Pelagic habitat

When I think of “a habitat”, the first thing that comes to mind from underwater is the benthic substrate – is it sand, is it rock, is it mud? Because that pretty much determines, what can and cannot grow there, or borrow there, or attach there.

With the pelagic habitat, it’s different. There is no bottom. It’s the water column between the surface and the bottom which makes the habitat. Of course, there are differences between pelagic habitat, different environmental factors that make every pelagic habitat unique (salinity, temperature, amount of light, amount of nutrients etc.) but it’s still all liquid and far away from the bottom.

One tiny fish in the water.
The baltic herring (Clupea harengus membras) swimming in the pelagic habitat. Photo by Pekka Tuuri.

Pelagic area is the vast amount of sea water which is far enough from the macrophyte communities near the shore and above the benthic habitats at the bottom. It is somewhat interlinked with another habitat, the sea ice, where the sea ice occurs at the open sea and not near the shore.

Sea and cloudy sky.
Calm sea close to Krunnit Nature Reserve. Photo by Suvi Saarnio, Metsähallitus.

In the last Finnish habitat type assessment 2018 the Bothnian Bay open sea was considered DD – data deficient. Sea ice in overall was considered VU, vulnerable, in Finland. It can be thought that during the winter, the uppermost part of the pelagic habitat turns into another habitat, the sea ice, but the part of the open sea that doesn’t freeze will still stay as the pelagic habitat. This year it seems that unless the weather takes a sudden turn to very cold, we are not going to have the pelagic habitat turned into the sea ice habitat.

Ice rafts and snow on sunny winter day.
Sea ice in Hailuoto. Photo by Pekka Lehtonen, Metsähallitus.

Who lives in the pelagic habitat then? It’s many of the fish (Baltic herring, vendace, white fish, to mention just a few), the plankton (once-celled algal plankton as well as the zooplankton) and also the Baltic Sea ringed seal, especially when the pelagic habitat turns into sea ice and the seals give birth to their pups there. What we’re lacking in the Bothnian Bay is for example the largest zooplankton in the Baltic Sea, the common or moon jellyfish Aurelia aurita. The salinity in the Bothnian Bay is not enough for the jellyfish, which is a conspicuous pelagic species that you can see with a naked eye and is not fast enough to swim away like most fishes.

The pelagic habitat of the Bothnian Bay differs from the rest of the Baltic Sea by being less salty, and also the primary production levels are not as high as elsewhere. Another special feature is that the sea freezes over most winters, this year being an exception.

Written by Essi Keskinen, Metsähallitus

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