Aarhus Universitets segl

Nr. 34: The marine environment 2011

Hansen, J.W. (red.) 2012: Marine områder 2011. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 154 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 34. http://www.dmu.dk/Pub/SR34.pdf


The nationwide status of the Danish marine environment in 2011 is summarised by subject below.


  • Air temperature was 1.3 °C higher in 2011 compared to the standard period 1961-1990 but on average level compared to the last 15 years.
  • Winds were generally weak and mainly from south-easterly directions.
  • Freshwater run-off to the inner Danish marine waters was 19 % above normal (1961-1990) and unusually high in late summer due to extensive rain.
  • Sea temperature at the surface was higher in 2011 than in the cold 2010 but on level with temperatures since 1989. Bottom water was relatively cold compared to recent years.
  • Sea water temperatures have increased about 1.5°C during the last 40 years.
  • The radiation in 2011 was average, but generally there is a tendency towards increased radiation in the latest 10 years during the summer months.
  • From the 1960s to the mid-1980s pH of sea water increased (approx. 0.3), but has since decreased equivalently despite an increase over the last couple of years. 

Nutrient loads and concentrations

  • The total deposition of nitrogen to the Danish marine waters was 79,000 tonnes in 2011.
  • On average 12 % 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 32 % decline.
  • Freshwater runoff from land to the Danish marine waters varies substantially from one year to another. In 2011 the runoff was 9 % more than the average for the period 1990-2010.
  • For Denmark as a whole the total phosphorus and nitrogen loads in 2011 were 2,600 tons phosphorus and 59,000 tons nitrogen.
  • Nitrogen and phosphorus loads have decreased about 50 % and 59 % 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 2011 were relatively low, especially for nitrogen, despite a relatively large run off.
  • During the early spring bloom of Chattonella sp. there was a large uptake of DIN and DIP in the open inner Danish waters. In return, the uptake of DSi was moderate resulting in relatively high concentrations of DSi.
  • Nutrient-rich water with low salinity, presumably due to a high fresh water input, entered the open inner Danish waters from the Baltic Sea in May and resulted in unusually high nutrient concentrations. However, there was no sign of the waters being affected significantly by water masses from the Jutland coastal current.
  • 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 and phosphorus limitation was high in 2011 in fjords and coastal areas as well as in open waters.
  • Concentrations of inorganic phosphorus and silicon in the bottom water have increased in recent years presumably due to increased nitrogen limitation and release from sediments. Changed nutrient ratios in the Baltic Sea – and probably also in the North Sea - add to this increase as well. 

Phytoplankton, zooplankton, and water clarity

  • Climate corrected water clarity (Secchi depth) has decreased significantly since 2000 in both open inner waters and in coastal areas. In 2011 the level was comparable to 2010 for the open inner waters and a little lower for the coastal waters.
  • In February and March, the Secchi depth was highly affected by an unusually heavy bloom of the potential toxic algae Pseudochattonella farcimen, which contributed to record low Secchi depths of ca. 4.5 m in the open inner waters and 2.5 m in fjords and coastal waters.
  • In 2011 chlorophyll concentrations in coastal areas increased from 2.9 µg l-1 in 2010 to 3.5 µg l-1 corresponding to the level of climate corrected chlorophyll concentration in the period 1989-2011. However, there has still been a decrease in the climate corrected chlorophyll concentration since 1989, although it is no longer statistically significant.
  • Chlorophyll concentrations in 2011 were unusually high in spring, whereafter the seasonal variation more or less followed the normal pattern although at a lower level than the mean for 1989-2011.
  • The seasonal variation of the biomass of diatoms in the open inner waters did not seem to be affected by the large bloom of P. farcimen. In fjords and coastal areas the spring bloom of diatoms was normal whereas the bloom of P. farcimen was unusually high although low compared to diatom bloom.
  • The summer biomass of multicellular zooplankton was unusually high in 2011 just as in 2010 primarily due to large biomasses of calanoid copepods in May to August. On the other hand, the biomass of single-celled zooplankton was lower than normal. 

