Boknis Eck Time Series Station (SW Baltic Sea): Measurements from 1957 to 2010

Salinity, temperature, and O2 have been recorded on a monthly basis at the Boknis Eck Time Series Station (BE; Eckernförde Bay, SW Baltic Sea) since April 1957 with only two major breaks (1976-78 and 1983-1985). Chlorophyll a measurements started in 1960 and nutrient data (NO2-, NO3-, NH4+, PO4 3-, SiO4 2-) are available since March 1979. Here we present a short introduction to the long-term trends observed at BE and selected results of ongoing projects covering different topics from the surface microlayer and the water column to the sediments at BE. On the basis of a preliminary analysis of the long-term records of surface water temperature, oxygen in 25m, and dissolved nutrients we conclude that BE is affected by both regional processes and global processes detectable as eutrophication and warming of the surface water, respectively. The number of events with extremely depleted O2 concentrations (hypoxia/anoxia) in the bottom layer has been increasing during the last 25 years. Moreover, BE is site of signifi cant emissions of climate relevant trace gases such as methane

Dissolved methane during hypoxic events at the Boknis Eck Time Series Station (Eckernförde Bay, SW Baltic Sea)

Dissolved CH4 was measured in the water column at the Boknis Eck (BE) time series station in the Eckernförde Bay (SW Baltic Sea) on a monthly basis from June 2006 to November 2008. The water column at BE was always supersaturated with CH4 and, therefore, CH4 was released to the atmosphere throughout the sampling period: the mean CH4 surface (1 m) saturation at BE was 554±317%. A pulse of enhanced CH4 emissions occurs when the CH4 accumulation in the hypoxic bottom layer during summer is terminated in late summer/autumn. We did not detect a straightforward relationship between periods of enhanced CH4 in the bottom layer and hypoxic events at BE: the sedimentary release of CH4 seemed to be mainly triggered by sedimenting organic material from phytoplankton blooms. We conclude that future CH4 emissions from BE will be determined by the intensity of phytoplankton blooms, which in turn will be influenced by eutrophication. However, hypoxic events seem to have only a modulating effect on the enhancement of sedimentary methanogenesis and the subsequent release of CH4 to the water column

Long-term trends at the Time Series Station Boknis Eck (Baltic Sea), 1957–2013: does climate change counteract the decline in eutrophication?

The Boknis Eck (BE) time series station, initiated in 1957, is one of the longest-operated time series stations worldwide. We present the first statistical evaluation of a data set of nine physical, chemical and biological parameters in the period of 1957–2013. In the past three to five decades, all of the measured parameters underwent significant long-term changes. Most striking is an ongoing decline in bottom water oxygen concentration, despite a significant decrease of nutrient and chlorophyll a concentrations. Temperature-enhanced oxygen consumption in the bottom water and a prolongation of the stratification period are discussed as possible reasons for the ongoing oxygen decline despite declining eutrophication. Observations at the BE station were compared with model output of the Kiel Baltic Sea Ice Ocean Model (BSIOM). Reproduced trends were in good agreement with observed trends for temperature and oxygen, but generally the oxygen concentration at the bottom has been overestimated