Underwater mosses

While mosses are commonly known from marshes, rocks, and forest floors, few know that there are also numerous underwater mosses in the Baltic Sea.

Although water mosses are found along the entire coast of Finland, the number of species and individuals is particularly high in the northern Bothnian Bay, where the water is almost fresh. In addition, this area lacks species such as bladder wrack (Fucus vesiculosus), mussels (Mytilus trossulus), and barnacles (Amphibalanus improvisus), which compete with mosses for the same hard anchoring surfaces in the more southerly sea areas.

In the Bay of Bothnia, water mosses grow more prominent than elsewhere and are the most significant species forming biomass on rock reefs. To some extent, they have replaced the missing bladder wrack’s role as a habitat former.

A bush of greater water moss.
Lush water moss communities provide food and protection for a diverse range of fish and invertebrates, especially in shallower brackish waters where the bladder wrack does not survive.

Greater water moss is the most widespread of the water mosses

The most prominent and widely distributed species of water moss on the Finnish coast is the greater water moss (Fontinalis antipyretica), also known as the antifever fontinalis moss. It can be found along the entire coastline from Hamina to Tornio. There are two other relatively common species of Fontinalis moss that are most often found in the Bay of Bothnia.

Some moss species have adapted to life in brackish water

Dozens of water mosses occur in the Bothnian Bay and occasionally in the Gulf of Finland. Although many water mosses are most at home in fresh and/or flowing waters, some species have also adapted to life in brackish water. Water mosses are a relatively unknown group thus far, and even more new species adapted to brackish water may yet be found.

While many of the water mosses that occur in the northern part of the Bay of Bothnia are classified as endangered, it may be more a question of a lack of information rather than the rarity of a particular species.

An enlarged view of the greater water moss, Fontinalis antipyretica, using a microscope.
Identifying water moss species almost aways requires a microscope. Pictured here is the greater water moss, i.e. Fontinalis antipyretica.

You can only find the small and beautiful water pocket moss by diving

Classified as endangered, the water pocket- or phoenix moss (Fissidens fontanus) is a small but beautiful species resembling a bird's feather. It is usually located at depths of one to seven metres on a relatively open rocky bottom.

It cannot be found by wading and it is too fine to be retrieved from the depths by raking. Similarly, it is too small to be seen when sampling by drop-video. The only way to find this tiny but surprisingly common moss is to scuba dive and search for it.

Water mosses also grow in deeper waters

Water mosses are perennial species and can overwinter under sea ice. They are extremely efficient users of sunlight and can be found up to nearly ten metres deep. They can even thrive in the humus-rich, river-influenced waters of the Bothnian Bay, occurring up to seven metres deep. 

The lush greater water moss of the Bothnian Bay shrinks as salinity increases

The greater water moss of the Bay of Bothnia resembles its counterparts in fresh and flowing waters. It forms lush, leafy, and abundant bushes with shoots up to tens of centimetres in length.

However, moving southwards towards the Kvarken area, the salinity grows and the outer appearance of the greater water moss changes: it becomes slender, relatively small, and lighter green in colour. Neither the greater water moss, nor other water mosses thrive as well in the waters between Hailuoto Island and the Kvarken as they do in the northern Bothnian Bay and within the Kvarken Archipelago itself.

Water mosses are the most diverse in the river estuaries of the
northern Bay of Bothnia

If you want a surefire way of finding water mosses, it is worth heading towards the giant river estuaries of the Kemijoki- and Tornionjoki Rivers, in the northern Bay of Bothnia. In these waters, as many as five or six species of water moss can be found on the stony reefs covered with giant moss meadows.

Brackish water mosses are relatively unknown

Water mosses occurring in brackish water have been studied a lot less than their counterparts from flowing water or other freshwater habitats. However, researching water mosses in brackish water almost always requires scuba diving, which is an expensive and slow method.

Water moss species:

  • Greater water moss or antifever fontinalis moss (Fontinalis antipyretica)
  • Water pocket or phoenix moss (Fissidens fontanus)
  • Long-beaked feather moss (Platyhypnidium riparioides
  • Showy feather moss (Oxyrrhyncium speciosum)