Gloyer, Michael (2021) Water transparency at the Boknis Eck Time Series Station (Eckernförde Bay, south-western Baltic Sea) from 1986 to 2020, 2021. (Master thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 65 pp.

Text MGloyer_MSc_2021.pdf - Published Version Restricted to Repository staff only Download (2MB) | Contact

Abstract

The water transparency depends on the amount of attenuating material in the water column, i.e. phytoplankton, suspended particulate matter (SPM) and coloured dissolved organic matter (CDOM). It can serve as a proxy for water quality. By lowering a Secchi disc, a circular white disc, into the water until it disappears from sight at the so-called Secchi depth, an approximate measure of the water transparency can be obtained. In this thesis, a Secchi depth time series from the Boknis Eck Time Series Station in the south-western Baltic Sea is analysed. The time series station at Boknis Eck is one of the longest running continually maintained time series stations in the Baltic Sea and thus provides valuable insight into the long-term development of this nearly enclosed brackish sea. In this thesis, it is shown that the Secchi depth at Boknis Eck exhibits a slight shoaling over the 35 years of available observations. Also the variability of the Secchi depth decreases, with a particularly strong decrease of high Secchi depth values. The dominant periodic signal in the Secchi depth time series at Boknis Eck is the seasonal cycle. The seasonal cycle is not constant, but changes over time, with summer Secchi depths increasing and Secchi depths in the rest of the months decreasing. Towards the end of the time series pronounced Secchi depth minima are located in March and October when the spring and autumn phytoplankton blooms occur. Remotely sensed data from the Sentinel-3 satellites can be used to obtain Secchi depth estimates for Boknis Eck, but they slightly underestimate the Secchi depth ariability. By comparing the observations from Boknis Eck to Secchi depth data from the Kiel Bay, it is shown that the Secchi depth at both locations covers a very similar range, but has a correlation coefficient of only 0.45. Chlorophyll a concentration, which is a proxy for phytoplankton biomass, is the only one of the investigated parameters (chlorophyll a concentration, water temperature, oxygen concentration, dissolved inorganic nitrogen, dissolved inorganic phosphorus, sunshine duration, North Atlantic Oscillation index) which shows a significant correlation with Secchi depth. However, it can only explain between 10% and 30% of the variability in Secchi depth at Boknis Eck.

Actions (login required)

View Item