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.

Abstract. The response of the phytoplanktonic community (primary production and algal biomass) to contrasted Saharan dust events (wet and dry deposition) was studied in the framework of the DUNE "a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem" project. We simulated realistic dust deposition events (10 g m−2) into large mesocosms (52 m3). Three distinct experimental dust additions were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition, DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R1, -R2: simulation of 2 successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentration (Chl a) were observed after a dry deposition event while a wet deposition event resulted in a rapid (24 h after dust additions), strong (up 2.4 fold) and long (at least a week duration) increase in PP and Chl a. We show that in addition to being a source of dissolved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of NO3− (net increases up to +9.8 μM NO3− at 0.1 m depth) to the nutrient depleted surface waters due to cloud processes and mixing with anthropogenic species such as HNO3. The dry deposition event was shown to be a negligible source of NO3−. By transiently increasing DIP and NO3− concentrations in P-N starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phytoplanktonic activity. Due to the higher input of NO3− relative to DIP, a wet deposition event resulted in a strong increase in the NO3−/DIP ratio from initially < 6 to over 150 at the end of the DUNE-2-R1 experiment suggesting a switch from an initial N or NP co-limitation towards a severe P limitation. We also show that the contribution of new production to PP increased after wet dust deposition events from initially 15% to 60–70% 24 h after seeding, indicating a switch from a regenerated-production based system to a new-production based system. DUNE experiments show that wet and dry dust deposition events induce contrasted responses of the phytoplanktonic community due to differences in the atmospheric supply of bioavailable new nutrients. Our results from original mesocosm experiments demonstrate that atmospheric dust wet deposition greatly influences primary productivity and algal biomass in LNLC environments, changes nutrient stocks and alters the NO3−/DIP ratio leading to a switch in the nutrient limitation of the phytoplanktonic activity.

The response of the phytoplanktonic community (primary production and algal biomass) to contrasted Saharan dust events (wet and dry deposition) was studied in the framework of the DUNE ("a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem") project. We simulated realistic dust deposition events (10 g m−2) into large mesocosms (52 m3). Three distinct dust addition experiments were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition; DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R1 and DUNE-2-R2: simulation of two successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentrations (Chl a) were observed after a dry deposition event, while a wet deposition event resulted in a rapid (24 h after dust addition), strong (up to 2.4-fold) and long (at least a week in duration) increase in PP and Chl a. We show that, in addition to being a source of dissolved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of nitrate (NO3−) (net increases up to +9.8 μM NO3− at 0.1 m in depth) to the nutrient-depleted surface waters, due to cloud processes and mixing with anthropogenic species such as HNO3. The dry deposition event was shown to be a negligible source of NO3−. By transiently increasing DIP and NO3- concentrations in N–P starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phytoplanktonic activity. Due to the higher input of NO3− relative to DIP, and taking into account the stimulation of the biological activity, a wet deposition event resulted in a strong increase in the NO3−/DIP ratio, from initially less than 6, to over 150 at the end of the DUNE-2-R1 experiment, suggesting a switch from an initial N or NP co-limitation towards a severe P limitation. We also show that the contribution of new production to PP strongly increased after wet dust deposition events, from initially 15% to 60–70% 24 h after seeding, indicating a switch from a regenerated-production based system to a new-production based system. DUNE experiments show that wet and dry dust deposition events induce contrasting responses of the phytoplanktonic community due to differences in the atmospheric supply of bioavailable new nutrients. Our results from original mesocosm experiments demonstrate that atmospheric dust wet deposition greatly influences primary productivity and algal biomass in LNLC environments through changes in the nutrient stocks, and alters the NO3−/DIP ratio, leading to a switch in the nutrient limitation of the phytoplanktonic activity.

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.