This is the largest and the deepest body of water in the Park. Ostrowite Lake is a mesotrophic, vendace-type lake. When it comes to hydrobotanic classification, it is considered to be a chara-type lake. Such designations are not very common. They are characterised by high water purity and high calcium content. Of note the Chara genus characteristic for the lake, create incredible underwater meadows.

Near the lake, nest some of the rarest birds of prey in Poland – the white-tailed eagle and the western marsh harrier. Also a couple of eagle owls have been observed making their home in this area. These species have been placed under strict and spatial protection (not only the species is protected, but also the area of its presence). You can also find here nests of the great reed warbler and the remiz. In autumn, flocks of cranes also gather here. Moreover, there are various species of fowl paddling about on the surface of the lake, including the goosander. Male birds of this specie have especially beautiful plumage. You can recognise goosanders by way of their characteristic crests, found both on males and females.

What is more, in March, you can observe the Eurasian bittern within the lake’s coastal rushes. It is a large bird, but not very easy to spot. However, it announces its presence with a resounding noise that resembles the sound that you can make while blowing upon the lip of an empty bottle.

Along the eastern border of the lake, the “Struga Siedmiu Jezior” trail (The Stream of the Seven Lakes) runs from Charzykowy to Chojnice. This last is a convenient start-point to get to this fabulous place.