Aarhus Universitets segl

No. 475: Marine areas 2020

Hansen J.W. & Høgslund S. (red.) 2021. Marine områder 2020. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 195 s. - Videnskabelig rapport fra DCE nr. 475. https://dce2.au.dk/pub/SR475.pdf


The monitoring in 2020 confirmed the observation in recent years of a stagnant development or decline of several biological parameters, while a few biological parameters showed progress.

The seasonal distribution of the precipitation was characterised by an unusually heavy rainfall in February and unusually low rainfall in September; the rest of the year the precipitation followed roughly the pattern of the normal seasonal variation. Freshwater runoff from land was high at the beginning of the year due to the unusually wet autumn in 2019 and the very wet start of 2020, while the runoff was relatively low in the last months of the year. The seasonal variation of the nitrogen input to the sea followed the same pattern as the runoff.

The concentrations of nitrogen were in particular relatively high in the surface water in the beginning of the year as a result of the large runoff and the mixing of bottom water. From April and the rest of the year, however, the concentrations of nitrogen were relatively low, especially in fjords and coastal waters. In most months, the concentrations of phosphorus were on level with the long-term mean, but the concentration was somewhat below the long-term mean during late summer and for most of the autumn in fjords and coastal waters. The concentration of silicon was unusually high in March in fjords and coastal waters and in April-June as well as October-December in the open inner waters, presumably due to low uptake by silica algae during these periods and influence from outflowing Baltic water. The annual means of nitrogen and phosphorus have not changed significantly for a number of years, and the concentrations in 2020 corresponded to the level in recent years. Hydrographic calculations show that, during the water flow through the inner Danish waters in 2020, there was a significant transport of nitrogen from the bottom water to the surface water, but the transport was lower than in 2019.

The combination of high temperatures in the bottom water, large nutrient inputs in the beginning of the year, relatively weak winds in spring and a late end of the previous year's oxygen depletion led to an early onset of oxygen depletion in a number of areas. The oxygen depletion was widespread and intense in 2020. However, in the shallow areas, strong winds resulted in a significant but temporary improvement of the oxygen conditions in July. The oxygen depletion in the deeper areas did not improve markedly until after a period of several consecutive days of strong winds in the beginning of November. Along with the distribution in 2016, the distribution in mid-September was the largest since 2008. Calculations of the retention time of bottom water show that the southern Little Belt is particularly vulnerable to the development of oxygen depletion.

Critical oxygen conditions were the cause of the poor state of the bottom fauna in some of the study areas in the fjords and coastal waters, although bottom trawling can be a contributory factor to the poor condition in the area near Løgstør. In the open inner waters, the composition of the bottom fauna shows that there generally are no problems with eutrophication in relation to the bottom fauna, but in some areas negative effects due to physical disturbance from bottom trawling appear.

A complete description of the seasonal variation of algae growth in 2020 cannot be made due to absence of spring data as a result of the Corona lockdown of the monitoring. The estimated annual production was – compared with the level of the last few years – a little above the mean for fjords and coastal waters and somewhat above the mean for the open inner waters. In both types of water, the concentrations of chlorophyll and Secchi depth corresponded with the level in the 1990s. In general, the environmental state of the water column has deteriorated since 2012.

The depth distribution and coverage of eelgrass generally developed positively for a number of years after 2007. In recent years, the development has been stagnant with a negative trend, both in terms of depth distribution and coverage. For the past 10 years, the coverage of macroalgae (seaweed) has increased in coastal waters, while the trend has stagnated in inner fjords, outer fjords, the Limfjord and on stone reefs. In 2020, the coverage of macroalgae on stone reefs in open waters was almost equal to that of the best years so far – 1997 and 2019.

The number of harbour seals has grown steadily since the they were protected in the 1970s, but in recent years it has stabilised or declined in most areas, which indicates that the population has reached the level of environmental sustainability. The number of visiting grey seals has increased significantly since 2005, but in recent years breeding has been limited (< 10 pups/year). The number of porpoises in the habitat areas in the North Sea is stable, whereas it is declining in the Skagerrak. In the inner Danish waters, the counting in 2020 indicated a significant decline, whereas the acoustic surveillance showed increased local porpoise activity in the Little Belt. The population of porpoises in the inner Baltic Sea is critically endangered.

In several cases, the concentration of heavy metals in mussels and fish was above the environmental quality requirements but below the threshold values for food. Brominated flame retardants were found in concentrations above the environmental quality requirements in all fish samples except one. The level of chlorinated pesticides, e.g. DDT, in the fish studied was below the environmental quality requirements, but for most it was above the background values. The level of tar substances was generally below the background values. TBT was not detected in mussels in 2020.

The development in recent years has shown that Danish waters are still very vulnerable to pressures and are still far from the goal of stable good environmental conditions. In addition to nutrient input (eutrophication), the state of the environment is also affected negatively by e.g. fisheries, climate change and hazardous substances. In summary, data from the national monitoring programme show that the marine environment responds positively to reduced pressure, but the changes at ecosystem level are slow.