The mires of Puurijärvi-Isosuo are part of the coastal Finland's zone of raised bogs. The barren centres of the mires are raised several metres higher than the more luxuriant, wet edges. Few pines stand on the centres, which consist of thick layer of turf growing sphagnum moss.
The mires of the National Park are almost in their natural state. However, the edges of some of the mires were drained before the National Park was established. Now these areas have been restored back to their natural state by blocking the ditches.
The centres of Isosuo and Ronkansuo Mires are open and full of puddles. Korkeasuo, Kiettareensuo and Aronsuo Mires are drier and more intricate. The edges of Isosuo and Korkeasuo Mires are formed of the riverside of Kokemäenjoki River in its natural state. Around the mires, there are some forests growing on mineral soil.
The mire vegetation is at its most beautiful in the beginning of the summer, when for example the Bog-Rosemary (Andromeda polifolia), the Marsh tea (Ledum palustre) and the Cloudberry (Rubus chamaemorus) are flowering. Along the duckboards, you can also see carnivorous plants: the Round Leaf Sundew (Drosera rotundifolia) and the Great Sundew (Drosera longifolia). Along the trail going to the campfire place on Isosuo Mire, you can also learn about peat digging.
The area (4,5 sq.km.) of Lake Puurijärvi is almost overgrown. In Finland it is one of the lakes with largest number of birds. It grows sedges (Carex), common reeds (Phragmites australis) and horsetails (Equisetum). On the lake shores, there are flood meadows and moist bushland.
In the 1700s, Lake Puurijärvi had clear water and hard bottom, but in the next century the draining of mires and clearing of rapids caused the water level to go down. Because the lake is so shallow, the landscape of Lake Puurijärvi changes greatly, depending on the water level. During dry spells, only little ponds of open water can be seen. There is a threat of the whole lake becoming overgrown and swampy. The challenge for the next few years is to restore the bird habitat of the lake.
Lake Puurijärvi is one of the important bird habitats in Finland (FINNIBA), and it has been proposed for inclusion in the Ramsar List of Wetlands of International Importance.
The traditional way of managing the shore meadows (1,0 sq.km.) is pasturing. This area is one of the largest traditional landscape sites in Satakunta region. Managing the landscape improves the conditions for the shore birds and the insects living on the open meadows on the shores.
The most abundant species on the mires of the National Park are the Meadow Pipit (Anthus pratensis) and the Yellow Wagtail (Motacilla flava). Also many wader species nest on the mires, such as the European Golden Plover (Pluvialis apricaria) and the Wood Sandpiper (Tringa glareola). On Isosuo Mire nest the Great Grey Shrike (Lanius excubitor) and the Red-throated Diver (Gavia stellata). Isosuo Mire is the area with most valuable set of bird species in the National Park. During the autumn and spring migrations, waders as well as geese and cranes (Grus grus) rest on the mires.
Lake Puurijärvi is an important stopping place for migrating water birds, such as geese and swans. In the spring, it is possible to see more than 400 Whooper Swans (Cygnus cygnus) and hundreds of Bean Geese (Anser fabalis) on the lake. About 35 species of birds nest around the lake and the surrounding flood meadows, for example the Bittern (Botaurus stellaris), the Whooper Swan, the Marsh Harrier (Circus aeruginosus), and the Black Tern (Chlidonias niger) which is rare in Finland. The lake is also an important preying area for many birds nesting in the surroundings, such as the Osprey (Pandion haliaetus) and the Northern Hobby (Falco subbuteo). Also the White-tailed Sea Eagle (Haliaeetus albicilla) can be seen above the lake preying on Muskrats (Ondatra zibethicus) or fish.
There is a diverse set of butterflies and moths living on the mires. Altogether 38 Lepidoptera species have been found in the area, including the Northern Grizzled Skipper (Pyrgus centaureae), the Freija´s Fritillary (Boloria freija), the Straw Belle (Aspitates gilvaria) and the Streaked Wave (Scopula virgulata). Also 24 different dragonfly species (Odonata) live there, for example Leucorrhinia albifrons.