Observations in the Western Baltic Sea

FIGURE 6.1 Current observations in the western Baltic Sea. DAS, DRS, OB, and W26 are permanent stations. The sampling locations are shown with their mooring identifiers. Mooring identifiers for Fehmarnbelt observations (dotted box) are given in Fig. 6.2. 

Trends for Cd and Cu were already evaluated in the Third Periodic Assessment by Schneider (HELCOM, 1996) between 1980 and 1993. Taking into account the seasonal biogeochemical cycling of metals (Schneider and Pohl, 1996) as well as analytical quality assurance aspects, a decrease in the surface water concentrations of Cd and Cu at a mean trend of —7%/year (Cd) and —5%/year (Cu) in the Baltic Proper and the western Baltic Sea was calculated. Trends could not be detected for Zn, Pb and Hg as the database was insufficient. These results were supported by investigations of Kremling and Petersen (1984), Kremling and Streu (2000) who observed a significant decrease of potentially hazardous heavy metals (5-6%/year for Cd and Pb and 1.4—1.8%/year for Cu and Zn) in Baltic Sea surface waters between two transects in 1982 and 1995. 

Satellite images of chi a are also used to observe the seasonal phytoplankton development. The maximum occurrence of cyanobacteria was determined in August in the period 1979-1986 (Siegel et al., 1999). fhe SeaWiFS time series (1998-2004) document that in several recent years, such as 1999, 2001, 2002, and 2003, the maximum was observed already in July. The differences in the temporal course, as well as in the spatial development of cyanobacteria blooms can be shown by comparisons of 2005 and 2006. In summer 2005, the maximum intensity was reached on July 13,2005, the warmest day of the year (23-25 °C) in the central Baltic. In summer 2006, the bloom started also in early July in the northern Gotland Sea, but disappeared quickly there and grew after mid-July mainly in the southern and western parts until the end of August (Fig. 15.7). 

However, two different aspects must be distinguished with respect to the decline of Fucus stands. Disappearance of shallow water stands are in most of the cases related to ice scouring or exposition to mechanical stress in general, so loss of these shallow water stands must be separated from changes in depth distribution (Eriksson, 2002 Schories et al., 2006a). With respect to shifts in depth distribution as observed especially in the western parts of the Baltic Sea and the Baltic proper, a link to changes in the nutrient regime are very likely. 

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