BSEP116B Biodiversity in the Baltic Sea - Helcom

3.3 Zooplankton communities
Zooplankton form an important link in the ecosystem
by transferring energy from primary producers
to fish. Zooplankton are suitable as indicators of
pelagic ecosystem conditions because, in addition
to their key role in energy transfer, they are
short-lived (maximum 1 year) and directly affected
by changes in hydrography. Hence, changes in
zooplankton assemblages are reflected directly and
rapidly in fish growth and stock conditions. Factors
that influence zooplankton may be bottom-up
processes, in which abiotic factors such as salinity
or temperature regulate the food web from lower
to higher levels. On the other hand, zooplankton
may also be affected by top-down processes,
whereby predators such as sprat or herring control
their abundance. These processes at higher levels
of the Baltic pelagic food web have been demonstrated
recently, also highlighting the top-down
control (e.g., Flinkman et al. 1998, Möllmann &
Köster 1999, Möllmann et al. 2005, Casini et al.
2006).
copepods, different meroplanktic larvae), or macroplankton
>2 mm (mysids, juvenile fish). Almost
800 ciliate species are found in open and coastal
waters of the Baltic Sea and the open Baltic also
harbours about 200 meso- and macrozooplankton
species (Telesh et al. 2008). Mesozooplankton is
the dominant group in the Baltic Sea in terms of
biomass.
Crustacean mesozooplankton, i.e., copepods and
cladocerans, are of particular interest because they
are key components of the pelagic food web, and
form the staple diet of planktivorous fish. Altogether,
there are fewer than 15 copepod genuses
and fewer than 20 cladoceran genuses present
in the Baltic Sea. As these species have different
adaptation capabilities in relation to salinity and
temperature, their distribution differs considerably.
A general trend of decreasing marine species
and increasing freshwater species towards the
north and east can be observed. A comprehensive
description of the Baltic Sea zooplankton is available
in Telesh et al. (2008).
There are no targets of the Baltic Sea Action
Plan (BSAP) that directly address zooplankton.
However, several targets are linked to the state of
the zooplankton community, primarily the overaching
goal “By 2021 all elements of the marine
food webs, to the extent that they are known,
occur at natural and robust abundance and
diversity”. In addition, all fish are planktivorous
as larvae, and some species remain as such all
their lives. This is the case for Baltic herring and
sprat, both of which are essential food sources
for salmon and gadoid fish in the Baltic. The
BSAP also aims at 50–80% of potential production
of wild salmon in river populations by 2015.
Furthermore, viable Baltic cod populations in their
natural distribution area should be reached by the
same year. This will be difficult to achieve if highenergy
marine copepods continue to decline, as
discussed in this section.
3.3.1 Status and trends
The Baltic Sea zooplankton community is a mixture
of neritic and euryhaline marine species and purely
freshwater species. Zooplankton are often classified
by size as nanoplankton 2–20 µm (flagellates),
microplankton 20–200 µm (ciliates, small rotifers),
mesoplankton 0.2–2 mm (big rotifers, cladocera,
Of copepods, truly marine species such as the
large-size genus Calanus do not occur in the
Baltic Sea, while in the North Sea, Northeast
Atlantic, and Polar Seas they are key species
as food for planktiovorous fish. Marine copepods
found in the Baltic are neritic, coastal, and
estuarine small-sized species, which all share
a common feature: the ability to adapt to low
salinity. The most pronounced marine species are
Pseudocalanus acuspes and P. elongatus, which
thrive in deep, saline water below the halocline.
Other neritic species are Centropages hamatus
and Temora longicornis, which are less dependent
on sub-halocline habitat than Pseudocalanus spp.
Acartia and Eurytemora are coastal and estuarine
marine species, but have even better adaptation
capabilities to the freshwater environment. Limnocalanus
macrurus is a stenothermic freshwater
species occurring in low-salinity deep water in
the Gulf of Finland, Åland Sea and Bay of Bothnia
ecosystems.
Most cladoceran species are of freshwater origin,
except for genera Evadne, Podon and Pleopis,
which are found in coastal ecosystems of adjacent
marine environments. A recent invader, Cercopagis
pengoi from the Pontocaspian region, is a brackishwater
species.
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