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Key messages: 1.1 The process of ocean acidification is now relatively well-documented at the global scale as a long-term trend in the open ocean. However, short-term and spatial variability can be high. 1.2 New datasets made available since Charting Progress 2 make it possible to greatly improve the characterisation of CO2 and ocean acidification in UK waters. 3.1 Recent UK cruise data contribute to large gaps in national and global datasets. 3.2 The new UK measurements confirm that pH is highly variable, therefore it is important to measure consistently to determine any long term trends. 3.3 Over the past 30 years, North Sea pH has decreased at 0.0035±0.0014 pH units per year. 3.4 Upper ocean pH values are highest in spring, lowest in autumn. These changes reflect the seasonal cycles in photosynthesis, respiration (decomposition) and water mixing. 3.5 Carbonate saturation states are minimal in the winter, and lower in 7 more northerly, colder waters. This temperature-dependence could have implications for future warming of the seas. 3.6 Over the annual cycle, North-west European seas are net sinks of CO2. However, during late summer to autumn months, some coastal waters may be significant sources. 3.7 In seasonally-stratified waters, sea-floor organisms naturally experience lower pH and saturation states; they may therefore be more vulnerable to threshold changes. 3.8 Large pH changes (0.5 - 1.0 units) can occur in the top 1 cm of sediment; however, such effects are not well-documented. 3.9 A coupled forecast model estimates the decrease in pH trend within the North Sea to be -0.0036±0.00034 pH units per year, under a high greenhouse gas emissions scenario (RCP 8.5). 3.10 Seasonal estimates from the forecast model demonstrate areas of the North Sea that are particularly vulnerable to aragonite undersaturation.

It is estimated that oceans absorb approximately a quarter of the total anthropogenic releases of carbon dioxide to the atmosphere each year. This is leading to acidification of the oceans, which has already been observed through direct measurements. These changes in the ocean carbon system are a cause for concern for the future health of marine ecosystems. A coordinated ocean acidification (OA) monitoring programme is needed that integrates physical, biogeochemical, and biological measurements to concurrently observe the variability and trends in ocean carbon chemistry and evaluate species and ecosystems response to these changes. This report arises from an OSPAR request to ICES for advice on this matter. It considers the approach and tools available to achieve coordinated monitoring of changes in the carbon system in the ICES marine area, i.e. the Northeast Atlantic and Baltic Sea. An objective is to measure long-term changes in pH, carbonate parameters, and saturation states (Ωaragonite and Ωcalcite) in support of assessment of risks to and impacts on marine ecosystems. Painstaking and sensitive methods are necessary to measure changes in the ocean carbonate system over a long period of time (decades) against a background of high natural variability. Information on this variability is detailed in this report. Monitoring needs to start with a research phase, which assesses the scale of short-term variability in different regions. Measurements need to cover a range of waters from estuaries and coastal waters, shelf seas and ocean-mode waters, and abyssal waters where sensitive ecosystems may be present. Emphasis should be placed on key areas at risk, for example high latitudes where ocean acidification will be most rapid, and areas identified as containing ecosystems and habitats that may be vulnerable, e.g. cold-water corals. In nearshore environments, increased production resulting from eutrophication has probably driven larger changes in acidity than CO2 uptake. Although the cause is different, data are equally required from these regions to assess potential ecosystem impact. Analytical methods to support coordinated monitoring are in place. Monitoring of at least two of the four carbonate system parameters (dissolved inorganic carbon (DIC), total alkalinity (TA), pCO2, and pH) alongside other parameters is sufficient to describe the carbon system. There are technological limitations to direct measurement of pH at present, which is likely to change in the next five years. DIC and TA are the most widely measured parameters in discrete samples. The parameter pCO2 is the most common measurement made underway. Widely accepted procedures are available, although further development of quality assurance tools (e.g. proficiency testing) is required. Monitoring is foreseen as a combination of low-frequency, repeat, ship-based surveys enabling collection of extended high quality datasets on horizontal and vertical scales, and high-frequency autonomous measurements for more limited parameter sets using instrumentation deployed on ships of opportunity and moorings. Monitoring of ocean acidification can build on existing activities summarized in this report, e.g. OSPAR eutrophication monitoring. This would be a cost-effective approach to monitoring, although a commitment to sustained funding is required. Data should be reported to the ICES data repository as the primary data centre for OSPAR and HELCOM, thus enabling linkages to other related datasets, e.g. nutrients and integrated ecosystem data. The global ocean carbon measurement community reports to the Carbon Dioxide Information Analysis Center (CDIAC), and it is imperative that monitoring data are also reported to this database. Dialogue between data centres to facilitate an efficient “Report-Once” system is necessary. Published Authors: Alberto V. Borges ● Carlos Borges ● Naomi Greenwood ● Susan E. Hartman David J. Hydes ● Caroline Kivimae ● Evin McGovern ● Klaus Nagel Solveig Olafsdottir ● David Pearce ● Elisabeth Sahlsten Carmen Rodriguez ● Pamela Walsham ● Lynda Webster Refereed Current 14.3 Inorganic carbon Best Practice Manual (incl. handbook, guide, cookbook etc)