The Development of the Baltic Sea Basin During the Last 130 ka - The Baltic Sea Basin - page 92

(North Atlantic Oscillation) has been reviewed and several correlations between cli-
mate and ecological changes have been observed, although the mechanism is not
understood (e.g., Ottersen et al. 2001 ) . At the Swedish west coast, a strong corre-
lation between phytoplankton biomass and NAO has been found, possibly caused
by an increased stratification in Skagerrak (Belgrano et al. 1999 ) . In the North Sea,
there has been an increase in phytoplankton season length and abundance since the
mid-1980s, interpreted as a response to climatic forcing (Reid et al. 1998 ) . Although
NAO is well known to influence climate conditions in the Baltic Sea, no direct links
between NAO, hypoxia, and inflow of saline water have been established.
The causes of hypoxia during the middle-late Littorina Sea are not fully under-
stood. An alternative trigging mechanism to widespread hypoxia during this time
period is increased anthropogenic forcing via eutrophication. It has been proposed
that hypoxia correlates with population growth and large-scale changes in land
use that occurred in the Baltic Sea watershed during the early Medieval expansion
between AD 750 and 1300 (Zillén et al. 2008 , Zillén and Conley 2010 ) . The large
land use changes increased soil nutrient leakage significantly in the Baltic Sea water-
shed, leading to high nutrient variability in the basin and associated hypoxia (Zillén
and Conley 2010 ) . The late Littorina Sea record of hypoxia in the Baltic Sea may
thus be due to multiple stressors, where both climate and human impacts may have
interacted. It is known that human activities have affected the Baltic Sea already AD
200 which is recorded as a change in the lead composition in the sediments from the
Eastern Gotland basin. This change coincides with a geographic shift in the Roman
lead mining from the Iberian Peninsula to other areas, e.g., Germany and the British
Isles during the first to third centuries AD (Bindler et al. 2009 ) .
Hypoxia again appeared in the Baltic Sea around the turn of the last century
with all sediments below 150 m in the Gotland Deep laminated (Hille et al. 2006 ) .
This period corresponds to a climate amelioration, which has lasted over most of
the twentieth century as well as the onset of the Industrial Revolution when the
European population increased rapidly (about six times since AD 1800) and techno-
logical advances in agriculture and forestry exploded (Zillén and Conley 2010 ) . The
eutrophication we now experience (e.g., Elmgren 2001 ) is caused by the increased
discharge of nutrients with a growing population and the use of synthetic fertilizers
on arable land after World War II (Elmgren 1989 ) , but these effects are also super-
imposed on effects caused by the ongoing climate warming (Andrén et al. 2000a ;
Leipe et al. 2008 ) . Revealing the relative importance between climate and anthro-
pogenic forcing on the Baltic Sea ecosystem is one of the major scientific challenges
for the future.
References
Adrielsson L (1984) Weichselian lithostratigraphy and glacial environments in the Ven-Glumslöv
area, southern Sweden. LUNDQUA Thesis 16
Andrén E, Shimmield G, Brand T (1999) Changes in the environment during the last centuries
on the basis of siliceous microfossil records from the southwestern Baltic Sea. The Holocene
9:25-38