Specialeforsvar: Emilie M. F. Kallenbach

Oxygen dynamics and ecosystem metabolism in Øle Å – a stream under restoration

Vejledere: Kaj Sand Jensen og Lars Båstrup-Spohr

Censor: Tenna Riis, Aarhus Universitet

Abstract
Along many streams natural riparian vegetation has been replaced by agriculture or forestry, with alterations in the stream as a consequence. This, due to a close and intricate interplay between streams and their riparian zones. However, because of an increased demand for recreational areas and economic difficulties of forestry, it is essential to achieve an understanding of how ecosystem will respond to deforestation and reestablishment of natural vegetation in the riparian zone.

In present study, 6 ha of a plantation of Norway spruce in the riparian zone of Øle Å was cut down to improve recreational areas, vegetation structure and stream condition. The purpose was to closely monitor and assess the response of an ecosystem to a restoration project; more specifically, this project aimed at investigating the changes in oxygen dynamics and metabolism within the stream in response to deforestation.

The monitoring program for this study started in 2013, one year before deforestation, and is still, two years after deforestation, ongoing. The deforestation took place in September 2014. The baseline measurements of ecosystem metabolism, oxygen condition together with a number of hydrological, physical and chemical parameters took place before deforestation. In addition, the upstream part of the stream where no deforestation took place, served as a reference. The single-station and two-station methods combined with a Bayesian approach were used to calculate the metabolic rates and to detect any metabolic response to deforestation.

We found low metabolic rates dominated by heterotrophic processes. A tight coupling between oxygen saturation and discharge in the highly variable hydraulic system was also found. This meant that hypoxic periods existed in the shaded parts of the stream when flow was stagnant.

Deforestation was found to have no effect on chemical, hydrological or physical variables, most likely because of the deforested area being too small, compared to the size of the catchment. Light was the only factor that increased in response to deforestation. After the deforestation, events of hypoxia no longer occurred, this made the stream more suitable as habitat for fish and invertebrates. GPP increased slightly in response to deforestation. No alterations in NEP or ER could be found.

In summary, the restoration project revealed that improved oxygen conditions can be achieved in stream running through a highly shaded spruce forest, with low availability of labile carbon, if the riparian zone is cleared. In addition, it provided unique insight in a stream ecosystem on a 3-year time scale.