The influence of riparian vegetation and season on stream metabolism of Valley Creek, Minnesota

Abstract

Climate change will undoubtedly influence stream ecosystems by affecting water temperature, discharge, and riparian vegetation phenology. It has been suggested that increasing the extent of riparian buffers may help to mitigate these impacts and contribute to ecosystem resilience. Measures of stream metabolism are one way to examine the changes in ecosystem function as they respond to climate change and management strategies used to respond to these impacts. In this study we examine the influence of riparian vegetation (open vs. forest) and season on ecosystem metabolism in a small stream in MN, USA. The stream was heterotrophic regardless of the type of riparian vegetation but the presence of a forested buffer depressed gross primary production (GPP) and also resulted in lower (less negative) ecosystem respiration (ER) and net ecosystem production (NEP). We also found significant daily and seasonal variation in GPP, ER, and NEP. Stream metabolism was more variable in the fall than in the summer. The study design was complicated by the fact that there were differences in groundwater dynamics between the two locations; the forested location was a losing stream while the open location was a gaining stream. Despite the fact that groundwater input likely influenced the oxygen dynamics of the open location, the impact of the forest buffer outweighed the impact of groundwater. The presence of a forested riparian corridor resulted in decreased GPP likely through the attenuation of light. The low O 2 groundwater input at the open location likely biased measures of ER and NEP. Modifying stream corridors with increased forest vegetation to offset the impacts of climate change will likely result in less in-stream primary production and a shift to a more heterotrophic system. This shift will undoubtedly influence many ecosystem functions in-stream systems.

Ecosystem structure and function in two branches of an eastern Minnesota, USA, trout stream

“…GPP increased over time in the South Branch but decreased in the North Branch. GPP in the North Branch also declined during the fall of 2014 (data from Hornbach et al 2016). The differences between the two branches could be due to different factors.…”

“…Groundwater inputs may strongly influence measurements of stream metabolism (Hall & Tank 2005;Hornbach et al 2016). While the South Branch received most of its groundwater from springs near the headwaters (Almendinger 2003), and thus had sufficient time to come into equilibrium with the atmosphere before it reached our site, the North Branch is a losing stream until it reaches a few hundred meters upstream of our site (Hornbach et al 2016).…”

“…Oxygen saturation reached 100% on most days in the South Branch while values rarely exceeded 97% in the North Branch. Hornbach et al (2016) estimated the impact of groundwater on the North Branch of Valley Creek and calculated the mean bias of groundwater on ER and GPP (ER uncorrected for groundwater/ER corrected) using methods in Hall and Tank (2005). Using their calculations, we can produce a rough estimation of how groundwater influenced our measurements for autumn 2015.…”

“…While the South Branch received most of its groundwater from springs near the headwaters (Almendinger 2003), and thus had sufficient time to come into equilibrium with the atmosphere before it reached our site, the North Branch is a losing stream until it reaches a few hundred meters upstream of our site (Hornbach et al 2016). There local groundwater input, indicated by a higher vertical hydraulic gradient, may reduce oxygen levels.…”

“…Despite the local groundwater input at the sampling site in the North Branch, the water temperature was lower in the South Branch (Table 1). The North Branch, just upstream of the sampling site, is a losing stream (Hornbach et al 2016) and thus the warmer water coming from Lake Edith into this branch was not cooled by groundwater before it reached our sampling site.…”