Oxygen deficiency

  • Due to some ice cover in coastal areas and generally weak winds, oxygen deficiency developed relatively early in exposed areas.
  • Oxygen conditions in 2011 were, however, relatively good as accelerating oxygen deficiency was halted by water exchange due to wind episodes and because the temperature in the bottom water of the open inner Danish waters was unusually low.
  • The estimated total area affected by oxygen deficiency in September 2011 was on the same level as in 2010 but smaller than in earlier years. Almost half of the area was affected by severe oxygen deficiency and in many cases followed by the release of toxic hydrogen sulphide.
  • 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, the archipelago south of Funen and the Sound.
  • In several areas oxygen deficiency was registered as early as mid-June. The last registrations of oxygen deficiency were made in November.
  • Along the open coastal areas and in the outer fjords, the depth limit of eelgrass has not shown any significant trends, while the eelgrass in the inner fjords and in the Limfjord overall has receded to shallower water.
  • There are large regional differences in the development of the depth limit of eelgrass, which primarily seems to be regulated by light.
  • In the Limfjord, though, there are positive signs, as the maximum depth distribution has increased by almost 30 % and the main coverage by almost 10 % from 2009 to 2011.
  • In the period from 1989 to 2011 the eelgrass show an overall tendency to cover still less of the bottom along the monitored transects. However, in recent years coverage has improved at 1-2 m depths in the outer fjords and in the Limfjord.
  • The lack of improvement of the eelgrass coverage, despite reduced nutrient loading, is presumable among other things related to the lack of improvements of the Secchi depth and that the eelgrass population has been reduced to an extent that reestablishment is problematic.
  • The total macroalgal coverage in deeper parts of selected stone reefs in the open parts of the Kattegat was not as good as in 2010 where a marked improvement had taken place compared to recent years.
  • For macroalgae on stone reefs there is no trend towards more extensive coverage or spreading to greater depths throughout the monitoring period from 1994 up until today.
  • Grazing of macroalgae by sea urchins is still a problem on many reefs in the Belt Sea, where vegetation has to a large extent disappeared at depths below the pycnocline.
  • After a marked improvement in bottom fauna biodiversity, abundance, and biomass in the open waters in 2010, the biodiversity and abundance in 2011 are again reduced to the levels before 2010.
  • For the fjords connected to the open inner Danish waters, the abundance and biodiversity is even below the level of the previous 13 years.
  • The coastal waters towards the Kattegat showed in 2011 as in 2010 a synchronous development with the changes in the open inner Danish waters, while this was not the case for fjords at the west coast, where the diversity generally was higher in 2011 (although not significant).
  • The reduced diversity and abundance but stable biomass in the fjords in the inner Danish waters can be interpreted as a result of no recruitment in recent years.

Bottom flora and fauna 

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 15,500 animals in 2011 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.
  • The grey seal population has shown improvement over the past 10 years with up to 68 animals in the Kattegat in 2008 and up to 57 animals in the Wadden Sea and 250 animals in the Danish part of the Baltic Sea in 2011.
  • Some grey seals are now breeding regularly in Denmark after approx. a hundred-year break.
  • In both the North Sea and the Skagerrak the harbour porpoise was observed in all Natura 2000 areas, although the area “Skagen Grenen” had a far higher abundance than the other four areas.
  • 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 summer and in Kalundborg Fjord during winter. In addition, there was a distinct diurnal variation in sound signals with most detection during night and fewest during daytime. 

Hazardous substances and biological effects

  • In 20 % of the common mussel samples and 86 % of the fish samples the content of heavy metals were higher than Environmental Quality Standards for biota (EQS) due to mercury (Hg) but no metals were above the EU criteria for content in food. Only the content of nickel (Ni) and copper (Cu) in the soft clams Mya arenaria from Ringkøbing Fjord were above the ‘good ecological status’ limit according to Norwegian assessment criteria.
  • Concentrations of metals in seawater from Skive Fjord in 2010/2011 were within the EU limits for nickel (Ni), lead (Pb), and cadmium (Cd).
  • The content of TBT in mussels is decreasing in the areas where time trends of significance are available due to the ban of antifouling paints. However, at more than 50 % of the stations, the concentrations are still above the ecotoxicological assessment criteria (EAC) so an impact from TBT in the marine environment can still be expected.
  • PAH in mussels were not above the EAC and in most cases corresponding to or below the ‘background’ criteria for the North Sea.
  • Chlorinated compounds in fish were generally below the EAC except for CB118 (50 % of fish above EAC) and HCB (25 % of fish above EAC), and also CB138 were above EAC in a few samples. Thus it cannot be ruled out that CB118 and HCB still have an impact in the marine environment.
  • It is possible to identify the geographic origin of flounder based on the composition of chlorinated compounds.
  • Time trends for hazardous compounds are in general decreasing especially for the organic compounds under regulation (PCB, and TBT) – most evident in Langerak (the Limfjord).
  • Imposex and intersex in sea snails due to exposure to TBT are still widespread and assessed as an environmental problem in Danish marine waters. Based on the OSPAR criteria for imposex and intersex, good ecological status is only found in the North Sea and the Skagerrak.
  • Levels of imposex and intersex are so high in the inner Danish marine waters that in relation to OSPAR and HELCOM criteria many areas have to be considered as not acceptable and even bad in certain contaminated harbours and marinas. However, the level has decreased markedly in recent years, which indicates that further improvements can be expected.
  • Investigations of lysosomal membrane stability in common mussels indicated that approximately one third of the investigated coastal areas were affected by hazardous substances and especially PAH.
  • Investigations 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 gall. A correlation between the activity of CYP1A and the concentration of PAH and PCB was found, whereas this was not the case for malformation in fry and the measured hazardous substances.