Oxygen crisis in the Northern Adriatic Sea

Eutrophication - an over-enrichment of nutrients - is increasingly recognized as a key threat to shallow marine coastal seas worldwide. The UNEP Global Environment Outlook Yearbook (UNEP 2004) put the resulting "dead zones" in the world`s oceans at the top of its list of emerging environmental challenges. The Northern Adriatic Sea provides a case study for the effects of eutrophication, including oxygen crises and extensive "marine snow" events that threaten organisms on the seafloor. The present project is designed to study the key symptoms and effects of oxygen deficiencies on the behaviour and mortality of these bottom-living organisms. The Department of Marine Biology (University of Vienna) will apply its long-term expertise in underwater experimentation to develop a state-of-the-art underwater device that will automatically document the effect of artificially induced and natural oxygen crises on the macroepifauna using oxygen sensors and a time-lapse camera. The community in the Gulf of Trieste is dominated by aggregated filter- and suspension-feeding organisms termed multi-species clumps or bioherms. We will analyze different types of bioherms and their reactions to low-oxygen conditions. This will yield a catalog of behavioral responses and mortality sequences which, when combined with information on community structure, will allow a better evaluation of the status and stability of the benthic system in the Northern Adriatic Sea. The study will include a cooperation with the Marine Biology Station in Piran, Slovenia, and will also provide useful predictive tools for the coastal management sector.

Of the many threats facing marine ecosystems, eutrophication - along with low dissolved oxygen events as its main symptom - is the only form of pollution that destroys entire coastal ecosystems. Today, more than 400 so-called dead zones suffering from regular or intermittent hypoxia (oxygen deficiency) or anoxia (no oxygen) have been identified worldwide. The North Adriatic Sea is one such area, and the present project is the logical continuation of decades of research by the Univ. of Vienna on the benthic communities here. The timing and magnitude of anoxia and benthic mortalities are poorly predictable and therefore difficult to study. This project was therefore designed to experimentally generate small-scale anoxia (0.25 m 2 /experiment) and document the behaviour of organisms on the sea floor in the Gulf of Trieste, Slovenia. The central element is the newly designed Experimental Anoxia Generating Unit (EAGU), a plexiglass chamber with an instrument lid consisting of a time-lapse camera, flashes, battery packs and sensors for oxygen, hydrogen sulphide, temperature and pH. This device is placed on aggregations of benthic organisms in 24 m depth. The project was highly successful in that it created anoxia and mimicked organismic responses as known from earlier oxygen depletion events. A total of 13 experiments ranging from 3-6 days yielded more than 10,000 photographs. For the first time, we were able to record the full range of behaviours for Adriatic key species from the onset of oxygen decline, including many previously undocumented ones, as well as mortality sequences. These reactions correlated precisely with sinking oxygen levels provided by the sensor data. Using the categories `beginning, moderate, and severe hypoxia` as well as `anoxia`, we clearly identified more tolerant (gastropods, anemones) and more sensitive (crustaceans) species. We also recorded the behaviour of many normally not visible infauna species, which emerged from their burrows onto the sediment surface. The unique aspect is that these experiments took place in the natural community setting, revealing intra- and interspecific reactions with the full complement of macrobenthic species. One unexpected results was that, at very low oxygen concentrations, certain more tolerant anemone species preyed on more sensitive species (brittle stars, crustaceans). This approach will allow us, based on different sensitivity to oxygen deficiency, to compile a catalogue of behaviours and to interpret the post-anoxia community composition on the sea floor. This project is a key step forward in providing advice to coastal managers, policy- and decision-makers about the status of these ecologically and commercially important shallow coastal seas, both in the Adriatic and worldwide. (website: www.marine- hypoxia.com)