Nature development in the project area in and around Egå Engsø, near Aarhus, Denmark
Remarkably diverse life thriving in and around a relatively new manmade lake, located on former agricultural land.
Authors: Nicolai Munk (Biologist, Greater Aarhus, Aarhus Municipality), Marta Baumane (Biologist, Amphi International), pictures and information is from the article: (Egå Engsø Plante- og dyrelivet i 2020 samt udviklingen i perioden 2007–2020, Grøn-consult) as well as survey results carried out by AMPHI International in 2020.
Egå Engsø is a large shallow lake, located in Aarhus, it was established in the autumn of 2006. The lake was formed by raising the water level in the Egå Stream, which ran through a low-lying area used for intensive agriculture. The lake was established to reduce the input of nitrogen to Aarhus Bay and create a habitat for aquatic plants and animals. After the lake’s establishment, a detailed study of the natural conditions in the lake and on the surrounding grasslands was carried out in 2007. The investigation showed that the lake had brought a great natural value to the area, as a diverse and species-rich plant and animal life had developed over the course of just one year. A diverse macroinvertebrate fauna and aquatic plant flora meant that there was also a species-rich breeding population of water-related bird species and many resting waterfowl. The areas were examined using the same methodology in 2020 to study how the habitats surrounding Egå Engsø as well as plant and animal biodiversity have changed since 2007.
Throughout the period 2007–2020, the aquatic vegetation has been species-rich and relatively stable in terms of species composition. Aquatic plant density, on the other hand, has varied quite a lot. There was a rapid immigration of aquatic plants to the entire lake in 2007, a mass occurrence of Elodea canadensis in 2008, following a low plant density in 2009 and a gradual increase within the last decade. Currently, the aquatic plant density in Egå Engsø can be classified as high.
Contrary, the vascular plant species richness in the areas surrounding the Egå Engsø has decreased. Many annual species were registered in the field study done in 2007 as large share of the area was used as arable land prior to restoration as well as due to the disturbance created during the establishment process of the lake. Even though the number of species has decreased, the vegetation has become more stable and new species have immigrated into the area since then. In the northern part of the lake, remnants of intensive grasslands are still present — these are areas where such sown grasses as Lolium perenne and Dactylis glomerata are dominant. Most of the grasslands are grazed by cattle, thus creating suitable habitats for various bird and invertebrate species. The unmanaged parts, mainly in the southern part of the lake, are dominated by tall herbs and grasses and various shrub species.
The Egå Engsø macroinvertebrate fauna is rich in species, as the lake has diverse substrate, high aquatic plant density and relatively good water quality. In addition, the water is rich in nutrients and calcium.
Regarding bird fauna in the period 2007 to 2020 there were 30 to 35 species of breeding birds associated with the lake and the grasslands, of which 14 of the species are on the Danish red list of threatened species. In the same period, a total of 232 species of resting birds were observed, of which 81 of the species are rare in Denmark. This means that the lake and the surrounding areas are very valuable for birdlife and especially waterfowl. The number and distribution of amphibian species, reptile species and mammal species have also increased in the period 2007 to 2020.
Egå Engsø thus has a great natural value — it is an area with a rich flora and fauna. This applies particularly regarding aquatic plants, aquatic insects, breeding and resting birds and mammals. It is remarkable that a shallow lake, restored more than 15 years ago and occurring near a city, can have such a high biodiversity.
The great species richness in Egå Engsø is mainly due to the fact that the lake is shallow and has a well-developed underwater vegetation with many underwater plants, which provides good living opportunities for many aquatic insects and fish. There are thus good food options for many waterfowl, and the shallow water depth means that the birds can forage all over the lake. In addition, cattle grazing on the grasslands means that a tall grass community does not develop in the transition zone from grasslands to lake, so the birds have free access to the lake. It is also of great importance that the birds in the majority of the lake can roost and forage in peace without human disturbance.
