Aarhus University Seal / Aarhus Universitets segl

No. 418: Marine areas 2019. NOVANA

Hansen J.W. & Høgslund S. (red.) 2021. Marine områder 2019. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 174 s. - Videnskabelig rapport fra DCE nr. 418. 



In 2019, the monitoring parameters did not show any unambiguous changes in relation to the development in recent years of the marine environment. Many parameters were thus unchanged, while some of them showed a decline and a few a progress.

Precipitation was very unevenly distributed over the year with unusually heavy rainfall in late spring (March) and in the autumn (September, October). Freshwater runoff from land was very low at the beginning of the year due to the unusually dry autumn in 2018, but it increased significantly with the heavy rainfall in March and in the autumn.

The concentrations of dissolved inorganic nutrients in the first months of the year were below the long-term mean. From April and the year out, the concentration of nitrogen was identical with the long-term mean, while the levels remained low for phosphorus and silicon. For all the nutrients, some of the monthly means were the lowest since 1989.

Hydrographic calculations show that, during the water flow through the inner Danish waters in 2019, there was a significant transport of nitrogen from the bottom water to the surface water.

The combination of high temperatures in the bottom water and a high concentration of phytoplankton in the beginning of the year led to an early onset of the oxygen depletion in a number of areas. The oxygen depletion was widespread and intense in 2019. Heavy winds in mid- September led to disappearance of oxygen in the shallow areas, while in the deep parts of the most affected locations it continued into December.

The growth of phytoplankton in 2019 was characterised by the uneven distribution of precipitation with low algae growth rates and chlorophyll concentrations in the growing season, while the last months of the year displayed increased algae growth and chlorophyll levels and more turbid water. Overall, the state of the water column in 2019 was similar to that prevailing in the last few years, with deteriorating conditions; however, in the open inner waters algae growth was significantly reduced.

The generally positive development of eelgrass in the beginning of this decade has stagnated relative to both depth distribution and coverage. For the past 10 years, the coverage of macroalgae (seaweed) has increased in coastal waters, while the trend has stagnated in inner fjords, outer fjords, the Limfjord and on stone reefs. In 2019, there was an increased coverage of macroalgae on stone reefs in the open inner waters.

The status of the bottom fauna in fjords and coastal waters was average with neither very good nor very bad conditions at most stations; however, at some stations the conditions for the bottom fauna were poor due to oxygen depletion. In the open inner waters, the status of the bottom fauna was generally relatively good. In the North Sea and the Skagerrak, the bottom fauna indicated relatively poor environmental conditions, and the species composition suggested that bottom trawling was the dominant pressure factor.

The number of harbour seals has grown steadily since the conservation in the 1970's; but in recent years it has stabilised in most areas, which suggests that the population has reached the level of environmental sustainability. The number of grey seals has increased significantly since 2005, but they only breed to a very limited extent (< 10 pups/year). The population of porpoises in the habitat areas in the North Sea/Skagerrak is stable, while it has declined slightly in the inner Danish waters, and in the inner Baltic Sea the species is critically threatened.

In several cases, the concentration of heavy metals in mussels and fish was above the environmental quality requirements, but below the threshold values for food. Brominated flame retardants were found in concentrations above the environmental quality requirements in several samples. The levels of TBT and tar substances were generally above the background values and in some cases above the environmental quality requirements. In some coastal-near areas, there were signs of impact of environmentally hazardous substances on eelpout, while this was not the case for mussels.

The development in recent years has shown that Danish waters are still very vulnerable to pressures and are still far from the goal of stable good environmental condition. In addition to nutrient losses (eutrophication), the state of the environment is also affected negatively by, for example, fisheries, climate change and environmentally hazardous substances. In summary, data from the national monitoring programme show that the marine environment responds positively to reduced pressure, but changes at ecosystem level are slow.