Marine chemistry data can represent measured or estimated values of pH, oxygen concentration, nutrient concentration, etc.
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The chemical composition of seawater is influenced by many processes, such as ocean currents, atmospheric conditions, seafloor processes (erosion, underwater volcanoes, hydrothermal vents), and not least human activity and climate change. Temperature and salinity are the core parameters that are frequently measured, as these parameters determine the density of seawater and thus can be used to model the large-scale density-driven currents in the world ocean.
pH changes due to ocean acidification has become increasingly important, as scientists gain a better understanding of how the ocean takes up excess carbon dioxide from human activities such as the burning of fossil fuels. When carbon dioxide (CO2) dissolves in sea water, the chemical process leads to the production of carbonic acid, which in turn increases the amount of hydrogen ions in the water, thus lowering the pH. This may have great implications on marine biology, as some species are highly sensitive to pH and may therefore not be able to survive in their habitats.
A good starting point for collecting and processing of chemical oceanographic data is to search the Ocean Best Practices repository, managed by the United Nations Intergovernmental Oceanographic Commission of UNESCO (IOC-UNESCO, https://www.oceanbestpractices.org/). This site contains over 1000 best practices documents written by experts in their fields, including instrument calibration practices, recommendations for performing measurements and more.
Managing ocean chemistry data typically means that measurements are incorporated in regional or global databases or ocean atlases. Some common metadata standards exist, but often it is up to the surveying organization to implement metadata requirements and impose specifics on formats. In the U.S., NOAA NCEI aim to collate and distribute all publicly available ocean chemistry data that are available, whereas in Europe, a large EU initiative called EMODnet (European Marine Observation and Data Network) is currently the main actor for making data available.