Ecology - Habitats

Watery habitats

 Places such as streams, lakes and bogs are home to many species of bryophytes. The most significant of the bog bryophytes are the mosses in the genus Sphagnum. Sphagnum bogs are estimated to cover between 1% and 2% of the world's land surface (more than the area covered by any other single plant genus) and have significant ecological roles. You'll find more in the SPHAGNUM SECTION. Some of the bryophyte species found near water can also tolerate drier areas while others cannot survive away from a moist environment. Many bryophytes are found in association with freshwater but there are no marine bryophytes. A few species are found in brackish water.

Taxithelium merrillii on mangrove mud

An example is the moss Taxithelium merrillii shown here carpeting mangrove mud in a north Queensland river estuary. In such an area the carpet is subject to tidal inundation but also receives a high input of fresh water. The photo was kindly supplied by the Australian bryologist Andi Cairns of Townsville.

You will probably have seen lush bryophyte colonies at the margins of streams or lakes, on boulders in streams and even on the rock faces of waterfalls. The bryophytes in such sites are frequently splashed with water and, from time to time, may even become submerged for relatively short periods. However many people are unaware that there are also permanently submerged bryophytes, particularly in lakes, some capable of growing many metres below the water's surface. Having said that, it's important to note that there are no obligatorily aquatic bryophytes. All the evidence points to the conclusion that the bryophytes found in water are essentially terrestrial species, albeit with varying degrees of adaptations to a watery environment. All the species found growing in water (even those found submerged) can also be found growing on land. Aquatic and terrestrial examples of the one species can certainly look different. Looking at just three aspects of bryophyte life, the requirements for gas exchange, photosynthesis and structural support for an aquatic bryophyte are clearly quite different to those for a terrestrial bryophyte. It is therefore not surprising to find some physical or physiological differences between land and water bryophytes.

Stream and lake bryophytes are typically attached to some substrate, usually stationary (such as the beds or sides of the streams or lakes) but some mosses have been found growing on the shells of living freshwater molluscs. Studies of Swedish lakes have shown that submerged bryophytes are common, with 65 species having been recorded in Swedish lakes. Mosses have been found to blanket the bottoms of some of the smaller lakes, at depths of one to two metres and, still in Sweden, mosses in the genera Calliergon and Drepanocladus have been found at depths up to fifteen metres and the leafy liverwort Marsupella aquatica to 30 metres. In Lac Léman in Switzerland a moss, probably in the genus Thamnobryum was found at a depth of 60 metres. In Crater Lake in Oregon, USA thick moss mats can be found at depths from 18 to 60 metres and green moss plants in the genera Drepanocladus and Fontinalis have been dredged from a depth of 120 metres. An important determinant of the depth to which bryophytes can grow is the clarity of the water. Bryophytes need light so, the clearer the water the better the light transmission. Studies have shown that high water pressures inhibit various plant functions, but that the aquatic bryophytes show a greater tolerance than do the aquatic angiosperms. Moving to the Southern Hemisphere, the moss Richardsiopsis lacustris (originally named as Sciaromium lacustre) is known only from Lake Titicaca and some nearby lakes in South America. These are very high altitude lakes, located between 3800 and 4200 metres above sea level. Aquatic mosses are found in various lakes on Signy Island in the sub-Antarctic, with the appropriately named Moss Lake showing the most spectacular development. For wet bryophytes the respiration rate increases sharply with temperature but photosynthesis cannot increase at the same rate. If a bryophyte is kept wet and the temperature is steadily raised, there would eventually come a point at which photosynthesis could no longer keep the cells supplied with the nutrients they need. Another point to note is that the concentration of carbon dioxide, essential for photosynthesis, is higher in cooler waters. These factors help explain why aquatic bryophytes are commoner in the non-tropical regions. Terrestrial bryophytes, well away from water, can dry out and go dormant in hot weather and so survive far higher temperatures than can aquatic bryophytes.

Under ice

For fully submersed bryophytes low temperatures are not a major problem. If the temperature drops enough to form ice, the ice forms as a covering on the water surface and thereby creates an insulating layer that protects the water below. Many Scottish lochs have peaty water but the higher lochs (especially those above 700 metres altitude) generally have clear water. A survey of several high altitude lochs found luxuriant bryophyte colonies and 15 species were collected, at depths ranging from one to twenty metres. The lochs were covered by about a metre of ice and varying depths of snow for up to seven months each year. Wave action and scouring by ice would make it difficult for bryophytes to colonize the first metre of the loch margins. Ice cover reduces the amount of light that can reach the water but there can be compensating factors. A study of the bryophytes in Colour Lake, at a latitude of 79° 25' in Canada, found that ice cover protected the lake from wind-induced sediment re-suspension and so helped maintain water clarity. At depths below 15 metres there was no difference in light levels. The following table shows the percentage of incident light penetrating the water surface and then the amounts of surface light that reached various depths in the lake:

As with other Arctic lakes, the floor of Colour Lake is dominated by bryophytes.   

Not necessarily passive bystanders

Aquatic bryophytes are not always merely passive users of water courses. They may influence their surroundings in various ways.

