Open ocean — ocean chemistry: dissolved oxygen and ocean acidity

The increase in greenhouse gas emissions and the temperature of the atmosphere, and changes in ocean currents induce significant modifications of the chemical properties of the ocean water, such as its oxygen content, acidity and salinity. Such changes can have significant impacts on marine biodiversity and ecosystem productivity (EEA, 2018: European waters — Assessment of status and pressures 2018).

Definition

The dissolved oxygen level index refers to the average concentration of free oxygen present in ocean water.

Index factsheet (ETC/CCA Technical Paper): Dissolved oxygen level

Relevance

The amount of dissolved oxygen in sea water affects the metabolism of marine organisms and provides information on marine ecosystem health. Reductions in oxygen content can lead to changes in the distribution of marine species, including the formation of ‘dead zones’. The primary cause of oxygen depletion is nutrient input from agricultural fertilisers, which cause eutrophication, but the effects of eutrophication are exacerbated by climate change, in particular increases in sea temperature and in water column stratification.

The index can support adaptation planning related to marine protected areas, fisheries and marine aquaculture, but can also inform required changes in agricultural activities on the surrounding land.

Past and future changes

In the Baltic Sea, oxygen concentrations in the water layer near the sea floor have decreased significantly since 1900 and oxygen-depleted zones in the Baltic Sea have increased more than 10-fold, with most of this increase happening after 1950. The Baltic Sea now has the largest dead zone in the world, due mostly to nutrient inputs from agricultural fertilisers and consequent eutrophication (EEA, 2017: Climate change, impacts and vulnerability in Europe 2016: An indicator-based report, Section 4.1.6). Oxygen depletion has also been observed in other European seas in recent decades.

Oxygen concentrations in the water layer near the sea floor have decreased at 11 % of stations in the Baltic Sea and at 9 % of stations in the Greater North Sea since 1990. The majority of other stations in European seas did not show significant trends in oxygen concentrations over this period, but data coverage is not sufficient in all regional seas.

Quantitative projections for future changes in ocean oxygen concentrations are not currently available.

Further information (EEA indicator assessment): Oxygen concentrations in European coastal and marine waters.

Definition

The ocean pH level index measures the acidity of ocean water. It is defined by the concentration of hydrogen ions dissolved in ocean water.

Index factsheet (ETC/CCA Technical Paper): Ocean pH level

Relevance

The index is relevant for assessing risks to marine ecosystems. Changes in the ocean pH level can directly influence marine calcifying organisms, such as reef-building corals, shellfish and plankton.

Past and future changes

Across the ocean, the pH of surface water has been relatively stable for millions of years, oscillating between 8.3 and 8.2. Ocean surface pH declined from 8.2 to below 8.1 over the industrial era as a result of an increase in atmospheric CO₂ concentrations. This decline corresponds to an increase in oceanic acidity of about 30 %. Observed pH reductions are nearly identical across the global ocean and throughout continental European seas, except for variations near the coast and the larger declines in the northernmost European seas (i.e. the Norwegian Sea and the Greenland Sea).

Models consistently project further ocean acidification worldwide. Ocean surface pH is projected to decrease to values of between 8.05 and 7.75 by the end of the 21st century, depending on future CO₂ emission levels. The largest projected decline represents more than a doubling in acidity with respect to present conditions.

Further information (EEA indicator assessment): Ocean acidification.