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.

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We present and compare small sediment-water fluxes of O 2 determined with the eddy correlation technique, with in situ chambers, and from vertical sediment microprofiles at a 1450 m deep-ocean site in Sagami Bay, Japan. The average O 2 uptake for the three approaches, respectively, was 1.62 ± 0.23 (SE, n = 7), 1.65 ± 0.33 (n = 2), and 1.43 ± 0.15 (n = 25) mmol m -2 d -1 . The very good agreement between the eddy correlation flux and the chamber flux serves as a new, important validation of the eddy correlation technique. It demonstrates that the eddy correlation instrumentation available today is precise and can resolve accurately even very small benthic O 2 fluxes. The correlated fluctuations in vertical velocity and O 2 concentration that give the eddy flux had average values of 0.074 cm s -1 and 0.049 µM. The latter represents only 0.08% of the 59 µM mean O 2 concentration of the bottom water. Note that these specific fluctuations are average values, and that even smaller variations were recorded and contributed to the eddy flux. Our findings demonstrate that the eddy correlation technique is a highly attractive alternative to traditional flux methods for measuring even very small benthic O 2 fluxes.

On the basis of in situ NO { 3 microprofiles and chamber incubations complemented by laboratory-based assessments of anammox and denitrification we evaluate the nitrogen turnover of an ocean margin sediment at 1450-m water depth. In situ NO N 2 production was attributed to prokaryotic denitrification (59%), anammox (37%), and foraminifera-based denitrification (4%). Anammox thereby represented an important nutrient sink, but the N 2 production was dominated by denitrification. Despite the fact that NO { 3 stored inside foraminifera represented ,80% of the total benthic NO { 3 pool, the slow intracellular NO { 3 turnover that, on average, sustained foraminifera metabolism for 12-52 d, contributed only to a minor extent to the overall N 2 production. The microbial activity in the surface sediment is a net nutrient sink of ,1.1 mmol N m 22 d 21 , which aligns with many studies performed in coastal and shelf environments. Continental margin areas can act as significant N sinks and play an important role in regional N budgets.

A laboratory-based optical pH sensor for 2-dimensional pH imaging at benthic interfaces is presented. The sensor consists of a single-layer hydrogel matrix embedding the fluorescent pH indicator 2′, 7′-dihexyl-5(6)-Noctadecyl-carboxamidofluorescein ethyl ester (DHFAE) and the phosphorescent ruthenium(II)-Tris-4,7-diphenyl-1,10-phenanthroline incorporated in nanoparticles, serving as an inert reference standard. The measuring principle is based on time domain dual-lifetime referencing (t-DLR). The fluorophore/phosphor couple is simultaneously excited by a green LED (λ max 530 nm) pulsed in the microsecond range, and the sensor emission is recorded by a fast-gateable CCD camera. For each pH image, intensity images in 2 time windows (1 during and 1 after the excitation phase) are taken. The ratio of these 2 images is proportional to the pH of the sample and not affected by the overall signal intensity. The sensor has a dynamic range suitable for marine conditions (pH 7.3 to 9.3) with an apparent pK a of 8.3. The sensor was long-term stable (months) when kept in darkness and had a response time of < 200 s when going from pH 8.3 to 7.6. Light proved to have a negative effect on the sensor performance due to photobleaching of the pH indicator, resulting in a negative drift in the signal ratio at higher pH after prolonged light exposure. The spatial resolution (83 by 83 μm/pixel) of the sensor was capable of resolving smallscale spatial variability in the pH at a heterogeneous sediment-water interface, and time series of calibrated pH images also expressed a marked temporal variability in the pH distribution across this interface. Hotspots with intensified microbial activity were observed, and pH minima along burrow walls of polychaetes indicated elevated diagenetic activity in these zones. Individually extracted profiles from the pH images agreed well with independently measured pH microelectrode profiles, confirming the robustness of the approach.

A carbon budget for the northwest European continental shelf seas (NWES) was synthesized using available estimates for coastal, pelagic and benthic carbon stocks and flows. Key uncertainties were identified and the effect of future impacts on the carbon budget were assessed. The water of the shelf seas contains between 210 and 230 Tmol of carbon and absorbs between 1.3 and 3.3 Tmol from the atmosphere annually. Offshelf transport and burial in the sediments account for 60-100 and 0-40% of carbon outputs from the NWES, respectively. Both of these fluxes remain poorly constrained by observations and resolving their magnitudes and relative importance is a key research priority. Pelagic and benthic carbon stocks are dominated by inorganic carbon. Shelf sediments contain the largest stock of carbon, with between 520 and 1600 Tmol stored in the top 0.1 m of the sea bed. Coastal habitats such as salt marshes and mud flats contain large amounts of carbon per unit area but their total carbon stocks are small compared to pelagic and benthic stocks due to their smaller spatial extent. The large pelagic stock of carbon will continue to increase due to the rising concentration of atmospheric CO 2 , with associated pH decrease. Pelagic carbon stocks and flows are also likely to be significantly affected by increasing acidity and temperature, and circulation changes but the net impact is uncertain. Benthic carbon stocks will be affected by increasing temperature and acidity, and decreasing oxygen concentrations, although the net impact of these interrelated changes on carbon stocks is uncertain and a major knowledge gap. The impact of bottom trawling on benthic carbon stocks Frontiers in Marine Science | www.frontiersin.org 1 March 2020 | Volume 7 | Article 143Legge et al.Carbon on the Northwest European Shelf is unique amongst the impacts we consider in that it is widespread and also directly manageable, although its net effect on the carbon budget is uncertain. Coastal habitats are vulnerable to sea level rise and are strongly impacted by management decisions. Local, national and regional actions have the potential to protect or enhance carbon storage, but ultimately global governance, via controls on emissions, has the greatest potential to influence the long-term fate of carbon stocks in the northwestern European continental shelf.

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