In the Baltic Sea, oxygen-depleted areas are widespread and have
increased in a relatively short period of time. In a laboratory
experiment, microcosms with natural soft-bottom communities were
exposed to (a) hypoxia (0.2 mg O-2/1) (b) hypoxia (0.2 mg
O-2/1)/normoxia (11-12 mg O-2/1) altered on a weekly basis (c) normoxia
for two months. Multidimensional scaling ordination separated all
treatments distinctly with communities in normoxia very similar to
field controls but different from hypoxic treatments. On the basis of
this experiment, we group the species into 'very sensitive': the
amphipod Monoporeia affinis, harpacticoids Microarthridion littorale
and Pseudobradya sp. were totally wiped out in pulses of hypoxia,
'sensitive': the surface dwelling nematode species Axonolaimus spinosus
and the meiofauna groups Oligochaeta and Turbellaria were found in
reduced abundance in the hypoxic treatments, 'less sensitive': Macoma
balthica, the ostracod species Paracyprideis fennica and the nematode
species Calomicrolaimus honestus, Paracanthonchus spp., survived
temporary hypoxia, but not permanent hypoxia, acid 'tolerant': the
ostracod species Heterocyprideis sorbyana, Candona neglecta, the group
Kinorhyncha, and the nematode species Leptolaimus elegans, L.
papilliger, Sabatieria pulchra were all tolerant to hypoxic conditions.
The different responses of the major species and taxa to long- and
shea-term hypoxic events, may be used as a tool in estimating the
duration of naturally occurring hypoxia. (C) 1998 Elsevier Science BN.
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