Aarhus University Seal / Aarhus Universitets segl

No. 6: The marine environment 2010

Hansen, J.W. & Petersen, D.L.J. (red.) 2011: Marine områder 2010. NOVANA. Tilstand og udvikling i miljø- og naturkvaliteten. Aarhus Universitet, DCE - Nationalt Center for Miljø og Energi. 120 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 6. http://www2.dmu.dk/Pub/SR6.pdf


The nationwide status of the Danish marine environment in 2010 is summarised by subject below.


  • The year 2010 was cold with relatively low air temperatures during most months and was thus the coldest year in 15 years.
  • The winds were generally weak alternating between easterly and westerly wind directions. During the late summer and autumn months the mean wind was relatively high.
  • Freshwater run-off was rather low during January and February as the precipitation fell as snow. On the other hand run-off was high in March because of the melting of snow. The pattern of run-off from April to November was normal, showing a marked increase over the period.
  • Sea water temperatures in 2010 were lower than in recent years. This was more pronounced in the surface water than in the bottom water. Despite this, sea water temperatures were still higher than normal throughout the year and on average ca. 2°C higher than air temperatures. Sea water temperatures have increased about 1.5°C during the last 40 years.
  • The radiation in 2010 was average, but generally there is a tendency towards increased radiation in the latest 10 years during the summer months.

Nutrient loads and concentrations

  • The total atmospheric nitrogen deposition to the Danish waters was 15 % lower in 2010 than in 2009.
  • On average 14 % of the atmospheric nitrogen deposition originates from Danish sources.
  • Since 1989 the atmospheric nitrogen deposition to the Danish waters has experienced a 33 % decline.
  • Freshwater run-off has varied substantially from one year to another. In 2010 run-off was close to the average for the period from 1990 and onwards.
  • For Denmark as a whole the total phosphorus and nitrogen loads in 2010 were 2,400 tonnes of phosphorus and 55,000 tonnes of nitrogen.
  • When variations in fresh water run-off are taken into consideration, nitrogen loads have decreased by 51 % and phosphorus loads by 64 % from 1990 to 2010.
  • A large part of the reduction in the phosphorus loads took place before the end of the 1990s and is primarily a result of improved wastewater treatment.
  • The reduction in the nitrogen loads is mainly a result of reductions in nitrogen losses from cultivated land. However, improved wastewater treatment has also contributed significantly to the overall decrease in nitrogen loads.
  • Overall, nutrient concentrations in 2010 were relatively low, in particular for nitrogen concentrations. By the end of the year the large freshwater run-off in November had increased nutrient concentrations to similar levels as in previous years.
  • During the spring bloom of diatoms, there was an exceptionally large uptake of all types of inorganic nutrient fractions.
  • In 2010 intruding bottom water with high salinity and high DIN levels from the Skagerrak was traceable in the Kattegat, the Sound, and in the Great Belt until September. There was no sign of the inner Danish waters being affected by water masses from the Jutland coastal current.
  • Potential nitrogen and phosphorus limitation in 2010 was high in fjords and coastal areas as well as in open waters.
  • Nitrogen and phosphorus concentrations show a clear decreasing trend since 1989, especially when variation in year to year freshwater run-off is taken into account. There is, however, a tendency of stagnation in phosphorus concentrations after 1997, and a similar stagnation in nitrogen concentrations after 2002.
  • Concentrations of inorganic phosphorus and silicon in the bottom water have increased during later years 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 2001 in open waters, coastal areas and in fjords. In 2010 the Secchi depth was on average 10 cm lower than in 2009.
  • In January and February 2010 the Secchi depths were very low, but began to rise hereafter and reached a maximum in May.
  • In 2010 chlorophyll concentration level in the fjords was slightly higher than in 2009. Despite this minor increase, the climate corrected chlorophyll concentration has decreased significantly in the fjords during the entire monitoring period.
  • Chlorophyll concentrations in open waters in 2010 were record high in January and February. The early peak concentration in February in connection with the spring bloom was dominated by the diatoms Thallassiosira nordenskioeldii and later by the large Rhizosolenia species R. setigera and R. hebetata. The rest of the year the chlorophyll concentrations followed the normal pattern, but at a level somewhat below the long-term mean.
  • After March the chlorophyll concentrations in the fjords were lower than the average concentration for the period 1989-2009. This was particularly the case for May and June, where the lowest concentration ever in the fjords for this time of year was measured.
  • Primary production generally followed the normal pattern, but peaked in June which is earlier than usual.
  • The contribution from diatoms to the total phytoplankton biomass in 2010 was larger than ever registered before in the monitoring period. The large contribution was mainly due to the massive spring bloom of diatoms in open waters and blooms in the fjords and coastal areas during summer and late summer.
  • Potential toxic algae were generally not registered in high concentrations in 2010 except for a few incidents of high concentrations of haptophytes and dinoflagellates.
  • The summer biomass of multicellular zooplankton was unusually high in 2010, primarily due to large biomasses of calanoid copepods in May and June. On the other hand, the biomass of single-celled zooplankton was smaller than normal.

