The environmental state of Mariager Fjord
Mariager Fjord, stretching 42 kilometers, is one of Denmark’s longest fjords. It extends from Hobro in the west to the Kattegat in the east.
The fjord’s physical shape, with a deep inner fjord and a shallow outer fjord, means that water exchange with the Kattegat is limited. The fjord is therefore naturally vulnerable to the inflow of nitrogen and phosphorus. The nutrient content in Mariager Fjord is among the highest in the inner Danish waters.
In terms of ecological status, oxygen levels, nutrients, plants, and animals, Mariager Fjord is unfortunately among the inner Danish waters highlighted as being in poor condition. Results from the Danish Environmental Protection Agency’s surveys in connection with the preparation of the River Basin Management Plan 2021–2027 assess that Mariager Inner Fjord has poor ecological status, while Mariager Outer Fjord is in bad condition.
Because of the fjord’s special shape, the deep part of Mariager is almost permanently oxygen-free, and only during strong high tides on the Kattegat coast does new oxygen-rich water enter from the Kattegat. Eelgrass has poor growth conditions in the fjord, bottom-dwelling animals have poor living conditions, and the high nutrient content in Mariager Fjord means that the production of phytoplankton is unusually high. Typically, between two and four times as much phytoplankton is produced in Mariager Fjord as in other Danish fjords and coastal areas.
Bottom plants
The bottom plants in Mariager Fjord consist of flowering plants and seaweed. Eelgrass and seagrass are the most widespread flowering plants. Surveys by the Danish Environmental Protection Agency show that eelgrass has poor growth conditions in the fjord. Eelgrass only extends to a depth of about one and a half meters. Annual free-floating seaweeds, such as sea lettuce, thrive in the nutrient-rich fjord, and dense growths of sea lettuce are often found in the fjord’s sheltered coves. Perennial seaweeds, on the other hand, have poor growth conditions in Mariager Fjord. This is partly due to the fact that there are very few large stones for the plants to attach themselves to.
Bottom-dwelling animals
Bottom-dwelling animals have poor living conditions in Mariager Fjord. Surveys by the Danish Environmental Protection Agency show that the number of species fluctuates between 2 and 22. This is a low number compared to other fjords in Denmark. The common species of bottom-dwellers in Mariager Fjord tolerate water with low oxygen levels. Species that require more oxygen are only found in small numbers in the fjord. Apart from blue mussels and occasionally sand gapers, not many large bottom-dwelling animals are found in the fjord. Blue mussels can tolerate short periods of oxygen depletion, and this species makes up the majority of the biomass of bottom-dwelling animals in the fjord.
Phytoplankton
According to the Danish Environmental Protection Agency, Mariager Fjord has a very high content of nutrients, which means that the production of phytoplankton is unusually high. Typically, between two and four times as much phytoplankton is produced in Mariager Fjord as in other Danish fjords and coastal areas. There are relatively few species of phytoplankton in Mariager Fjord. However, the species that do occur can cause very strong blooms, visible to the naked eye as the water turns reddish or brownish.