Seafloor Desertification – A Future Scenario for the Gulf of Finland? - The Baltic Sea Basin - page 370

Fig. 17.5 White bacterial growth on top of core MGGN-2004-1 from station E3 in the central
Gulf of Finland, in autumn 2004. Bottom depth 89 m (Photo J. Hämäläinen/GTK)
17.5 Worst Scenario
In the Gulf of Finland, ferromanganese concretions are formed in rather shallow
water depths. Due to global warming, permanent anoxia in the Baltic Sea might fur-
ther expand in future. Thus, even the areas of Fe/Mn concretion growth might be
affected, which may cause dissolution of already formed concretionary matter. This
will further release more phosphorus into the water column as the concretions nor-
mally act as good phosphorus traps. The concentrations of phosphorus in the Baltic
Sea Fe/Mn concretions can be up to 2-3% (Winterhalter 2004 ) or even up to 7.16%
in the Gulf of Finland (Zhamoida et al. 1996 ) . It has been calculated that the con-
cretion fields of the eastern Gulf of Finland, only, contain more than 330,000 tons
of P 2 O 5 (Zhamoida et al. 2007 ) , which is equal to some 175,000 tons of elementary
phosphorus (Fig. 17.6 ) . If we speculate that all concretions would be dissoluted in
extremely anoxic conditions, this new phosphorus input would strongly contribute
to eutrophication and a further seafloor desertification of the Gulf of Finland, a
situation probably never seen before during postglacial times. Important to remem-
ber in this scenario are also the heavy metals, which normally are well trapped
in the concretions in rather high concentrations. According to Emelianov ( 2004 ) ,
it seems that especially the concretions on the shallow bottoms (27-53 m) have
high heavy metal concentrations. The concentrations of most metals, except cop-
per, are 1.5-5 times higher in the shallow Gulf of Finland concretions compared
to average concentrations in the Gulf of Finland seafloor surface gyttja clays (cf.
Vallius and Leivuori 1999 , 2003 ) . During extreme anoxia, the heavy metals incorpo-
rated in the dissolving concretions would be released and their concentrations would
rapidly increase in the near-bottom waters. The near-bottom waters would then be
overloaded with nutrients as well as thousands of tons of heavy metals (Zhamoida
et al. 2004 , Emelianov 2004 ) .