Geoscience Reference
In-Depth Information
Chapter 17
Seafloor Desertification - A Future Scenario
for the Gulf of Finland?
Henry Vallius, Vladimir Zhamoida, Aarno Kotilainen, and Daria Ryabchuk
Abstract
The Gulf of Finland is a shallow semi-enclosed sea area which due to
strong anthropogenic pressure and poor water exchange is very sensitive to eutroph-
ication. During its whole postglacial history, the seafloor of the gulf has been period-
ically anoxic, and anoxia below halocline can thus be seen as a natural phenomenon.
During the last decades, however, this has been accompanied by a yearly repeated
seasonal anoxia in the shallower basins above halocline. This yearly repeated shal-
lower anoxia is triggered by substantial eutrophication of the sea and is a clear signal
of anthropogenic pressure. The seasonal anoxia has during the last decades propa-
gated to basins with water depths less than 20 m. The areal coverage of anoxia has
thus expanded substantially. Phosphorus which is bound to oxic seafloor sediments
is easily released during episodes of anoxia, which further intensifies eutrophica-
tion. It has been estimated that the concretion fields of the eastern Gulf of Finland,
only, contain more than 330,000 tons of P
2
O
5
which is equal to some 175,000 tons
of elementary phosphorus. In case of shallowing of the area of permanent anoxia,
these concretion fields would become anoxic, which would lead to rather rapid dis-
solution of the concretions and a release of a large amount of phosphorus together
with the heavy metals which today are bound to the concretions.
Keywords
Gulf of Finland
·
Baltic Sea
·
Ferromanganese Concretions
·
Marine
sediments
·
Anoxia
·
Heavy metals
·
Phosphorus
17.1 Introduction
The Baltic Sea is a European epicontinental sea, one of the world's largest brack-
ish water areas. It has connection to the Atlantic Ocean only through the narrow
Danish Sounds. The water depth is moderate, only 52 m on average. There is no
