Algae and diatoms

We all know that increased amounts of filamentous and blue green algae mean eutrophication. This much is clear. All kinds of algae grow much faster than vascular plants and because of that, algae can make use of the excess amounts of nutrients available during the growth season. Algae are tough competitors because they are such fast growers and during competition, they can grow on top of other species (epiphytes) or in open water (blue green bacteria). They easily outcompete many other species in a growth competition.

This, however, is only one part of the truth. All algae that can be found in the Bothnian Bay right now seem to be native species to the area. This means that they belong here (for example the exciting species Aegagrophila linnaei), and they are meant to be here and when they exist, the nature is in its current balance.

Green freshwater macroalgae. They consist of stiff branched one cell wide filaments, which gives them a brush-like appearance.
Aegagrophila linnaei. Photo by Niina Syrjälä, Metsähallitus.

The issue is not the existence but the amount of algae that grows in the Bothnian Bay. If the area is getting excess nutrients and nice warm temperatures and eutrophication kicks in, some species of algae start growing more than others and at the expense of others. Poor competitors can’t utilize nutrients or growth space as efficiently as algae and so their biomass grows. In the end, thick layers of green filamentous algal mass can cover everything else and suppress the growth of other species (for example Vaucheria sp).

Yellow-green algae that grow long, tube formed and unbranched filaments. Grows straight up from the soft sea bottom like green spiky hair.
Vaucheria sp. Photo by Essi Keskinen. Metsähallitus.

There is one group of algae which we usually don’t think when we are talking about algae – diatoms, or Bacillariophyceae, tiny microalgae that can mostly be seen by microscope. No, wait, now I’m lying – they CAN be seen by naked eye, but only as brown bits or sludge on top of everything and covering everything under the surface of the Bothnian Bay sea. They can be identified with a good microscope only, and their taxonomy is a special field of algal biology.

Diatoms can be found almost everywhere – from oceans to brackish water seas to fresh water bodies to sea ice to even damp soil. Some of them might even thrive in atmospheric moisture only. Diatoms are part of the world’s oceans’ phytoplankton (approximately 45 % of the oceanic primary production comes from diatoms) so they contribute much even though they are so small.

In the Bothnian Bay, a scuba diver notices early that everything underwater is covered by brown sludge, or a slick layer of brown stuff. This is all diatoms. They can be either pelagic (freely floating in the water column) or attached or resting on surfaces like rocks, other vegetation or any other structure under water.

Bothnian Bay has a smaller number of other algal species compared to other sea areas around the Finnish and Swedish coasts, but that only highlights the importance of the vast amount of diatoms in the Bothnian Bay. So far, no indicator species have been identified among the species of diatoms in the Bothnian Bay, but that is probably only due to lack of resources – time, money and research.

Microscope picture of red algae. The long, branched, threadlike filaments bear dense whorls of branchlets, resembling beads on a string.
Batrachospermum sp. Photo by Essi Keskinen. Metsähallitus.

When we are looking for indicator species for the Bothnian Bay in the future, one possibility would be to look at the vast number of diatom species.

If you want to know more about the species of the SEAmBOTH area, see species guides on the Results page!

Written by Essi Keskinen, Metsähallitus


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