Large shallow bays

Large shallow inlets and bays is a Natura 2000 habitat that can be found all around the Baltic Sea. There are several large shallow bays in the SEAmBOTH area as well, for example the Liminganlahti Bay in Finland and Råneå Bay in Sweden.

Two mappers working in shallow water.
A very shallow coast in Halosenlahti Bay, Haukipudas, Finland. Photo by Ashley Gipson.

If you read the Natura 2000 habitat description for the large shallow bays, it says that the freshwater influence is usually limited. This means that if you have trouble trying to separate large bays and river estuaries from each other (because sometimes the river estuaries can be shaped like large shallow bays), one way to do this is to see, if there are large rivers discharging their water to the area.  Flads and lagoons, on the other hand, are smaller than large bays, and they have a threshold, a shallower area, between the lagoon and the sea, which restricts the water exchange between the sea and the lagoon. Bays don’t have a threshold like this.

Flad surrounded by forest, sea on the background. Aerial picture taken with a drone.
Flads have treasholds which separate them from the sea. A flad in Maakrunni, Ii, Finland. Photo by Jaakko Haapamäki.

Large shallow bays are one of the most important and valuable habitats both in the SEAmBOTH area and in the Baltic Sea marine nature, according to Zonation analysis and Elina Virtanen from SYKE.

What makes the large shallow bays so valuable then? There are a myriad of fish species that use these areas as their spawning grounds, many of the bays are important resting, feeding and nesting grounds for migrant and domestic bird species, and the macrophyte species diversity is often very high in large shallow bays. In the SEAmBOTH area many of the threatened aquatic plants can be found in the large shallow bays, for example Alisma wahlenbergii and Hippuris tetraphylla, as well as the directive species beetle Macroplea pubipennis.

Pike close to the bottom, pondweeds on the background.
A young pike. Photo by Suvi Saarnio.

And why are all these things concentrated in the large shallow bays? Some of the reasons are these: even though the bays are directly connected with the sea and the water flow is not restricted like with the lagoons, bays are still often somewhat sheltered from the strongest winds and wave action. Also, like the name suggests, shallow bays are SHALLOW, and that invites a higher biodiversity compared to many deeper areas where the light just doesn’t reach all the way to the bottom and enable photosynthetic plants to grow there. In addition to lush vegetation, many bays have a rich benthic fauna. Birds are also attracted to the shallow areas and the wetlands around the bays.

The unfortunate thing is that most of the bays are already altered by man and there are a lot of different human pressures that can limit the marine biodiversity, such as dredging, coastal building, harbours and recreational boating. Not to mention the eutrophication development in the whole of the Baltic Sea. In addition to flads, lagoons and river estuaries, large shallow bays are the most threatened, and most valuable, of all marine habitats.

Despite all the threats, large bays still host a beautiful underwater nature, as can be seen in this video from Swedish bays.

Essi Keskinen


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