Ørnsø receives most of its water from River Funder Å, which, like the Arnakke well and several other wells in the area, is highly ferrous. The water from Lake Ørnsø runs with River Lyså on to Lake Silkeborg Langsø.

Fish

The fish stock in Lake Ørnsø was examined in 1998 by Aarhus County. With 15 different species of fish, the lake is among the most species-rich Danish lakes. Among others you can find whitefish, smelt, ruffe, zander, pike and burbot. The dominant species of fish are roach, which is typical of nutrient-laden lakes with unclear water.

Water quality

The water in Ørnsø is unclearly large parts of the year due to algal growth. It is especially diatoms that color the water brown. Blue-green algae appear to a greater or lesser extent in late summer. Transparency in the summer period was last measured at 1.5m in 2008. The biologists estimate that it is up to 2 - 2.5m to get the water plants back in the lake. Around 1900 there was widespread underwater vegetation in Lake Ørnsø. Studies in the mid-1950s and 1990 did not find underwater plants at all.

Supply of phosphorus

It is especially the nutrient salt phosphorus, which is the cause of the many algae in Ørnsø. Phosphorus is an essential component of wastewater from fish farms, households and in the farmer's fertilizer to the cultivated fields. In the 1950s some fish farms were established along River Funder Å, which emitted large amounts of phosphorus. Today there is a better treatment of wastewater from the few remaining fish farms along Fonder Å. Earlier, large amounts of domestic waste water were also diverted to Ørnsø from wastewater treatment plants in the surrounding area. Today, the waste water from urban development has ceased and the municipality is continuously working to reduce the environmental impact from so-called rain-based outflows.

Is the lake getting better?

Phosphorus, which has been discharged into the lake through time, is accumulated on the lakebed, from which it is released each year as temperature rises for the benefit of the algae. Addition of phosphorus from the surroundings with the watercourses also contributes. Although the water quality in the lake has improved, the lake remains relatively polluted, measured on the content of algae and nutrients. The positive development seems to have stopped, and the inpact with phosphorus to the lake has been fairly unchanged in recent years. It may take a lot of years before Lake Ørnsø again becomes clear water.