Investigating hypoxia in aquatic environments: diverse approaches to addressing a complex phenomenon

In this paper we provide an overview of new knowledge on oxygen depletion(hypoxia) and related phenomena in aquatic systems resulting from the EU-FP7project HYPOX ("In situ monitoring of oxygen depletion in hypoxicecosystems of coastal and open seas, and landlocked water bodies",http://www.hypox.net). In view of the anticipated oxygen loss in aquatic systemsdue to eutrophication and climate change, HYPOX was set up to improvecapacities to monitor hypoxia as well as to understand its causes andconsequences.
Temporal dynamics and spatial patterns of hypoxia were analyzed in fieldstudies in various aquatic environments, including the Baltic Sea, the BlackSea, Scottish and Scandinavian fjords, Ionian Sea lagoons and embayments,and Swiss lakes. Examples of episodic and rapid (hours) occurrences ofhypoxia, as well as seasonal changes in bottom-water oxygenation instratified systems, are discussed. Geologically driven hypoxia caused by gasseepage is demonstrated. Using novel technologies, temporal and spatialpatterns of water-column oxygenation, from basin-scale seasonal patterns tometer-scale sub-micromolar oxygen distributions, were resolved. Existingmultidecadal monitoring data were used to demonstrate the imprint of climatechange and eutrophication on long-term oxygen distributions. Organic andinorganic proxies were used to extend investigations on past oxygenconditions to centennial and even longer timescales that cannot be resolvedby monitoring. The effects of hypoxia on faunal communities andbiogeochemical processes were also addressed in the project. Aninvestigation of benthic fauna is presented as an example ofhypoxia-devastated benthic communities that slowly recover upon a reductionin eutrophication in a system where naturally occurring hypoxia overlapswith anthropogenic hypoxia. Biogeochemical investigations reveal that oxygenintrusions have a strong effect on the microbially mediated redox cycling ofelements. Observations and modeling studies of the sediments demonstrate theeffect of seasonally changing oxygen conditions on benthic mineralizationpathways and fluxes. Data quality and access are crucial in hypoxiaresearch. Technical issues are therefore also addressed, including theavailability of suitable sensor technology to resolve the gradual changes inbottom-water oxygen in marine systems that can be expected as a result ofclimate change. Using cabled observatories as examples, we show how thebenefit of continuous oxygen monitoring can be maximized by adopting properquality control. Finally, we discuss strategies for state-of-the-art dataarchiving and dissemination in compliance with global standards, and howocean observations can contribute to global earth observation attempts.