MAP Technical Reports Series No. 106 UNEP

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6.2.1 Effect of oxygen deficiency on the benthic communities in the Gulf of Trieste
The detailed in-situ observations in the Gulf of Trieste in 1983 (Stachowitsch, 1984)
were able to detect individual behaviour patterns and the sequence of mortality of benthic
species due to oxygen deficiency, and will be used as a basic framework for comparing data of
benthic community deterioration in the seas.
The animal groups chosen include sponges, polychaetes, anemones, bivalves,
crustaceans, echinoderms and fish.
Sponges
A characteristic feature in the Gulf of Trieste was a mucus layer covering sponges and
other sessile macroepifauna organisms. All sponges had died within the first two days after the
onset of community deterioration. Sponges (35% of total biomass in the community investigated)
are a major component of the multi-species clumps typical of the deeper Gulf.
Polychaetes
The large polychaetes Eunice aphroditois and Dasybrancus caducus emerged on the
second and third days after the first signs of community stress: most individuals were dead on
day 4.
Anemones
Both in the Gulf of Trieste and in the Limfjorden, anemones were the most resistant to
oxygen depletion. Calliactis parasitica, normally found attached to hermit crab shells in the gulf,
detached on days 3 and 4 and could be observed lying on the sediment with tentacles extended.
After one week, however, virtually all species, including the large Cerianthus sp., had died.
Bivalves
In the Gulf of Trieste the first signs of stress in bivalves were pointed out in Cardium sp.
accompanied by extension of the siphons. Similar behaviour was recorded in Limfjorden, where
the siphons of Cardium edule and Syndosmya alba stretched a few cm above the sediment,
those of Mya arenaria 10-20 cm above the black mud (Jørgensen, 1980). This was followed by
the critical condition of these species (the siphons of Mya extending 20-30 cm above the
bottom). These bivalves were considered to be able to survive for an additional week in this
state. However, substantial early losses of Mytilus edulis were reported in dense beds on the
soft mud due to extremely high respiration even under normal conditions, coupled with H 2S
sensitivity.
Crustaceans
In addition to the early mortality of small shrimp and the crabs Pilumnus spinifer and
Pisidia longicornis associated with sponges, a number of larger forms were affected both in the
Gulf and in the other areas under consideration here.
The first signs of unusual conditions were various reports of lobster (Homarus
americanus) leaving their shelters and gathering on the highest parts of shipwrecks, a
phenomenon paralleled by thousands of dead lobsters outside their dens and aggregated on the
highest parts of outcrops in the North Adriatic Sea (Stefanon and Boldrin, 1982).