Lennartz, Sinikka T. (2013) Time-series analysis and development of a 1D box model to simulate trace gas concentrations at the Time Series Station Boknis Eck, Baltic Sea. (Master thesis), TU Braunschweig, Braunschweig, 96 pp.

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Abstract

The goal of the study was to statistically evaluate the long-term trends at Time Series Station Boknis Eck (Southwestern Baltic Sea) in the period of 1957 to 2013 as well as to develop a simple 1D box model to simulate concentrations of methane and nitrous oxide in the mixed layer. The seasonal Mann-Kendall-test and wavelet analysis were used to detect changes in mean and seasonality of nine physical and biogeochemical parameters. Significant changes were present in all analysed parameters and allow the conclusion that the Baltic Sea is subjected to extensive changes. The trends of increasing oxygen depletion with simultaneously decreasing nutrient and chlorophyll a concentrations, the increase of spring temperatures and the intensified stratification are in agreement with previously found trends in large parts of the Baltic Sea. The results indicate that changing climatic factors may counteract the decline in eutrophication and contribute to oxygen depletion in the bottom water. Furthermore, two 1D box models were developed that simulated the concentrations of methane and nitrous oxide in the mixed layer in dependance of temperature, salinity, wind speed, oxygen and chlorophyll a concentration. The simulated concentrations were in average in the range of the standard deviation of the measured concentrations and reflected the seasonality well. For the concentration of both gases, production processes below the mixed layer boundary with susequent transport to the mixed layer determined the concentration there. For methane concentrations at the surface, the sediment played the major role as anoxic conditions there enabled the formation of methane that was then transported to the surface. Nitrous oxide was mainly produced in the water column below the mixed layer, the production being highly dependent on the oxygen concentration in the bottom water. For both trace gases, the emission to the atmosphere acted as the main sink. The development of the concentration of the trace gases under a scenario of continuing long-term trends is estimated to result in a stronger emission of methane and nitrous oxide. To determine the future development of trace gas emissions as well as the general state of the ecosystem at Boknis Eck, it is crucial to continue the monthly monitoring at Boknis Eck.

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