The view above the sea surface gives only a hint of the landform underwater. A diver enters a kingdom of rocks and hills. Shaped by the Ice Age, the undulating moraine ridges offer an excellent soil for different algae, aquatic mosses and rockweed (Fucaceae). Amidst the boulders scattered here and there, it feels as though you have arrived at an underwater labyrinth.

In Kvarken, both saline and fresh water species live side by side, as it is located where the fresh water of Bothnian Bay meets the more saline water of the Bothian Sea. On the other hand, for many sea species, the water in Kvarken is not salty enough, while for many freshwater species, it is too salty. Due to its low saline levels, Kvarken is the northernmost habitat for various sea species.
Benefiting from eutrophication, filamentous algae have taken over areas from other species. From the ecosystem’s point of view, however, they are important survivors. Filamentous algae can live in conditions impossible for other species, thus protecting the invertebrates in the area. Slightly deeper, we are met by gold-coloured bladder wrack (Fucus vesiculosus) swaying in the water. Bladder wrack is one of the most important species in the Baltic Sea, as it provides food and shelter for a large number of plants and animals. Red algae (Rhodophyta) also form important habitats for other species, as they survive in very low light, and are therefore able to live deeper than other algae.
Charophyte algae (Charophyta), a group of freshwater green algae, live in sea bays and other calm areas, and provide important information on water quality. The most common vascular plants include sago pondweed or fennel pondweed (Stuckenia pectinata), claspingleaf pondweed (Potamogeton perfoliatus) and different types of watermilfoil (Myriophyllum). Their communities are valuable spawning grounds and growing areas for fish. In addition, the plants filter water flowing from the coast to the sea and stabilize the seabed with their roots.
The shallow open waters and bays of the Kvarken Archipelago are crucially important for many fish. Approximately 40 to 50 fish species live in the area and Kvarken is the northernmost habitat for many species including the great sand eel (Hyperoplus lanceolatus) and broadnosed pipefish (Syngnathus typhle). The rarest species of fish in the area is the critically endangered grayling (Thymallus thymallus). This type of grayling spawns in brackish water and is native to the Gulf of Bothnia, which is its only habitat in the world.

In spring, as soon as the ice has melted, nature provides the stage for a fascinating play. Numerous fish, such as the northern pike (Esox lucius), perch (Perca fluviatilis), ide (Leuciscus idus) and other cyprinids (Cyprinidae) swim to the coastal waters of the Kvarken Archipelago to spawn. Perfect places for spawning can be found in estuaries, and flads and gloes, which have been playfully named fish nurseries.

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The ferry between Korsbådafjärden and Stora Segelsören operates from May to September