Oxygen deficiency

  • Due to ice cover and weak easterly winds, oxygen deficiency developed early.
  • Oxygen conditions in 2010 were relatively good, as accelerating oxygen deficiency was halted by wind episodes, which led to the exchange of water masses.
  • The estimated total area affected by oxygen deficiency in 2010 was smaller than in earlier years. Half of the area was affected by severe oxygen deficiency in many cases followed by the release of hydrogen sulphide.
  • The main affected areas – with respect to the duration and intensity of oxygen deficiency – were the southern Little Belt, including the fjords of southern Jutland, the archipelago south of Funen and the Sound. In the Limfjord the oxygen deficiency was less extensive and of shorter duration than normal, because of periods of windy weather.
  • In several areas oxygen deficiency was registered as early as late May. The last registrations of oxygen deficiency were made in November 2010.

Bottom flora and fauna and seals

  • 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.
  • In the Limfjord, though, there are positive signs, as the maximum depth distribution has increased by > 20 % and the main coverage by 10 % from 2009 to 2010.
  • In the period from 1989 to 2010 the eelgrass show an overall tendency to cover still less of the bottom along the monitored transects.
  • 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.
  • Total macroalgae coverage on the deepest parts of a selection of stone reefs in the open parts of the Kattegat was markedly better in 2010 than in previous years.
  • 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.
  • The biodiversity, abundance, and biomass of bottom fauna have increased markedly since 2008. This applies to the open waters and the open coastal waters that connect to the Kattegat and the Belt Sea including Aarhus Bight, southern Kattegat and the northern part of the Sound.
  • The significant development of the bottom fauna is attributed to a large recruitment having taken place since 2008.
  • In the fjords the development has in most cases been negative or neutral, i.e. a decrease or no changes in biodiversity, abundance, and biomass of bottom fauna. There was no improvement in 2010 except for a substantial increase of the biomass of common mussels in the central parts of the Limfjord.
  • The deterioration of the bottom fauna, which has been reported in previous years in Roskilde Fjord and especially in the Isefjord, still seems to be present, even though some improvements may have occurred in the Isefjord.
  • Harbor seal is the most common seal in Denmark and the population has increased from approx. 2,000 animals in 1976 to 15,700 animals in 2010 due to a hunting ban in 1977 and the establishment of a number of seal sanctuaries where access is prohibited.
  • Grey seal has shown improvement over the past 10 years with up to 67 grey seals at Rødsand, ca. 80 around Christiansø and up to 76 in the Kattegat. The grey seal is now breeding regularly in Denmark after approx. a hundred-year break – albeit modestly.

Heavy metals and environmentally hazardous substances

  • Investigations of lysosomal membrane stability in common mussels indicated that approximately 50 % of the investigated coastal areas were affected by hazardous substances.
  • Investigations of eelpout showed marked differences in the degree of effects between areas, both concerning presence of malformation in fry and CYP1A enzyme activity. The highest effect levels were observed in certain fjords with a generally increased impact from urban and industrial areas.
  • In 33 % of the common mussel samples, the mercury (Hg) content was higher than the ‘good ecological status’ limit, and 58 % of the sediment samples showed Hg contents above the American Environmental Protection Agency (US EPA) Effect Range Low (ERL), which defines the threshold limit value for ecotoxicological effects on sediment-living organisms.
  • During the monitoring period the content of copper (Cu) in common mussels has varied. In Roskilde Fjord the Cu-content and the condition of the common mussels are correlated.
  • In 2010 a large proportion of the sediment samples contained metal concentrations higher than the ERL for all metals except for arsenic (As).
  • In 5 % of the common mussel samples, the TBT-content was above the quality criteria for food and 95 % had TBT-content, which was higher than the ecotoxicological assessment criteria (EAC).
  • For sediment only one out of seven stations had TBT-concentrations which were above the HELCOM limit for significant impact.
  • Concentrations of CB118 in fish were above the EAC in Nivå Bight only, which means that effect of PCB in the Sound cannot be ruled out. In the Wadden Sea and the Great Belt concentrations of CB118 were below the EAC in all individuals, and effects of PCB or HCB are therefore not expected.

PAH poses the largest problem with respect to the sediments, where 42 % of the samples had concentrations above the ERL. Effects of PAH – Indeno(1,2,3-cd)pyrene and Benz(ghi)perylene in particular – cannot be ruled out.