Articles | Volume 11, issue 22
Research article
 | Highlight paper
24 Nov 2014
Research article | Highlight paper |  | 24 Nov 2014

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

S. T. Lennartz, A. Lehmann, J. Herrford, F. Malien, H.-P. Hansen, H. Biester, and H. W. Bange

Abstract. 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.

Short summary
A time series of nine oceanic parameters from the coastal time series station Boknis Eck (BE, southwestern Baltic Sea) in the period of 1957-2013 is analysed with respect to seasonal cycles and long-term trends. Most striking was a paradoxical decreasing trend in oxygen with a simultaneous decline in eutrophication. Possible reasons for this paradox, e.g. processes related to warming temperatures such as increased decomposition of organic matter or altered ventilation, are discussed.
Final-revised paper