Effects of shading on stream ecosystem metabolism and water temperature in an agriculturally influenced stream in central Wisconsin, USA

Abstract

Headwater streams with a forested riparian zone and canopy cover have regulated stream temperatures and stream ecosystem metabolism because of shading. Agricultural practices have dramatically altered riparian corridors with the removal of mature vegetation, which consequently alters natural stream processes. The objective of this study was to investigate the effects of shading on stream temperature and ecosystem metabolism in an agriculturally influenced, low order stream. We examined three stream reaches; two in a recently restored stream (Lost Creek) and one in a nearby-unimpacted reference stream (Little Plover River). The two reaches in Lost Creek included a 100-m artificially-shaded stream reach and a control reach immediately upstream of the shaded reach. The Little Plover River included one reference reach with a fully forested canopy. Both streams are located in agricultural watersheds and based on phosphorus levels would be classified as meso-eutrophic. Each stream reach was assessed during stable flow conditions once each month between June and October of 2016. Rates of gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP) were determined using the open system, two-station diel dissolved oxygen change method, and reaeration (k) was calculated using the nighttime regression method. Based on stream discharge data, we corrected metabolism values for groundwater when inputs exceeded 7% (Hall and Tank 2005). We also measured water temperature, PAR (photosynthetically active radiation), soluble reactive phosphorus (SRP), periphytic chlorophyll a content, and organic matter content at each site as potential controlling variables. Surprisingly, no significant differences were observed in daily maximum water temperatures among the three sites; however, GPP levels were significantly lower (ANOVA F2,14 = 45.45; p < 0.001) in the artificially shaded reach and the reference site compared to the control indicating that shading reduced GPP. ER was not significantly different among the reaches and remained relatively low, which resulted in the control reach having a highly positive and significantly higher NEP (ANOVA F2,14 = 17.656 p < 0.001) compared to the shaded and reference reaches. Redundancy analysis revealed that GPP and NEP were strongly correlated with PAR. Our findings suggest that shading can improve dissolved oxygen dynamics in an agriculturally-influenced stream by lowering GPP.