0.39 & 6.49 & 1.18 & 1.62 & 1.09 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of the reaeration coefficient (\(k\)), gross primary productivity (GPP), ecosystem respiration (ER), net ecosystem metabolism (NEM), and production to respiration ratio (P/R) of 12 headwater streams in the Grand River Watershed. all collected samples (TN = 2.69 mg/L, NH4 = 0.185 mg/L, NO2-NO3 = 1.66 mg/L, TP = 0.27 mg/L, SRP = 0.18 mg/L, TDP = 0.19 mg/L) were comparable to results of studies on other small agricultural streams in the North American Great Lakes region. For example, [PERSON] _et al._[21] reported concentrations of 0.087, 0.179, and 1.101 mg/L for TDP, NH4, and NO3 respectively, among seven agricultural streams with abundant BMPs. Additionally, [PERSON] _et al._[54] found an average TN concentration of 2.68 mg/L and TP concentration of 0.08 mg/L along a gradient of agricultural catchments in Southern Ontario (6.7%-100% cover).