Aarhus University Seal / Aarhus Universitets segl

No. 355: Marine Areas 2018. NOVANA

Hansen J.W. & Høgslund S. (red.) 2019. Marine områder 2018. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 156 s. - Videnskabelig rapport fra DCE nr. 355.


In 2018, the monitoring parameters did not show unambiguous changes compared to the development in the marine environment during recent years. Many parameters were thus unchanged while others showed decline and a few progress.

The autumn in 2017 and the start of 2018 were very rainy which caused a high freshwater run-off. From May to August, it was unusually warm and dry and this resulted in high water temperatures and a very low freshwater run-off during the summer and autumn. The input of nutrients from land to sea followed the seasonal variation of the run-off. This resulted in concentrations of inorganic nutrients on level with the long-term average during the first months of the year. During the rest of the year, the concentrations were low – in some cases, the lowest since 1989.

Light winds and very warm weather during the last part of the spring lead to an early start of oxygen depletion in some areas and it developed markedly in area of distribution and severity during the summer. At the end of July and the beginning of August, the oxygen depletion was very serious in several areas. A short storm in mid-August improved the conditions in the shallow areas, whereas the oxygen conditions in the vulnerable, deeper areas did not improve until after a storm at the end of September.

The growth of phytoplankton and the concentrations of chlorophyll in 2018 were comparable with the last couple of years but had increased compared with 2012-2013, while the water clarity was on average for the period since the mid-1990s. The conditions of the water column have thus all in all been worse in recent years compared with 2012-2013.

The generally positive development of the eelgrass during the last ten years has stagnated both for depth distribution and coverage. In coastal waters and outer fjords, the coverage of macroalgae (seaweed) has increased during the last ten years, but the development has stagnated in inner fjords, in Limfjorden and on stone reefs.

The state of the benthic fauna in fjords and coastal waters was very variable, and even within the same area, some locations remained in good condition and others in poor condition. In the open inner waters, the benthic fauna were generally in a relatively good condition. In the North Sea and in the Skagerrak, the benthic fauna indicated relatively poor environmental conditions, and the species composition implied that bottom trawling is the dominating pressure factor.

The number of harbour seals in Danish waters has increased steadily since the 1970s, but in recent years, it has stabilized in most of the areas which indicates that the population is getting close to the carrying capacity of the environment. The number of visiting grey seals increased significantly from 2005 to 2015, but has since then stagnated. The grey seal breeds only to a limited extent in the Danish waters and in 2018 only at one locality. The occurrence of porpoises in the habitat areas in the inner Danish waters is stable, whereas it has declined since 2011 in the habitat areas in the North Sea. The population in the inner Baltic Sea is still critically endangered.

Concentration of most of the hazardous substances were in line with or lower than previous measurements. Mercury, lead, cadmium and brominated flame retardants were in several places at a level which might cause problems to the marine environment, and increased content of the tar compound anthracene in sediment is a widespread problem.

Despite improvements in recent years of some of the parameters, the development in the last couple of years has shown that the Danish waters are still very vulnerable to pressure on the environment and still far away from the goal of a stable, good environmental status. In addition to input of nutrients (eutrophication), the environmental state is also negatively impacted by e.g. fishery, climate change and hazardous substances. In summary, data from the national monitoring programme show that the effort to secure a better marine environment has paid off, but the marine environment is still under strong pressure and is only responding slowly to mitigation measures.