Abstract. In this paper we provide an overview of new knowledge on oxygen depletion (hypoxia) and related phenomena in aquatic systems resulting from the EU-FP7 project HYPOX ("In situ monitoring of oxygen depletion in hypoxic ecosystems of coastal and open seas, and landlocked water bodies", www.hypox.net). In view of the anticipated oxygen loss in aquatic systems due to eutrophication and climate change, HYPOX was set up to improve capacities to monitor hypoxia as well as to understand its causes and consequences.Temporal dynamics and spatial patterns of hypoxia were analyzed in field studies in various aquatic environments, including the Baltic Sea, the Black Sea, Scottish and Scandinavian fjords, Ionian Sea lagoons and embayments, and Swiss lakes. Examples of episodic and rapid (hours) occurrences of hypoxia, as well as seasonal changes in bottom-water oxygenation in stratified systems, are discussed. Geologically driven hypoxia caused by gas seepage is demonstrated. Using novel technologies, temporal and spatial patterns of watercolumn oxygenation, from basin-scale seasonal patterns to meter-scale sub-micromolar oxygen distributions, were resolved. Existing multidecadal monitoring data were used to demonstrate the imprint of climate change and eutrophication on long-term oxygen distributions. Organic and inorganic proxies were used to extend investigations on past oxygen conditions to centennial and even longer timescales that cannot be resolved by monitoring. The effects of hypoxia on faunal communities and biogeochemical processes were also addressed in the project. An investigation of benthic fauna is presented as an example of hypoxia-devastated benthic communities that slowly recover upon a reduction in eutrophication in a system where naturally occurring hypoxia overlaps with anthropogenic hypoxia. Biogeochemical investigations reveal that oxygen intrusions have a strong effect on the microbially mediated redox cycling of elements. Observations and modeling studies of the sediments demonstrate the effect of seasonally changing oxygen conditions on benthic mineralization pathways and fluxes. Data quality and access are crucial in hypoxia research. Technical issues are therefore also addressed, including the availability of suitable sensor technology to resolve the gradual changes in bottom-water oxygen in marine systems that can be expected as a result of climate change. Using cabled observatories as examples, we show how the benefit of continuous oxygen monitoring can be maximized by adopting proper quality control. Finally, we discuss strategies for state-of-the-art data archiving and dissemination in compliance with global standards, and how ocean observations can contribute to global earth observation attempts.

Benthic fluxes of nutrients and metals were measured in the coastal zone of the north-western Black Sea, which is influenced by the Danube and Dniestr rivers. The results from the benthic flux chambers deployed during two EROS 21 cruises in summer 1995 and in spring 1997 yield information on benthic nutrient cycling and diagenetic pathways at the sediment-water interface. This information is discussed in the light of benthic activity as well as pore-water data. The benthic recycling of nutrients varied seasonally due to the availability of oxygen and organic material, and spatially due to river influence. Areas of high benthic fluxes near-shore and of low benthic fluxes offshore on the shelf were distinguished. Nutrients and suspended particulate matter discharged by the Danube is kept nearshore by the coastal current. The oxygen concentrations in the bottom water varied strongly between the two seasons. In summer, the bottom water at the near-shore stations turned anoxic. In spring, the higher oxygen concentrations are related to more intense mixing of the water column due to stormy periods and high river discharge. Highest oxygen concentrations were found on the offshore continental shelf. However, the benthic oxygen consumption rates in spring (13-23 mmol m 2 day 1 ) were as high as in summer. Areas with highest nutrient concentrations in the overlaying bottom water were found at the Danube delta front and Danube prodelta. On the Danube delta front and the Dniestr mouth, ammonia fluxes were lower in spring (1·1-1·7 mmol m 2 day 1 ) than in the summer (2·6-4·4 mmol m 2 day 1 ) due to higher nitrification rates and the lower influx of organic matter. In spring, the concentrations of dissolved iron and manganese in the bottom water were one order of magnitude lower than during the summer. This decrease in flux rates is related to the higher oxygen concentrations in the bottom water in spring. A rough comparison of the river's nutrient load and the benthic recycling over a certain area showed that the near-shore benthic phosphate and silica recycling account for 50% and 35% of the Danube input in summer, respectively. The fluxes of ammonia from benthic recycling and from the Danube discharge were at the same order of magnitude in both seasons. Nitrogen is introduced by the rivers mostly as nitrate. Benthic recycling is the dominant source of ammonia. The Danube input is phosphate deficient. In brief, benthic nutrient recycling is an important factor in sustaining high productivity of the system.

