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,H Biester,A Lehmann,H W Bange,H.-P Hansen,J Herrford,F Malien

doi:10.5194/bg-11-6323-2014

Abstract

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.

Highlights

Long-term observations in oceanography are crucial when it comes to improving the understanding of the state of ecosystems and monitoring their long-term developments
Observations at the Boknis Eck (BE) station were compared with model output of the Kiel Baltic Sea Ice Ocean Model (BSIOM)
We present and statistically evaluate a monthly data set of nine physical, chemical and biological parameters observed over a time span of 56 years at Boknis Eck

Summary

Introduction

Long-term observations in oceanography are crucial when it comes to improving the understanding of the state of ecosystems and monitoring their long-term developments. They have been a core strategy in the last 50 decades and are still considered to have high priority today Long-term monitoring is especially important for a dynamic system such as the Baltic Sea, where high variations, which are effective on different timescales, are triggered by natural and anthropogenic causes. Major saltwater inflows triggered by westerly winds transport large amounts of fully saline North Sea water into the Baltic Sea (Hanninen et al, 2000; Lass and Matthäus, 1996)

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