Aarhus University Seal / Aarhus Universitets segl

No. 77: The marine environment 2012. NOVANA

Hansen, J.W. (red.) 2013: Marine områder 2012. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 162 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 77. http://dce2.au.dk/pub/SR77.pdf


The nationwide status of the Danish marine environment in 2012 is summarised by subject below.


  • Air temperature was 0.6 °C higher in 2012 compared to the standard period 1961-1990 but slightly colder than compared to the last 12 years.
  • Winds were generally weak although on average level for recent years. The first three months of the year were dominated by westerly winds, whereas the wind the rest of the year generally was from south-westerly directions.
  • Precipitation was 819 mm in 2012, which was 15 % above normal (1961-1990).
  • Sea temperature was a little higher than normal, but colder when compared to the latest 10-15 years.
  • Sea water temperatures have increased about 1°C during the last 40 years.
  • The radiation in 2012 was average, but generally there is a tendency towards increased radiation in the latest 10 years during the summer months.
  • Since the mid-1980s pH of sea water is reduced (approx. 0.2), although pH has increased in fjords and coastal waters during the last couple of years. 

Nutrient loads and concentrations

  • The total deposition of nitrogen to the Danish marine waters was 81,000 tonnes in 2012.
  • On average 13 % of the atmospheric nitrogen deposition to the Danish marine waters originated from Danish sources.
  • Since 1989 the atmospheric nitrogen deposition to the Danish waters has experienced a 34 % decline.
  • Freshwater runoff from land to the Danish marine waters varies substantially from one year to another. In 2012 the runoff was 9 % more than the average for the period 1990-2011.
  • For Denmark as a whole the total phosphorus and nitrogen loads in 2012 were 2,600 tons phosphorus and 59,600 tons nitrogen.
  • Nitrogen and phosphorus loads from land have decreased about 50 % and 56 %, respectively, since 1990 when corrected for variation in freshwater runoff.
  • The reduction in phosphorus load is primarily a result of improved wastewater treatment, whereas the reduction in nitrogen load is mainly a result of reduced leaching from cultivated land, although also improved wastewater treatment has contributed significantly to the decrease in nitrogen load.
  • Overall, nutrient concentrations in 2012 were very low, actually the lowest so far for organic N and TN.
  • Phosphorus concentration was comparable to the level in recent years, but for fjords and coastal waters it was significantly lower than in the beginning of the 1990s.
  • Bottom water entering the open inner Danish waters in February and March was low in DIN and DSi indicating an origin from the central part of the North Sea.
  • Nitrogen and phosphorus concentrations show a clear decreasing trend since 1989, especially when taking into account variations in year to year freshwater run-off. There is, however, a tendency to stagnation in phosphorus concentrations after 1997, and only a slight decrease in nitrogen concentrations after 2002.
  • Potential nitrogen limitation was very high in 2012 in fjords and coastal waters as well as in open waters. This has resulted in a relatively large excess of phosphorus, and accordingly a decrease in the potential phosphorus limitation.
  • Silicate concentration was on average level for the last 10-15 years. 

Phytoplankton, zooplankton, and water clarity

  • Lower chlorophyll concentrations and primary production and higher water clarity (Secchi depth) in 2012, especially in the open inner waters, indicated an improvement in the marine environmental status.
  • Chlorophyll concentration in fjords/coastal waters was reduced significantly from 1989 to 2012, whereas the change for Secchi depth has been more variable.
  • Data from a number of monitoring stations demonstrated that the distribution of primary production in the water column was dependent on the nitrogen load with a higher share of production below the mixed surface layer at lower nitrogen load.
  • Biomass of diatoms in 2012 was relatively high like in 2010 and 2011 but has generally been very variable from year to year. Diatoms have become increasingly dominant during the monitoring period.
  • There were no significant reports of toxic or in other ways harmful species of phytoplankton in 2012.
  • Biomass of micro zooplankton was lower than previous years and shows a general decrease during the last five years. 

Oxygen deficiency

  • Oxygen conditions in 2012 were relatively good as accelerating oxygen deficiency was halted by water mixing and water exchange due to wind episodes.
  • The estimated total area affected by oxygen deficiency in September 2012 was on the same level as in 2010 and 2011 but significantly smaller than in earlier years. More than half of the area was affected by severe oxygen deficiency.
  • The main affected areas – with respect to the duration and intensity of oxygen deficiency – were the Limfjord, the Mariager Fjord, the southern Little Belt, including the fjords of southern Jutland, and the archipelago south of Funen. In some of the areas toxic hydrogen sulphide was released and dead fish were observed.
  • In most areas oxygen deficiency was registered from August-September and disappeared completely during October. 

