EPIFAUNA-DOMINATED BENTHIC SHELF ASSEMBLAGES AND OXYGEN CRISES: LESSONS FROM THE MODERN ADRIATIC SEA

The northern and central Adriatic shelf serves as a rare modern equivalent to typical Palaeozoic and Mesozoic epeiric seas. It extends over more than 300 km and the northern part has an average water depth of only ca. 30 m. Distinct oceanographic features include seasonally high productivity and a stable thermocline, where mixing of the water body is mostly due to episodic storms of variable intensity. These characteristics, intensified by ongoing anthropogenic eutrophication, result in frequent bottom water oxygen deficiencies and mass mortalities of benthic organisms, which have also been suggested as typical features of ancient epeiric seas.

The present project focuses on the northernmost part of the Adriatic Sea, the Gulf of Trieste, whose benthic fauna is characterized by Palaeozoic-style benthic ecology of high-biomass, epifaunal, sedentary suspension feeders (including sponges, ascidians and anemones). This peculiar epifauna is developed on densely burrowed muddy soft bottoms, where it colonizes shell grounds to form bioherms. These in turn are a habitat for numerous other invertebrates (e.g., ophiuroids, echinoids, crabs, molluscs) and fish. This unique setting serves as a case study for behavioural modifications and mortalities of the benthic fauna during hypoxia and anoxia.

The occurrence of such ecosystem collapses is difficult to fully document in the field. We therefore deployed a specially developed underwater chamber – equipped with time-lapse camera, flashes and sensors – to artificially induce and analyse anoxia in situ. The photo documentation, combined with oxygen- and hydrogen sulphide data, yields a catalogue of behavioural responses and mortality sequences of epifauna and infauna. First results indicate that, with decreasing oxygen content, most animals try to reach higher substrates. However, the various taxa show different susceptibilities to oxygen deficiencies. Regular and irregular echinoids, ophiuroids and small crustaceans, for example, are very sensitive to hypoxia, whereas ascidians, anemones and large gastropods show much higher levels of resistance. We conclude that the development of this peculiar epifauna is related to seasonal high productivity, and that retreat from the sediment-water interface to more elevated structures helps to survive hypoxia-related risks.