Physical and chemical changes in the ocean over basin-wide zones and decadal or longer time-scales: perspectives on current and future conditions - Ocean Sustainability in the 21st Century - page 100

3.7 Eutrophication, oxygen levels, and biodiversity
Ocean biological and ecosystem conditions are influenced by climate change, and are also
influenced in several other direct ways by human actions. A comprehensive and integrated
approach is required in order to obtain an understanding of ocean changes and their signi-
ficance for our life-support systems. Nutrient enrichment of marine systems, in particular
through excess use of nitrogen fertilization, has been well documented in many coastal areas
and shelf seas. This has driven an exponentially increasing spread of dead zones with dis-
solved oxygen levels below 2 ml/l (hypoxia) since the 1960s, for instance in the Baltic Sea,
the Kattegatt, Black Sea, Gulf of Mexico, and East China Sea (Diaz and Rosenberg, 2008 ) .
Natural nutrient enrichment throughocean circulation, coastal andequatorial upwelling sup-
porting highly productive zones, can generate low or intermediate oxygen levels at varying
depths in the water column. In these areas the ecosystem and benthos seem, however, to be
adapted to low levels of dissolved oxygen. This is not the case in the estuarine and coastal
areas subject to excess nutrient enrichment from land run-off. In these areas the low oxygen
levels lead to mass mortality and major changes in community structure and biodiversity.
Observations made over several decades show a decline in oxygen levels since the
1940s onwards, and localized declines were documented in the Baltic Sea in the 1930s.
However, it was only in the 1960s that hypoxia became more widely observed. Paleo-in-
dicators suggest that hypoxia has not been a naturally recurring condition in many of the
systems. Since the 1960s the number of dead zones has approximately doubled each decade,
according to Diaz and Rosenberg ( 2008 ) .
The Baltic Sea is an interesting case because of its importance for the surrounding
countries, its physical, chemical, and biological characteristics, strong river inflows, topo-
graphy with several basins separated by sills, stratification, and estuarine characteristics,
with narrow and shallow connections to the ocean (e.g. Kullenberg, 1983 ).
The impact of climate change on the Baltic Sea ecosystem over the past 1000 years
was recently elucidated by Kabel et al . ( 2012 ) . They demonstrated the role of a surface