Bottom flora and fauna

  • Along the open coastal waters and in the outer fjords, the depth limit of eelgrass has not shown any significant trends since 1989, while the eelgrass in the inner fjords and in the Limfjord overall has receded to shallower water.
  • During recent years an improvement has taken place for eelgrass in some fjords that was continued in 2012. In the Limfjord the maximum depth distribution has increased by 50 % and the main coverage by almost 20 % from 2009 to 2012. The depth distribution has also significantly improved in Roskilde Fjord, Isefjord, Flensborg Fjord, and Als Sound in 2012. In contrast, Aarhus Bay and Odense Fjord inter alia have experienced a negative change.
  • In the period from 1989 to 2012 the eelgrass shows an overall tendency to cover still less of the bottom along the monitored transects. However, in recent years coverage has improved at depths of 1-2 m especially in the Limfjord and to a lesser extent in the other fjords.
  • The lack of improvement of the eelgrass coverage, despite reduced nutrient loading, is presumable among other things related to the general lack of improvements of the Secchi depth and that the eelgrass population has been reduced to an extent where reestablishment is problematic. However, recent years’ positive trend indicates that there is an improvement underway.
  • The total macroalgal coverage in deeper parts of selected stone reefs in the open parts of the Kattegat was generally good in 2012, which is in accordance with the improvement of the Secchi depth in the open inner Danish waters in the first half of the year.
  • Grazing of macroalgae by sea urchins is still a problem on several reefs in Danish marine waters.
  • In inner fjords, outer fjords, coastal waters and on reefs there have been positive trends for the average cover of macroalgae, whereas the trend has been negative for the Limfjord.
  • The alpha diversity increased with latitude both in Habitat areas and in Water Framework Directive areas from the Baltic Sea in the south to the North Sea in the north with an average factor of about 2, most likely due to increasing salinity.
  • With reservations to large scale year to year variation the alpha diversity is generally higher in the Habitat than in the Water Framework Directive areas.
  • For the coastal hard bottom fauna localities the Limfjord diverged from the other localities. In particular there were many species and high coverage of ascidians. Furthermore, three of four invasive or introduced species were only registered there.
  • The similarity analysis generally showed gradual transitions for the other localities. Æbelø, however, stood something out from the nearest localities probably due to random differences in coverage for the most dominant and widely distributed species. 

Seals and porpoise

  • The harbor seal is the most common seal in Denmark and the population has increased from approx. 2,000 animals in 1976 to 16,100 animals in 2012 due to a hunting ban in 1977 and the establishment of a number of seal sanctuaries where access is prohibited.
  • The harbor seal is divided into four populations: the Wadden Sea, the Limfjord, the Kattegat, and the western part of the Baltic Sea with growth rates since the epidemic in 2002 of 13 %, 10 %, 8 %, and 9 %, respectively.
  • The grey seal population has shown improvement over the past 10 years and in 2012 up to 78 animals in the Kattegat, up to 76 animals in the Wadden Sea, and 147 animals in the Danish part of the Baltic Sea in 2012 were registered.
  • Some grey seals are now breeding regularly in Denmark after approx. a hundred-year break.
  • The population of harbour porpoise in the inner Danish waters was estimated to approx. 18,500 animals in July 2012.
  • Registrations from the Great Belt and Kalundborg Fjord showed that the porpoises are present all year round, but that the abundance is highest in the Great Belt during the first half of the year and in Kalundborg Fjord during winter. 

Hazardous substances and biological effects

  • Levels of heavy metals in mussel samples were generally a little higher in 2012 than in 2011. The concentration of mercury (Hg) was above EU’s Environmental Quality Standard (EQS) in 60 % of the mussel samples and in all fish samples.
  • Levels of tars (i.e. PAHs) in mussel samples were higher for the light compounds but otherwise corresponding to the level in 2011 and for most PAHs the level was between OSPAR’s Background Assessment Criteria (BAC) and Environmental Assessment Criteria (EAC).
  • The concentration of organotin in mussel samples and eelpout corresponded to the level in 2011 and was lower than previous years in accordance with the general decreasing level of TBT in Danish waters. In approximately 60 % of the mussel samples the concentration of TBT was, however, still higher than OSPAR’s EAC, thus negative biological effects of TBT cannot be ruled out.
  • Organochlorines (PCBs) in fish samples were found in concentrations comparable to the level in 2011. Only concentrations of the congener CB118 were higher than OSPAR’s EAC in some samples indicating the risk of adverse biological effects of the more dioxin-like PCBs in the ecosystem.
  • Phenanthrene and anthracen in sediment samples were the only PAH compounds where concentrations in more than 10 % of the cases exceeded the OSPAR EAC after normalization to 2.5 % TOC and where negative effects of PAH cannot be ruled out.
  • There was a decreasing time trend for concentrations of hazardous substances when normalised to percentage of total organic carbon (TOC) or fine-grained material as recommended by OSPAR.
  • Investigation of lysosomal membrane stability in blue mussels indicated that almost all of the investigated coastal waters were affected by hazardous substances and especially PAH.
  • Investigation of eelpout showed marked differences in the degree of effects between areas, both concerning presence of malformation in fry, CYP1A enzyme activity, and PAH metabolites in the bile. A correlation between the activity of CYP1A and the concentration of PAH and PCB was found, whereas no correlation could be documented between malformated fry in eelpout and hazardous substances.