Seabirds – a part of the sea

When thinking about the sea one probably first thinks about the sound of waves and after that, the calls of seabirds. Birds are a large part of the nature at sea, but seldom a thought is given to how they are a part of the underwater ecology. So, let’s give some thought to how birds might affect and are affected by what happens under the water in the northern Bothnian bay.

A lot of seabirds nest on small and isolated islands, because of the low level of predation here. In our area for instance razorbills, black guillemots and different seagulls and terns can be found nesting on barren islands in the outer archipelago. Other seabirds prefer to nest in reed-rich bays, also because of low levels of predation here. Species that we can find nesting in our shallow bays are ducks, swans, mergansers and grebes. Busy nesting sites can have an effect on underwater nature, as nutrients are accumulated around these sites. The nesting birds not only rely on the sea for its sheltering qualities, but also for feeding, and for that they themselves venture under the surface.

A small island formed of big rocks. Great cormorants and seagulls sitting on the rocks and flying above.
Seabirds colonize small treeless islands to avoid predation.  The great cormorants seen on the photo are not as common in the northern Bothnian Bay as in other parts of the Baltic Sea but do also breed here. (Photo by Petra Pohjola, County Administrative Board of Norrbotten)

An obvious way that seabirds affect underwater ecology is by feeding on fish, bottom fauna and underwater vegetation. By specializing on specific prey, birds can influence the whole food chain. For instance, feeding on small fish can lessen the feeding pressure on animal plankton, which in turn feed on algae. Grazing by birds can influence the composition and height of plant species in shallow bays. Grazing at these shallow sites can also momentarily affect the visibility, as the birds cause resuspension of soft sediments. The birds themselves are affected by visibility, as fishing birds that rely on vision have a lot harder time finding food in blurry water, or as food can become sparse for grazing birds as the water becomes too murky for plants to grow. 

Whooper swan's nest on water. A couple resting there.
The whooper swan breeds in sheltered bays and grazes on underwater vegetation. In other parts of the Baltic Sea the mute swan is more typical for sea habitats but in our waters the whooper swan is more common. (Photo by Petra Pohjola, County Administrative Board of Norrbotten)

By moving from one spot to the next, birds are important for many underwater species in the dispersal to new habitats. This has both negative and positive effects on underwater nature as birds can transport both invasive species as well as threatened ones. This transportation can occur through the consumption of spores and later releasing them through defecation, or involuntarily as plant parts temporarily tangle on the birds. For instance, oospores of charophytes can survive through the digestion tract of birds and hence birds can be valuable in the dispersal of charophytes. The invasive waterweeds spread through loose plant parts, which can tangle on the feet of birds and so invade new areas.

Although we seldom see marine birds as a part of underwater nature, they very much are so.

5 black guillemots on water.
Black guillemots are good divers and fish for bottom dwelling fishes.
(Photo by Petra Pohjola, County Administrative Board of Norrbotten)

Written by Petra Pohjola, County Administrative Board of Norrbotten


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