During geological field work on Yakobi Island, in southern Alaska, one of the participants noted the behaviour of two leafy liverworts, Nardia compressa and Scapania paludosa in a small stream that flowed through a steep ravine. The liverworts grew in or by the stream and even submerged. The plant stems grew densely intertwined and this manner of growth allowed the liverworts to impound the swiftly flowing rivulet in a series of terraced pools which were formed by small dams, 30-50 centimetres tall. Those dams were composed of living or dead liverworts. Water didn't flow over the dams but filtered through them and continued liverwort growth would eventually cover the pools. At times water would flow through "blowout" holes through the mat, possibly when heavy runoff created sufficient water pressure, though frost action could be an alternative explanation. Over time vascular plants colonize the liverwort mats. You'll see Nardia compressa called Alicularia compressa in some of the older literature.

Flow in streams may vary between gentle, low speed flow to turbulent, high speed flow and organisms living in streams must contend with the drag forces induced by flowing water. Such forces would tend to dislodge stream organisms from their substrates. The higher the drag force the more likely the dislodgement. One group of New Zealand researchers investigated drag forces on bryophyte-bearing stream rocks and then the drag forces on those same rocks with the bryophytes removed. Drag forces were measured over a range of flow speeds in a laboratory flow tank and the bryophytes involved were the hornwort Anthoceros laevis; the mosses Blindia lewinskyae, Bryum blandum and Fissidens rigidulus; and two leafy liverworts: Cryptochila grandiflora and an unidentified species in the genus Lophocolea. All samples were collected in the South Island. The Anthoceros, Blindia and Cryptochila were collected from steep streams in the high rainfall area of the west coast. The Fissidens and the Lophocolea were from a slightly lower rainfall area of the central Southern Alps and the Bryum was from low-rainfall areas in the gently rolling foothills of the east coast. For each of the Anthoceros, Fissidens and Lophocolea there was little or no difference between bare rocks and bryophyte-bearing rocks. Drag forces on the rock with the Bryum colony were about 10% higher than the drag forces on the denuded rock. In the remaining cases the drag forces were much lower when the rocks had their bryophytes: on average about 40% in the case of Blindia and about 30% for Cryptochila. In effect those two bryophytes streamlined the rocks. In their published report the researchers suggested that some bryophytes may...

...actually increase substrate stability by lowering the drag of the rocks on which they grow...Indeed these plants may create stable areas of streambed, and consequently enhance their own survival.

The drag forces were measured on isolated rocks but, given the streamlining, trailing growth of the Cryptochila and Blindia, the authors felt confident that similar results would be found in real streams. Reduced drag is likely to have at least one other effect as well. Aquatic bryophytes support high invertebrate densities, often more than could be explained by just the increased surface area available for invertebrate colonization. The authors noted an earlier suggestion that aquatic bryophytes create calmer regions around them and that such calmer regions, with their increased algal growth and trapping of micro-debris, would promote invertebrate colonization.

Tufa is a rock often formed from calcium carbonate precipitation by evaporation of water rich in dissolved calcium, for example water flowing from a limestone area. Bryophytes and algae in calcium-rich waters may act as centres around which tufa is formed. Exposed bryophyte or algal surfaces provide areas where thin films of calcium-rich water can lose carbon dioxide by evaporation, leaving a tufa encrustation. Bryophytes in various tufa areas have been found to have their lower parts encased by tufa "cement" but with the plants vigorous enough to have the growing points constantly above the increasing tufa layer. In an Indian study some flowering plants were also found to help in tufa formation, but only "...when their exposed parts become covered with an algal or bryophytic felt...". Tufa formation is not necessarily wholly inorganic since photosynthetic activity can actively promote the formation of tufa. An English study of mosses from a tufa site in Yorkshire concluded that up to about 10% of the tufa deposit could be explained by moss photosynthesis. It is possible that moss photosynthesis is a more significant contributor to tufa deposition at another English site, Matlock Bath in Derbyshire, where the springwater is warmer by 5°C – but that is still to be analysed.

Floating bryophytes

The majority of the bryophytes of watery habitats grow attached to some substrate but there are also a few floating species. An example is the thallose liverwort Ricciocarpos natans . It can be found on the surface of still water, for example in ponds or in the quiet margins of streams and often grows in association with vascular aquatic plants such as duckweed or the floating fern Azolla . The liverwort is virtually cosmopolitan and can also grow as a terrestrial form, for example when small water bodies dry up . When growing on land the species often grows in a rosette shape and might then be taken for a species of Riccia and, indeed, on land it can be found growing with species of Riccia. Spore capsules are produced within the thallus but are reported to be rarely seen. The species reproduces vegetatively by means of thallus fragmentation and either spores or thallus fragments may overwinter, the latter even if frozen for several weeks. Overseas literature reports this liverwort as supposedly widely dispersed by birds, with a Scandinavian book noting "...especially by swans...". On this theme it's interesting to see that the Australian bryologist George Scott has written that in Australia Ricciocarpos natans is found "...on still lakes, especially those with large bird populations...".