Zooplankton of the open Baltic: Extended Atlas - IOW

1. INTRODUCTION
Since the last glaciation, the Baltic Sea has undergone several
evolutionary stages between a huge marine bay and a large freshwater lake
during which a number of different ecological systems developed and were
successfully replaced in this area (Jansson, 1972).
Today the Baltic Sea is the largest brackish water area in the world
characterized as a temperate shelf sea with permanent salinity stratification, a
horizontal salinity gradient and low water turnover of 35 years. It is a shallow
sea with a mean depth of 62.1 m, the greatest depth 459 m, an area of 415,266
km 2 (Baltic Proper itself is 211,069 km 2 ), a volume of ca. 22,000 km 3
(HELCOM, 2001; Wulff et al., 2001; Schiewer, 2008). The presence of
shallow sills at the western inlets causes stable water stratification. The Baltic
can be best compared to a stratified fiord with a rich supply of fresh water
from the rivers.
Due to the humid climate, there is an estuarine circulation with outflow
of low-saline water above the halocline and powerful periodic injections of
North Sea water below the halocline which greatly affect the salinity of the
deep water layers.
The location of the Baltic Sea in the northern high latitudes means a
pronounced seasonality in temperature and light regime. The vegetation
season lasts longer in the southern areas than in the north of the sea. Ice
coverage is occasional in the southern Baltic. The average temperature of the
surface waters in summer is 16° C, and a thermocline is formed at the depth
of 10 to 30 m. The water below thermocline is usually colder than 7-8° C
during the whole year. The water stratification delimits vertical exchange of
water masses and pelagic organisms thus creating layers with different
ecological characteristics.
In terms of salinity, the regime ranges between oligohaline (0.5 PSU)
and mesohaline (18 PSU) conditions, with an average of 7-8 PSU in the major
open Baltic waters (HELCOM, 2001). Climate change and decadal scale
variability of these parameters modifies the hydrographic characteristics
accordingly (BACC, 2008; Feistel et al., 2008).
Since the end of the XIX century many authors have tried to divide the
different Baltic Sea areas with respect to hydrographical conditions, and their
ideas have differed greatly in many cases (Ekman, 1931, and Wattenberg,
1949, cited after Ackefors, 1969).
In the present study we use the following names of the different subareas
of the Baltic Sea and a classification which is largely based on the
division of the sea made by Ackefors (1969):
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