Responses of Baltic benthic invertebrates to hypoxic events

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. All rights reserved.