General trends

Temperatures

Sea surface temperatures (SST) show a trend to warmer temperatures especially during the last decade which is illustrated by the fact that cold events with winter SST below 0°C have not been recorded during the last 14 years.

Oxygen

Besides the obvious warming of the surface layer there is a trend to increasing number of anoxia, here defined as O2 below the detection limit of the Winkler method (i.e. ~2 µM): During the period 1957-1983 only one anoxic event (1980) has been recorded, whereas during the period 1986-2010 10 anoxic events (1989, 1991, 1992, 1994, 2001, 2002, 2003, 2005, 2007, 2008) have been recorded.

Nutrients

The long-term trends of the winter means (DJFM) of dissolved inorganic phosphate (DIP = PO43-) and total dissolved inorganic nitrogen (tDIN = NH4+ + NO2 + NO3) in the surface layer (0-10m) show a pronounced trend of decreasing DIP and tDIN which started in the mid-80s. DIP concentrations seem to have reached a plateau since about 10 years. In contrast the decrease of tDIN seems to continue until today.  Interestingly the present DIP concentrations are comparable to those concentrations observed at time when the BE time series has been started back in 1957. Despite the fact that a pronounced decreasing trend of the winter nutrient concentrations has been detected, the concentrations of DIP and tDIN remain on relatively high level.

Water transparency

The observed minimum water transparency (measured with a Secchi disk; data are available since 1986) during the months of the spring bloom (February-March) showed a slight decreasing trend from 5 m - 8 m (1986-1991) to 2 m - 6 m (1991-2010). This may indicate an intensification of the spring blooms during the last two decade.

NAO

The mean winter SST (December-March, DJFM) show a significant positive correlation (R = 0.52, n= 40, p <0.01) with the mean DJFM North Atlantic Oscillation (NAO) index. However, the correlation at BE is not as strong as observed for other basins of the Baltic Sea.

 

Summary

The long-term trends for SST, O2 and nutrients at BE are in general agreement with the trends observed in other coastal areas of the various basins of the Baltic Sea.

The observed long-term trends at BE are affected by both regional processes (i.e. eutrophication) and global processes (warming due to climate change).


Monthly sea surface temperature measured in 1 m depth at BE since 1957.

 


Monthly O2 values measured in 25 m depth at BE since 1957.

 


Mean winter (DJFM) concentrations (red circles) of DIP (lower panel) and tDIN (upper panel). The black solid lines indicate 5-year moving averages. Please not that (i) no DIP measurements are available btween1978 and 1988 and (ii) no tDIN measurements are available before 1986.