Aarhus University Seal / Aarhus Universitets segl

No. 123: Marine waters 2013. NOVANA

Hansen, J.W. (red.) 2015: Marine områder 2013. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 142 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 123. 


The nationwide status of the Danish marine environment in 2013 is summarised by subject below.


  • Air temperature was 0.7 °C higher in 2013 compared to normal (1961-1990) but slightly colder than compared to the last 12 years. The first six months were relatively cold, whereas the last six months were quite some warmer compared with the standard period.
  • The wind speeds were generally low throughout the year. Although a few strong storms at the end of the year caused complete mixing of the water column. The weak wind in January-April, mainly from eastern directions, probably contributed to a steady transport of water from the Baltic Sea up through the inner parts of the open Danish waters.
  • In 2013 the weather was generally dry and sunny. The number of sun hours was 19 % above normal and the amount of precipitation was 6 % below normal.
  • Sea temperature was a little higher than normal, but colder when compared to the latest 10-15 years.
  • Sea water temperatures have increased 1-1½ °C during the last 30-40 years.
  • Since the mid-1980s, pH of sea water has been reduced (approx. 0.2 in open inner waters and approx. 0.1 in fjords and coastal waters). 

Nutrient concentrations

  • Overall, nutrient concentrations in 2013 were very low, especially for organic nitrogen and total nitrogen.
  • Phosphorus concentrations were comparable to the levels in recent years, but significantly lower than at the beginning of the 1990s for fjords and coastal waters.
  • Concentrations of inorganic nutrients were relatively high in the beginning of the year due to outflow of water from the Baltic Sea and a reduced uptake by phytoplankton due to a late spring bloom.
  • Potential nitrogen limitation was very high in 2013 in fjords and coastal waters and comparable to the level in recent years in the open inner waters. This has resulted in a relatively large excess of phosphorus, and accordingly a decrease in the potential phosphorus limitation.
  • Nitrogen and phosphorus concentrations show a clear decreasing trend since 1989, especially considering 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.
  • The reduced nutrient concentrations can to a large extent be attributed to waste water treatment of phosphorous and a reduced nitrogen surplus from agricultural production.
  • Silicate concentration was on the average level for the last 10-15 years. Concentrations varied strongly during the year, especially in the open inner waters depending on the influence from the Baltic Sea and the North Sea. 

Phytoplankton, zooplankton, and water clarity

  • Higher water clarity (Secchi depth) and lower chlorophyll concentrations and primary production (algae growth) in 2013, especially in the open inner waters, indicated an improvement in the state of the marine environment.
  • In fjords and coastal waters, chlorophyll concentration was reduced significantly from 1989 to 2013, whereas the change in Secchi depth has been more variable.
  • In the open inner waters there has been a tendency to increasing chlorophyll concentration and decreasing Secchi depth. This trend was, however, broken by the low chlorophyll concentration and the high Secchi depth in 2012 and 2013.
  • Algae growth was on a relatively low level in 2013 and has decreased significantly over time due to lower input of nutrients. The algae growth was somewhat higher in the fjords than in 2012, most likely caused by increased release of nutrients from the sediment due to widespread oxygen depletion. This demonstrates the vulnerability of the marine environment.
  • There were reports of increasing amounts of toxic species of phytoplankton (Dinophysis sp.) in 2013 compared to 2012.
  • The biomass of ciliates was higher than in 2012, but the biomass of the remaining micro zooplankton was on the same low level as in recent years. 

Oxygen deficiency

  • Oxygen depletion in 2013 was relatively significant in a number of coastal waters (incl. fjords) due to a warm summer with relatively weak winds, whereas oxygen depletion was not intensive or widespread in the open parts of the Danish waters.
  • The estimated total area affected by oxygen deficiency in September 2013 was larger than in the last three years but less than in the years up to 2010. Almost 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, the archipelago south of Funen, and Fehmarn Belt/Lübeck Bight. In the Little Belt including some fjords and in the archipelago south of Funen toxic hydrogen sulphide was released from the sediments.
  • Oxygen depletion started at the beginning of June (the Limfjord) and late June (Aabenraa Fjord and the archipelago south of Funen). In most areas, oxygen deficiency was registered from July-August and disappeared completely in connection with a violent storm in late October.
  • Oxygen concentrations have generally been decreasing during the period of monitoring in the open parts of the inner Danish waters, but during the past 10 years there has been a positive development. 