The influence of the River Danube as a major source of nutrients and suspended solids to the continental shelf of the Black Sea has been analyzed. In the framework of the EC project EROS-2000, sediment cores from 33 stations on the northwestern continental shelf were sampled in August 1995. Spatial patterns in structural sediment characteristics, macrobenthos composition and benthic mineralization rates were examined using ordination techniques. Three general areas could be distinguished according to the distribution of abiotic sediment characteristics and macrobenthos community:(1) The area just in front of the Danube Delta where large amounts of nutrients and suspended solids are discharged. High sedimentation rates of fine-grained sediments and high benthic mineralization rates characterized this area. The macrobenthos community was dominated by deposit feeders.(2) The northern part of the continental shelf where an anticyclonic gyre is located. The majority of the Danube discharges are transported to this region. This area was characterized by low sedimentation rates. However, the deposited material contained a larger fraction of fresh organic matter compared to the delta area, resulting in high benthic mineralization rates. Suspension feeders dominated the macrobenthos community. (3) The southern part of the continental shelf was characterized by low sedimentation rates and low rates of benthic mineralization. In this area suspension feeders dominated the macrobenthos community. Oxygen fluxes into the sediment ranged between 2 and 52 mm01 0' m-' d-' (average 21 mm01 O2 m-' d-l) decreasing with water depth. Macrobenthos accounted for 20% of the total benthic oxygen consumption. In the northern part of the continental shelf and in the coastal stations, microorganisms, and micro-and meiobenthos dominated benthic community respiration, while macrobenthos became relatively more important in terms of oxygen consumption in the southern part of the continental shelf.

In this paper we synthesize the new knowledge on oxygen and oxygen-related phenomena in aquatic systems, resulting from the EU-FP7 project HYPOX ("In situ monitoring of oxygen depletion in hypoxic ecosystems of coastal and open seas, and land-locked water bodies", www.hypox.net). In view of the anticipated oxygen loss in aquatic systems due to eutrophication and climate change, HYPOX was set up to improve capacities to monitor hypoxia as well as to understand its causes and consequences. Temporal dynamics and spatial patterns of hypoxia were analysed in field studies in various aquatic environments, including the Baltic Sea, the Black Sea, Scottish and Scandinavian fjords, Ionian Sea lagoons and embayments, and in Swiss lakes. Examples of episodic and rapid (hours) occurrences of hypoxia as well as seasonal changes in bottom-water oxygenation in stratified systems are discussed. Geologically-driven hypoxia caused by gas seepage is demonstrated. Using novel technologies, temporal and spatial patterns of water-column oxygenation, from basin-scale seasonal patterns to meter-scale submicromolar oxygen distributions were resolved. Existing multi-decadal monitoring data were used to demonstrate the imprint of climate change and eutrophication on long-term oxygen distributions. Organic and inorganic proxies were used to extend investigations on past oxygen conditions to centennial and even longer timescales not resolved by monitoring. The effects of hypoxia on faunal communities and biogeochemical processes were also addressed in the project. An investigation of benthic fauna is presented as an example of hypoxia-devastated benthic communities that slowly recover upon a reduction in eutrophication in a system where natural and anthropogenic hypoxia overlap. Biogeochemical investigations reveal that oxygen intrusions have a strong effect on microbially-mediated redox cycling of elements. Observations and modeling studies of the sediments demonstrate the effect of seasonally changing oxygen conditions on benthic mineralization pathways and fluxes. Data quality and access are crucial in hypoxia research. Therefore, technical issues are addressed, including the availability of suitable sensor technology to resolve gradual changes in bottom-water oxygen that can be expected as a result of climate change in deep-sea waters. Using cabled observatories as examples, we show how the benefit of continuous oxygen monitoring can be maximized by adopting proper quality control. Finally, we discuss strategies for state-of-the-art data archiving and dissemination in compliance with global standards and how ocean observations may contribute to global earth observation attempts

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