Hansen J.W. & Høgslund S. (red.) 2023. Marine områder 2021. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 220 s. - Videnskabelig rapport fra DCE nr. 529. http://dce2.au.dk/pub/SR529.pdf.
Several biological parameters showed progress in 2021 compared to 2020. However, overall observations from the past 10 years show a stagnant development or a decline, especially for parameters in the water column and phytobenthos.
The air temperature was average with only minor deviations from what was expected throughout the year. Sea temperatures were slightly above normal, but somewhat lower than the record highs of 2014 and 2020. The wind was generally light and only few months reached normal levels. In terms of precipitation, 2021 was a very normal year, however, May was unusually wet, while February and June were relatively dry. Freshwater runoff from land was low in the first months of the year, slightly increased in May and followed the normal seasonal pattern for the remainder of the year.
Nitrogen concentrations were relatively low in most months, and in some months the lowest recorded to date. However, the concentrations were relatively high in the January due to calm wind conditions, which caused reduced water exchange and, thus, a greater significance of the runoff from land. The monthly means of phosphorus were also generally low in 2021. However, the concentration of dissolved inorganic phosphorus was at par with the long-term mean in the open inner waters except for January, where the concentration was very high, as was the concentration of total phosphorus. The concentration of dissolved silicon followed the normal seasonal pattern in fjords and coastal waters. In the open waters, the concentration was very high at the beginning of the year and in April-June, which is due to the impact of various water masses. Annual means for nitrogen and phosphorus have not changed significantly since 2003 and 1998, respectively. However, in 2021 the concentrations of nitrogen and phosphorus, excluding dissolved inorganic phosphorus, were at the low end of the level for the past several years. The relatively high level of dissolved inorganic phosphorus is probably due to a combination of reduced algal growth and changed procedures for storing water samples.
The modelled hydrography in the inner Danish waters showed that there is large vertical transport from the bottom water to the surface water, especially in the Kattegat and the Belt Sea. This upward transport of water adds significant amounts of nutrients to the surface water, where nutrients stimulate algal growth. The retention times calculated on the basis of the water transports show that the retention time is particularly long in the southern Little Belt, which contributes to the area's vulnerability in relation to developing oxygen depletion.
The combination of several factors caused oxygen depletion to start later in 2021 than in 2020. Oxygen depletion also developed relatively slowly in most places due to the relatively cool spring and, periodically, a lot of wind. The spread and intensity of oxygen depletion increased significantly from the end of the August to the end of the September. In shallow areas, winds caused an improvement in oxygen conditions at the beginning of October. In the deeper areas, oxygen depletion only became less significant after several days of continuous wind at the end of November, at which time there were still widespread areas with moderate oxygen depletion. The extent of oxygen depletion in September was the third largest since 2008 and the sixth largest since the worst oxygen depletion to date in 2002. The extent of oxygen depletion in September has been increasing since 2010. In Southern Little Belt, the length of the oxygen depletion season has increased by just over two months since the mid-1980s.
Critical oxygen conditions caused poor conditions for the bottom fauna in some of the study areas in fjords and coastal waters. In the open inner waters, the species composition of the bottom fauna indicates that generally there are no problems associated with eutrophication in relation to the bottom fauna, but in some areas there are negative effects of physical disturbance from bottom trawling. Monitoring of the bottom fauna in the North Sea indicates that the communities here are also affected by physical disturbance.
In 2021, algal growth was among the lowest measured in the monitoring period, both in fjords and coastal waters and in the open inner waters, and the water was clearer than usual with low chlorophyll concentrations for a major part of the year in the open inner waters. However, in general the state of the environment in the water column has deteriorated since 2012, as the concentration of chlorophyll has increased and the water has become more turbid.
Observations of eelgrass showed improvement in the inner and outer fjords and status quo in coastal waters and the Limfjord compared to 2020. Despite the progress made in 2021, monitoring over the past ten years shows that the positive development that existed for a number of years after 2007 has stagnated or turned to a decline, both in regards to depth distribution and the degree of coverage. Similarly, the degree of coverage of macroalgae (seaweed) has stagnated in all water types and in stone reefs. Studies of deep-lying stone reefs in the North Sea and Skagerrak showed signs of physical disturbance of the reefs.
The number of harbour seals has grown steadily since the conservation in the 1970s, but in recent years it has stabilised or declined in most areas, which indicates that the population has reached a level of environmental sustainability. The number of visiting grey seals has increased significantly since 2005, but in recent years the grey seal has only bred to a very limited extent (< 10 pups/years). The presence of porpoises in the habitat areas in the North Sea is stable, whereas it is declining in the Skagerrak. Acoustic monitoring of porpoises showed progress in five out of six Natura 2000 areas.
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. The level of chlorinated pesticides, e.g. DDT, in the examined fish was below the environmental quality requirements, but for the majority above the background assessment criteria. Values of organic fluorinated compounds (PFAS) in fish were below the EU's environmental quality requirements.
Effects of environmentally harmful substances were observed in many Danish ports, where there were signs of hormonal disturbances caused by TBT. Measurements of lysosomal membrane stability and PAH-specific effect indicators also showed ecotoxicological effects above the background levels at the studied stations outside the ports.
Organic matter and nutrients in coastal sediments have been monitored over a period of twenty years (from 1999-2003 to 2017-2021). Results from the monitoring indicates that most of the exchangeable nitrogen that was added to the sediment in the period up to the mid-1990s has been converted and released before the monitoring of the sediment pools began in the period 1999-2003.
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 the nutrient input (eutrophication), the state of the environment is also affected negatively by e.g. fisheries, climate change and environmentally 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.
