Zooplankton of the open Baltic: Extended Atlas - IOW

• Baltic Proper – the area east of the Belt Sea and the Sound,
limited at the north by the Åland Sea and the Archipelago Sea, at
the east – by the Gulf of Finland;
• Western Baltic Sea includes Kiel Bight and Mecklenburg Bight;
• Northern Baltic Sea includes the Åland Sea, the Archipelago
Sea, and the Gulf of Bothnia;
• Southern Baltic Sea – the area of Gdansk Basin;
• Eastern Baltic Sea is attributed to the Gulf of Riga and the Gulf
of Finland.
This is a very general division of the Baltic Sea; moreover, it is defined
to a great extent by the availability of published data on zooplankton species
composition.
As any other attempt to classify the natural systems, including water
bodies, the proposed subdivision of the Baltic Sea is largely conventional and
the real borders between the areas mentioned above do not exist. This is
especially true for the pelagic communities that may be driven by water
masses to significant distances.
The shallowness and the consequently vast area occupied by the coastal
ecosystems in the Baltic Sea (Schiewer, 2008) are the major reasons for the
intensive mixture of the coastal and open-water plankton communities and for
the penetration of brackish water, euryhaline and also freshwater species of
zooplankton far into the open Baltic waters (Telesh et al., 2008a). Thus, the
strict definition of the “open Baltic Sea” in respect to the pelagic fauna can
hardly be given.
Due to peculiarities of the salinity regime, the pelagic ecosystem
component in the Baltic Sea consists mainly of plankton communities
dominated by euryhaline species. The organisms in the Baltic are well
adapted to the brackish water environment, but only a few true brackish water
species have developed here. The present species composition is a result of
the selection process, where organisms with a high osmotic resistance have
been able to survive.
Since the publication of the “species minimum curve” by Remane
(1934, 1940), it has been generally accepted that “the number of species in the
Baltic is small” (Jansson, 1972, p. 12). This conclusion was commonly
applied to and supported mainly by the data on benthic macrofauna
(Zenkewitch, 1963). Meanwhile, already in the 1960-s Hans Ackefors
proposed that “if the microfauna in the water and at the bottom are
included the number of species will be much higher” (Ackefors, 1969, p.
5). In other words, according to an exceptionally evocative affirmation of
Jansson (1972), “the diversity is there but it is found on a microscale with a
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