Low dissolved oxygen events in the Adriatic

In the marine environment, no other environmental variable has changed more dramatically in recent decades than the dissolved oxygen (DO) content. Today, nearly 415 eutrophic and hypoxic coastal systems have been identified worldwide, affecting a total area of more than 245,000 km. Regular, severe oxygen deficiencies since the mid- 1970s in the Northern Adriatic have had profound consequences on the ecosystem. Even though low DO events are increasing here and elsewhere, their onset and extent remain difficult to predict and study. Rapid mortality events further hinder comprehensive documentation. In the framework of an ongoing precursor Austrian Science Fund (FWF) project (P17655-B03), we developed an underwater-chamber (EAGU; Experimental Anoxia Generating Unit) equipped with camera, flashes and a sensor array. This system autonomously generates small-scale oxygen deficiencies and quantifies benthic responses. The present project plans to expand upon the current EAGU approach, which was designed to focus on the well- developed macroepifauna. We will attempt to incorporate key representatives of the macroinfauna and meiofauna (foraminiferans and harpacticoid copepods) and will include sediment geochemistry. We will also take the EAGU concept one step further by evaluating post-anoxia developments in both the macroepifauna and meiofauna. This is a unique effort to examine detailed responses at all relevant levels - individuals, species, functional groups, community - under natural conditions. This study is a cooperation between the Dept. of Marine Biology (Faculty of Life Sciences) and the Dept. of Paleontology (Faculty of Earth Sciences), both Univ. of Vienna. We have also assembled a top-notch team of international collaboration partners (Marine Biology Station Piran, Slovenia; University of Gent, Belgium; University of Angers, France; University of Plymouth, UK) who will help work up and interpret the samples and images/films. This project will be a significant contribution to understanding anoxia as a key, and growing, threat to shallow coastal ecosystems. It will put an ecological framework on past hypoxia-tolerance experiments and contribute to the ongoing debate on valid tolerance thresholds and indicator organisms. The project will also add concrete input into the discussion - both in the current and historical context - on the issue-complex of climate change, eutrophication and biodiversity loss. Our multidisciplinary results will be of interest to marine researchers, environmental scientists and policy makers in the increasing number of regions affected by dead zones worldwide. The results can be used by coastal managers to make informed decisions on the status of the affected marine habitats, help formulate strategies to avoid catastrophic mortalities and promote recovery, and provide criteria for determining the location and borders of protected areas. Finally, mariculture enterprises in Slovenia and immediately adjoining Croatian and Italian waters can directly benefit by being able to better judge the local impact of oxygen crises on their ecosystem, organisms and livelihoods.

The worlds oceans face an increasingly long list of threats. Only one form of pollution eutrophication (over-enrichment with organic wastes) is actually causing entire marine ecosystems to collapse today. The most clear-cut symptom of eutrophication is oxygen deficiency. This kills off the animal communities in large areas of the sea floor. Today, more than 500 such so-called dead zones are known worldwide, one of which is the northern Adriatic Sea.Our project used a specially designed instrument to test the effect of such oxygen crises on the sea floor in the Adriatic Sea. This experimental approach was necessary because such ecosystem collapses are difficult to predict and their occurrence and development can only rarely be documented. Our project successfully created oxygen crises on small areas of the bottom (50 x 50 cm) in 24 m depth. We were able to document the effect on the chemistry of the sediment, on two groups of microscopic animals (copepods and foraminiferans), and on the full range of larger organisms that make up the animal community here (including sponges, brittle stars, sea squirts). Finally, we documented the events immediately after such oxygen crises (scavenging and predation of the killed organisms) and examined the recovery process for up to 2 years after such events. We used a broad range of techniques from sediment samples and sediment chemistry analyses to time-lapse photography. The results showed that the changes in the sediment were also influenced by decaying organisms on the sediment surface, that the copepods were very sensitive to lack of oxygen (with only one group being able to survive for up to 2 weeks), and that the foraminiferans were much more tolerant, with some surviving even 10 months of no oxygen and co-occurring toxic hydrogen sulfide. We were able to document a full range of behaviours and the sequence of death of the larger fauna. After such events, the sequence of scavengers/predators that arrived was fishes, hermit crabs and snails. The unique feature of our project is that our experiments were done in the natural environment and in the community setting. We were unable to record any recovery of the bottom fauna on our experimental plots even after 2 years. This underlines that the destruction of the animal communities through oxygen deficiency is very fast (days), but that recovery is very long (years to decades). This supports the classification of such coastal communities as sensitive and underlines the necessity to take action to ensure that further disturbances, such as destructive commercial fishing, do not compound the damage and prolong the recovery times.The results have been published in several scientific journals, in one published and another upcoming book chapter as well as in a special issue of the journal Biogeosciences (9 contributions from researchers in 6 countries in this international multi-disciplinary effort). Our website: www.marine-hypoxia.com