Gotland Sea

The first cmises were limited to sea areas between the Fehmarnbelt and the island of Bornholm until the summer of 1953. Later, some expeditions as forerunners of the seasonal cruises were extended to the Gotland Sea beginning in May 1955 and to the Gulf of Finland beginning in November 1955. The transition area between the North Sea and the Baltic was included in March 1959 by measurements with r/v Joh. L. Kruger in the Kattegat and the Skagerrak. 

The frequency of ice occurrence in the outer sea is 10-20% from about 57 N northward, 25% from about 58 N, and 50% on the northern and northeastern margins of the area. The frequency of ice occurrence in the other, southerly part of the Gotland Sea is extraordinarily low. Only a combination of several unfavorable factors (particularly low salinity, particularly low heat storage, long-term inflow of extremely cold air masses from the east as early as December and through March, high radiation of heat) can lead to ice formation in this area. During the past century, there have been only three winters with major ice formation in the entire sea area (1939/1940, 1940/1941, 1946/1947), and within the past 50 years only two winters (1955/1956 and 1986/1987) with some drift ice occurrence, which is a frequency clearly below 5%. 

In the southern and middle parts of Kalmar Sound including the coasts, ice forms in over 75 % of all winters. In the northern part of the Sound and at the coast of East Sweden up to the northern boundary of the western Gotland Sea, ice formation occurs in 60% of winters, and in all sheltered areas as well as in the fairways leading inland in 70-95% of winters. On the west coast of Gotland, ice forms in about 20% of all winter seasons. 

The average number of days with ice per winter is 20-30 in the southern and northern inshore waters of Kalmar Sound, and 40-50 days in the largest part of the Sound. At the northern coast of East Sweden, up to the northern boundary of the Gotland Sea, about 20 days of ice is to be expected, 40-50 days in all sheltered waters, and 50-75 days per winter in fairways extending far inland (e.g., Oxelosund, Stockholm). In extreme ice seasons, the maximum number of days of ice in all areas exceeds 60, in sheltered areas over 100, and from 120 to 130 days in the fairways to inland harbors. At the west coast of Gotland, maximally 70 days of ice has been recorded in winter. 

In Siegel et al. (2006), the regional differences were additionally discussed separately for the winter and summer months. In the northern Baltic Sea, particularly in Bothnian Bay, the SST every year in February is below 0°C and the water is often covered by ice. From the western part (Pl) to the northern Gotland Sea (P16), the mean SST is around 2°C. The warmest February was in 1990 with SST between 3 C and 4 C in the main parts. The coldest was in 1996 in the southwestern Baltic and in 2003 in the northern Baltic. The... 

January Februgty March April May June Jnly August September October November December FIGURE 9.8 Slope of trends of different months (1990-2005, upper panel) in regions Mecklenburg Bight (3), Arkona Sea (6), Gotland Sea (15), Bothnian Sea (19), and Bothnian Bay (23). 

TABLE 9.4 Correlation between SST and Climate Indices NAO, AO, and WIBIX for the Arkona Sea (6), Bornholm Sea (10), Eastern Gotland Sea (15), and Bothnian Sea (19) for March (Coefficients of Determination... 

A correlation analysis between the winter SST and the climate indices NAO, WIBIX, and AO showed the highest coefficients of determination of 0.86 for the Arkona Sea between SST (March) and NAO and WIBIX. They were slightly reduced for the Bornholm Sea and eastern Gotland Sea. The atmospheric forcing of the entire winter season drives the water temperature of the Baltic Sea with minimum temperature in March. Therefore, the best correlation was found for the SST of March compared to the winter season. 

TABLE 12.6 Winter Nutrient Concentrations in the Mixed-Surface Layer (0-10 m) in the Eastern Gotland Sea (January-April Six Stations)—Based on HELCOM, lOW, and SMHI Data (Nausch et al., 2004)... 

Pohl, C., Loftier, A., Schmidt, M., Seifert, T, 2006. A trace metal (Pb, Cd, Zn, Cu) balance for surface waters in the eastern Gotland Sea, Baltic Sea. Journal of Marine Systems, 60, 381-395. 

FIGURE 14.11 Vertical flux of particulate organic nitrogen into a sediment trap located at 180-m water depth at station BMP J1 in the central Gotland Sea. Note that in the first 2 years some periods are missing because of initial technical failures of the instrument. 

The horizontal distribution of the cyanobacteria blooms can be followed well by satellite imagery (e.g., Kahrti et al., 1994). In summer 2005, the bloom covered the entire Baltic Proper, but the western parts were excluded. In summer 2006, the bloom started as usual in the northern Gotland Sea, but was terminated very fast in the central Baltic and proceeded into the southern and western Baltic. Both bloom developments are discussed in detail in Section 15.4.2. 

Bornholm and Gotland Seas for the benefit of dinoflagellates (see Section 15.5.1). 

FIGURE 15.6 Seasonal patterns of chi a (representative monthly means of decades or a half decade) in (a) Mecklenburg Bight (n = 229), (b) Arkona Sea (n = 746), (c) Bornholm Sea (n = 436), and (d) Eastern Gotland Sea (n = 339), based on averages of 0-10 m depth. Note that data from June 2000-2005 and some from September 2000-2005 are missing. 

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). 

Spring Bloom The main trend observed by Wasmund and Uhlig (2003) was the significant reduction in the share of diatoms in the spring blooms in many areas northern Baltic Proper, Gotland Sea, Belt Sea, and Kattegat. In the southern Baltic Proper, the... 

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