Flads, lagoons

A small flad in the Bothnian Bay national park is separated from the sea by a shallow threshold and a narrow mouth. Photo Metsähallitus

Have you ever heard of land uplift or land upheaval phenomenon? Or more correctly “post-glacial rebound”? It means that when the 2-3 km thick Ice age glacier was pressing the Scandinavia, it was so heavy that the ground was actually pushed downwards and it’s still bouncing back, after 10 000 years since the ice melted. If you want to brush up your knowledge about the geological history of the northern Bothnian Bay, read the blog from last year.  

Water temperature is often higher in the lagoons because water exchange between the sea and the lagoon is restricted, like here on the north side of Hailuoto island. Photo Metsähallitus.

Because of this phenomenon, new land is constantly rising from the sea. Flads, or small lagoons, are bays which are forming by land uplift. They are small lagoons which are almost completely cut off from the sea. There is a threshold between the sea and the flad and this threshold suppresses the exchange of water between the lagoon and the sea. This is why the temperature is usually higher in the flads in early spring. 

In the Kvarken area and in the SEAmBOTH project area, the land uplift is especially fast, and since the shores are really shallow, new land is forming very quickly. When the land uplift succession continues, the flad will become a glo lake when it’s completely cut off from the sea. Since the land uplift is so fast in this area of the Baltic Sea, the flads are a short-lived habitat which come and go in the history of the landscape. 

Vegetation in a flad is often lush and variable because lagoons are more sheltered than rest of the coast line. In this little flad in Maasarvi in the Bothnian Bay national park you can see at least Potamogeton perfoliatus, Potamogeton pusillus, Sagittaria sp, Callitriche hermafroditica and filamentous algae. Photo Metsähallitus.

Flads are nurseries and spawning grounds of many fish. The fish larvae, or baby fish, are in a safe place in the lagoons, where the temperature is nice and warm and which are quite sheltered from direct wave action. This is a reason why so many vascular plants and Charophytes can also be found in flads, and they are a heaven for many species of water fowl as well. Birds find nesting in or around the lagoons safe with plentiful buffet table under their beaks and many threatened species also thrive in the lagoons. 

Small flads are a specially protected nature type in Finland, and they are also considered as an important Natura 2000 nature type around the shores of the Baltic Sea. 

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Flada or lagoons can be found in stony or sandy shores, or they can even be found in bedrock shores south of the SEAmBOTH project area where bedrock is found. Lagoon bottom is quite often muddy because flads are sheltered from most of the wave action. Photos Metsähallitus.

Many of the flads are not in their natural state any more because people find them attractive as well. Lagoons offer safe harbors for small recreational boats and nice swimming places for holidaymakers with their summer cottages next to the flads. The threshold is often a burden for a recreational boater, so it’s dredged away and the water exchange with the sea is enhanced. This alters the ecology of the whole lagoon and the ecosystem services it provides. Often diches from the surrounding fields are lead to flads, and this will bring more freshwater, nutrients and solids into the lagoon. 

A small but important little flad from the Maasarvi island in the Bothnian Bay national park. Almost 15 species of vascular plants and algae can be found in this small lagoon, where also some threatened vascular plants can be found. Photo Metsähallitus.

Keeping some of the flads in their original state is important so that we will ensure that fish spawning grounds will exist in the future as well. 

Written by Essi Keskinen, Metsähallitus

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