Bottom flora

  • 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.
  • Within the latest five years, an improvement has taken place for eelgrass in some fjords. These improvements were further developed in 2013. In the Limfjord, the maximum depth distribution has increased by 46 % and the main coverage by 31 % from 2008 to 2013. The depth distribution has also significantly improved in Kalundborg Fjord, the Sound, the Isefjord and Odense Fjord in 2013. In contrast, Als Sound, Kolding Fjord, and Vejle Fjord have experienced a negative change.
  • The eelgrass shows an overall tendency to cover still less of the bottom along the monitored transects in the period from 1989 to 2013. However, during the past five years coverage has improved at depths of 1-2 m and 2-4 m in the Limfjord and in the outer parts of the fjords, respectively, as well as at depths of 4-6 m at the open coast.
  • The reduced nutrient loading seems to begin to have positive effects on the eelgrass distribution in some areas. This may i.a. be related to the improved water clarity during recent years. However, the eelgrass populations in many areas have diminished to an extent that the positive development in eelgrass distribution is slow and delayed compared to the reduction in nutrient loading.
  • The development in eelgrass distribution and coverage fluctuated during the period of monitoring (1989-2013). Therefore, the positive trends in recent years (2008-2013) should be interpreted with caution.
  • For the monitoring period (1989-2013) there has been a positive trend for the total cover of macro algae in inner fjords, outer fjords, coastal waters and on reefs, whereas the trend has been negative for the Limfjord. Similarly, there has been a significant positive trend for the cumulated cover in fjords and open coastal areas except for the Limfjord, where also this trend has been negative.
  • The total macroalgal coverage in the deeper parts of selected stone reefs in the open parts of the Kattegat was generally good in 2013, which is in accordance with the improvement of the Secchi depth in the open inner Danish waters in the first half of the year. 

Bottom fauna

  • In both 2011 and 2013 the species richness (alpha diversity), biomass, and to a lesser extent the abundance for bottom fauna in fjords and coastal areas increased with latitude from the Baltic Sea in the south to the North Sea in the north, whereas there was no significant difference between the years for any of the parameters.
  • The long-term development of species richness, abundance, and biomass clearly varied between the fjords and the open coastal areas during the monitoring period (1989-2013). All parameters decreased in fjords, whereas there were no changes for the open coastal areas. This supports the hypothesis that reduction in nitrogen loading (oligotrophication) has had an effect on bottom fauna.
  • In the open waters, the biomass in 2013 was the highest since 1995 due to a large number of clams, especially black clams. In the period 1994-2013 there was no clear trend for biomass.
  • The species richness and the abundance were halved from 1994 to 2008. In 2010 the species richness increased significantly, whereas the number of species in 2011 and 2013 was close to the average level for 1994-2008.
  • The highest species richness and abundance of bottom fauna in the open waters was in the mid 1990’s, a period with high nutrient concentrations. However, it is most likely but uncertain whether the subsequent decrease in the species richness and abundance was due to reduced nitrogen loading (oligotrophication), as the changes are smallest in the shallower and typical more coastal parts of the open waters, and as the biomass does not show any clear trend.
  • The species richness and the abundance are highly dependent on inter annual variation in recruitment. However, the cause of these variations in recruitment is not known. 

Seals and porpoise

  • The harbour seal is the most common seal in Denmark and the population has increased from approx. 2,000 animals in 1976 to approx. 17,000 animals in 2013 mainly due to a hunting ban in 1977 and the establishment of a number of seal sanctuaries where access is prohibited.
  • The harbour 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 11 %, 8 %, 12 %, and 14 %, respectively.
  • The grey seal population has shown improvement over the past 10 years and in 2013 up to 76 in the Kattegat, up to 87 in the Wadden Sea, and 404 in the Danish part of the Baltic Sea were registered.
  • Some grey seals are now breeding regularly in Denmark after approx. a hundred-year break.
  • The number of harbour porpoises in the North Sea and Skagerrak registered in 2013 was approximately half as many as found in 2011 and 2012. There is no clear explanation for the fewer registrations in 2013.
  • Registrations from the Great Belt, Kalundborg Fjord, Little Belt and Flensborg Fjord showed that the porpoises are present all year round, but that the abundance varied seasonally. 

Hazardous substances and biological effects

  • Levels of hazardous substances in samples from mussels and fish were generally low and below EU’s Environmental Quality Standard (EQS) and other criteria except for mercury (Hg), tars (PAH), and possibly perfluorinated compounds (PFAS – see below).
  • For dioxin, tars (i.e. PAHs), brominated flame retardants (HBCDD and PBDE), and PFAS in biota only one PAH exceeded EQS. High concentrations of PFAS were found in liver, but as the EQS is for whole organisms a conversion factor is needed before the significance of the concentrations in liver can be assessed.
  • In 61 % of the soft tissue samples from mussels and in all fish samples the level of Hg exceeded EQS.
  • Despite the ban on use of organotin (TBT) in anti-fouling bottom paint 50-75 % of the mussel samples exceeded the OSPAR Environmental Assessment Criteria (EAC) for ‘good’ status. TBT and Hg are the most common reason to classification below ‘good’ status due to the load of environmentally hazardous substances.
  • In 40% of the sediment samples, the concentration of zinc (Zn), cobber (Cu), and cadmium (Cd) was so high that negative effects could not be ruled out.
  • For PAHs and naphthalene, OSPAR has criteria for nine substances. Of these it cannot be ruled out that there is risk for impact of sensitive species due to benzo(ghi)perylene (34 % of samples) and indeno(1,2,3-cd)pyrene (6 % of samples) in the sediment.
  • Imposex and intersex in snails, which are caused by TBT, are still widespread and an environmental problem in Danish waters, primarily in inner Danish waters. However, the level has decreased markedly in recent years, which indicates that further improvements can be expected.
  • 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.