When you think of reefs, you most probably think of tropical coral reefs with lots of colourful fish and clear blue water. Maybe some turtles and sharks as well.

Reefs in the Northern Bothnian Bay are nothing like this. The reefs we have are basically just piles of rock. The bedrock is under much too thick a layer of moraine (till) to be exposed from underneath the many meters of sand and rocks. If you ask someone to define the Natura 2000 nature type “reef – 1170”, the person or a handbook would say that the reef is composed of either rock, boulders, stones or of biogenic concretions (corals or tube worms or something like that). It should rise from the bottom (a simple flat rocky bottom won’t do) and it should be permanently under water as well. The vegetation can be in layers, with the green filamentous algae at the surface, with a layer of Fucus species and then a layer of red algae, or it can be almost completely covered in blue mussels and all other invertebrates, which are often associated with the blue mussel beds.

Pile of rocks and a transect line.
Reef rising up from the flat sandy bottom (photo by Linda Jokinen, Metsähallitus).

If you now look at the reefs we have up here in the SEAmBOTH project area, you’ll notice first that we completely lack both the blue mussels and the Fucus. We are almost devoid of red algae as well, and that leaves our reefs with some filamentous algae, water mosses, polyp animals and a sponge animal, Ephydatia fluviatilis. Mostly the reefs in the Northern Bothnian Bay have just a few bushes of water mosses and a few small invertebrates here and there with a layer of 1 cm long filamentous algae on the rocks.

Rocks under water, covered with vegetation.
Reef with filamentous algae and water moss (photo by Jan Ekebom, Metsähallitus).
Close up of a reef with polyp animals and sponge animals, Ephydatia fluviatili (photo by Essi Keskinen, Metsähallitus).

Our reefs may not rival those of the tropics and the coral reefs in color, but they still form an important habitat for many species. Since the definition of a reef is that it rises from the seabed, it usually means that everywhere around the reef, there is mostly just flat sand or mud, which is not at the top on the list of underwater marine species hotspots. But the reef then – it offers a growing platform for water mosses and algae, which need a hard bottom to attach themselves, and so do the polyps and the sponge animal as well. The vegetation then attracts other invertebrates, for example snails, that graze on the algae, or fish, which lay their eggs on the water mosses, or invertebrates and fish which come to feed on the grazers or eggs and so on.

The reef forms an oasis in the desert of muddy sea bottom.

Picture collection of reef bottom and different species.
Example of a dive transect done on a reef (image by Johanna Kantanen, Metsähallitus).

Written by Essi Keskinen, Metsähallitus.


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