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

In this paper we provide an overview of newknowledge on oxygen depletion (hypoxia) and related phenomenain aquatic systems resulting from the EU-FP7project HYPOX (“In situ monitoring of oxygen depletion inhypoxic ecosystems of coastal and open seas, and landlockedwater bodies”, www.hypox.net). In view of the anticipatedoxygen loss in aquatic systems due to eutrophication and climatechange, HYPOX was set up to improve capacities tomonitor hypoxia as well as to understand its causes and consequences.Temporal dynamics and spatial patterns of hypoxia wereanalyzed in field studies in various aquatic environments, includingthe Baltic Sea, the Black Sea, Scottish and Scandinavianfjords, Ionian Sea lagoons and embayments, and Swisslakes. Examples of episodic and rapid (hours) occurrences ofhypoxia, as well as seasonal changes in bottom-water oxygenationin stratified systems, are discussed. Geologicallydriven hypoxia caused by gas seepage is demonstrated. Usingnovel technologies, temporal and spatial patterns of watercolumnoxygenation, from basin-scale seasonal patterns tometer-scale sub-micromolar oxygen distributions, were resolved.Existing multidecadal monitoring data were used todemonstrate the imprint of climate change and eutrophicationon long-term oxygen distributions. Organic and inorganicproxies were used to extend investigations on past oxygenconditions to centennial and even longer timescales thatcannot be resolved by monitoring. The effects of hypoxia onfaunal communities and biogeochemical processes were alsoaddressed in the project. An investigation of benthic fauna ispresented as an example of hypoxia-devastated benthic communitiesthat slowly recover upon a reduction in eutrophicationin a system where naturally occurring hypoxia overlapswith anthropogenic hypoxia. Biogeochemical investigationsreveal that oxygen intrusions have a strong effect on the microbiallymediated redox cycling of elements. Observationsand modeling studies of the sediments demonstrate the effectof seasonally changing oxygen conditions on benthic mineralizationpathways and fluxes. Data quality and access arecrucial in hypoxia research. Technical issues are thereforealso addressed, including the availability of suitable sensortechnology to resolve the gradual changes in bottom-wateroxygen in marine systems that can be expected as a result ofclimate change. Using cabled observatories as examples, weshow how the benefit of continuous oxygen monitoring canbe maximized by adopting proper quality control. Finally,we discuss strategies for state-of-the-art data archiving anddissemination in compliance with global standards, and howocean observations can contribute to global earth observationattempts