Lisa Brase
Published: 2017
Total Pages: 0
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Summary: The increase of reactive nitrogen due to anthropogenic nutrient inputs is a widespread problem in the aquatic environment which can lead to significant alterations of the nitrogen cycle and thus to a general increase in eutrophication. Processes of N-turnover, such as nitrification and denitrification, can be influenced by an increased level of reactive nitrogen and contribute to an intensified production of the climate-relevant greenhouse gas N2O (nitrous oxide). This thesis demonstrates human-driven accelerations of the nitrogen cycle and their effect on water column reactive N internal processing and on N2O production. In a small river, it is demonstrated that even an intensification of internal N-turnover processes cannot counteract additional nutrient inputs. Furthermore, by using high-resolution measurements of N2O, it is shown that such enhancement of nitrification and denitrification contributes to substantial N2O production and resulting emissions. The first part of this thesis (chapter 2) describes the influences of an anthropogenic gradient on internal nutrient cycling processes, i.e. nitrate production and consumption, in a small river. It is shown that contribution of nitrate due to nitrification decreases with increasing eutrophication, although sedimentary nitrate production is enhanced and contributes to nitrate concentration in the river. Similarly, additional nutrient increase leads to an increased nitrate consumption rate in the river sediment, regardless of seasonality. Although nitrate removal always exceeded internal nitrate production, the filter capacity of the sediment is limited and overwhelmed by surplus N inputs. Besides their impact on the water column nitrate inventory (or the lack thereof), nitrification and denitrification are significant sources of nitrous oxide (N2O). Both processes have been investigated individually in the Elbe estuary, but their integrated effect on N2O concentration in the contemporary estuary is unclear. By using transect measurements, the Hamburg port region was identified as a hot-spot of biological N2O production, as demonstrated in chapter 3. This is mainly due to nitrification, but also denitrification can contribute to additional N2O in the area of lowest measured oxygen values. Relating to the entire Elbe estuary freshwater area, and contrary to measurements in the late 80s, internal N2O processes appear to have changed from denitrification to nitrification as the main N2O contributing source. It is notable, that N2O saturation did not decrease since the middle of the 90s, even though a continuous nutrient decrease occurred since the late 80s. Since the port of Hamburg was identified as the area with highest N2O production, N2O dynamics in this area are examined in chapter 4 by stationary measurements in a tide controlled context. Stationary measurements showed an increase of N2O concentration with ebb tides and, as already concluded from transect measurements, N2O production in this low oxygen area is mainly attributed to in-situ production, i.e. by means of nitrification and denitrification. An increased remineralization and abiotic factors, such as a decrease in oxygen concentration and a lower discharge can further lead to an intensified internal N2O production by fueling nitrogen turnover processes. In addition, a small contribution of allochthonous N2O can be allocated to N2O derived from harbor basins and/or riparian zones and thus is a minor N2O source. The research conducted within the present thesis confirms the port of Hamburg as a hot-spot of biological N2O production and as a constant net source of N2O emissions to the atmosphere.