Lobelia dortmanna L., Water Lobelia

Account Summary

Growth form and preferred habitats

A characteristic and interesting, if neither strikingly beautiful nor conspicuous flower of shallow water around oligotrophic, acid, often gravelly, sandy, or more rarely peaty lakeshores, Water Lobelia is an aquatic, rosette-forming, 'isoetid' perennial (ie with leaves like those of Isoetes). Its leaves are linear, dorsi-ventrally compressed, 2-8 cm × 2-4 mm, each with two internal, uninterrupted longitudinal air channels (Farmer 1989).

L. dortmanna is most frequently found in lakes on higher ground and can grow in water up to 2 m deep, but it is more commonly found at much shallower depths. The deeper the water the fewer flowers L. dortmanna produces, but since it is not very gregarious, tending to produce separate, individual, small rosette plants, it rarely develops anything approaching a colourful floral display. Indeed, the reddish, emergent, leafless flowering stems (scapes) are frequently far more noticeable than the small, pale bluish or lilac flowers, held about 25 cm above the water surface. It can occur in waters in the pH range 5.5-8.0, on substrates from coarse gravel (possibly the most common situation) to silty peat (Farmer 1989). Webb & Scannell (1983) suggested that L. dormanna was a strict calcifuge, but that applies to the nature of the sediment rather than the characteristics of the water (Farmer 1989).

L. dortmanna is described as a slow-growing perennial of low competitive ability (Preston & Croft 1997), a view which is supported by Farmer (1989), but the fact that while it does occur amidst taller emergent aquatic plants in swampy sites, it is much more frequently found in open water situations, especially in relatively exposed, upland lakes and lakelets. The rate of growth measured in terms of production to biomass ratio (P:B) (ie the rate of organic matter increase per unit dry weight) is the slowest measured for any B & I aquatic vascular plant, averaging just 0.64 (Farmer 1989).

The key to understanding the occurrence of Water Lobelia appears to be its intolerance of competitive shading. In both more acidic and more productive, less nutrient-limited conditions, it suffers competitive decline due to overgrowth and shading by epiphytes and/or phytoplankton (Farmer 1989). Thus, the observed species decline in eastern parts of its B & I distribution may in part be due to eutrophication from the intensive agriculture in these areas of the country, together with general urban and industrial development involving drainage and pollution, leading to consequent losses of suitable aquatic habitats (Preston & Croft 1997). It should not be forgotten, however, that the kind of shallow lake basins that L. dortmanna inhabits are always more or less short-lived, since they naturally infill with dead organic matter growing in and around them, gradually evolving into dry-land habitats by being invaded first by fen scrub, that eventually develops into woodland, if succession is not somehow deflected.

Gas exchange and metabolism

Since it mainly occurs in relatively shallow lakes, the vegetative leaf rosettes of some Water Lobelia populations may regularly become exposed to the air through at least part of the summer months. As the short, stiff 'isoetid', dwarf, rosette-forming leaves (ie Isoetes-like), of L. dortmanna lack stomata and are covered by a thick cuticle, they provide high resistance to gas exchange and water loss. Continuous air passages run between leaves, each with two large lacunae, and the root tips which feature many small lacunae. The air passage lacunae may account for up to 50% of the total cross-sectional area of the leaf.

Chloroplasts are lacking in the epidermis, unlike in many aquatic plants, but they are concentrated in the cortex, close to the source of the CO₂. There is also a low stem:root ratio (0.6), when compared to other aquatic species (Richardson et al. 1984). Although the leaves may become exposed to the air for part of the year, virtually all metabolic gas exchange remains via the roots embedded in carbon dioxide-rich soil, into which a substantial proportion of the oxygen produced by photosynthesis is released (Pedersen & Sandjensen 1992).

L. dortmanna absorbs most of its CO₂ from the sediment via the lacunae in the root system. The thick cuticle and lack of stomata in the leaves maintain high carbon dioxide concentrations (about 23 times air level) within the leaf lacunae for use in photosynthesis, while the release of oxygen by the roots, and low oxygen consumption rates in the sandy or muddy acidic sediments, result in the presence of dissolved oxygen in the pore water, with higher daytime concentrations than at night. This root oxygenation may assist the plant in the acquisition of limiting nutrient resources in its typical, infertile, oligotrophic habitat (Pedersen & Sandjensen 1992). These latter authors also suggest that the special morphology and predominant root exchange of metabolic gases in L. dortmanna, may well have evolved on land before the species became an aquatic macrophyte!

Unlike some of its associated isoetid species (eg Isoetes aquatica (Quillwort) and Littorella uniflora (Shoreweed)), L. dortmanna does not exhibit CAM-like metabolism (Richardson et al. 1984; Farmer & Spence 1985)

Flowering and vegetative reproduction

L. dortmanna plants flower mainly in July and August, but flowering may continue into October. The flowers appear to attract very few (if any) insect visitors, and reportedly they self-pollinate while in bud (ie they are cleistogamous). The plant may even do this while the flower bud is underwater and still produce viable seeds (Farmer 1989). Ripe capsules from which seed may be shed can occur from mid-June onwards. The capsule is ovoid and drooping. Seed set is thus assured and, being of dwarf rosette habit, the species has no special organs of vegetative reproduction. Its relatively high volume of seed production is the main method of the species' increase and long-distance spread (Preston & Croft 1997).

Salisbury (1942) reckoned fruiting specimens showed a range from one to six capsules per plant, but the average number was two. He estimated the mean seed output per capsule to be 118 ± 9.5, with counts varying from 41 to 175. This meant the normal output was 236 seeds; the maximum output in his study did not exceed 700 seeds per plant. Capsules in Wales produced up to 250 seeds, and in Scotland figures around 300 seeds have been reported, with similar output for submerged capsules (Woodhead 1951; Farmer & Spence 1987).

Seeds are minute, >1 mm long, ellipsoid, finely netted, grey, soon turning purple-red (Butcher 1961; Farmer 1989). Seed dispersal requires either the disintegration of the capsules in the water or a slow liberation from the pores at the top of the fruit. Seeds float very briefly and then sink. The rapid sinking of the seed restricts its dispersal distance and helps keep them on the littoral lakeshore where favourable germination sites are located. Seed may or may not require a dormant period, but L. dortmanna can give very good levels of germination, even at very low light levels, although germination itself is a slow process and growth can be halted by anoxia, requiring a second dormancy to be broken before resumption of growth (Farmer & Spence 1987). According to the survey of soil seed banks in NW Europe, seed survival is short-term persistent, ie more than one year but less than five. There was only one estimate published in the listing, however (Thompson et al. 1997).

Having said all of this regarding seed production, new plantlets are formed at the base of old flowering shoots by the development of axillary buds, and they, or indeed whole plants that become dislodged from the lake floor and are mobile and floating, could well play some role in dispersal within connecting water-bodies. However, on account of where and how the axillary vegetative buds are formed, L. dortmanna cannot asexually propagate without first flowering (Aberg 1943; Farmer & Spence 1987). Also, the survival of these vegetative offsets may well be rather low (Szmeja 1987). Although Szmeja made further population studies of Lobelia and other 'isoetids' (ie Isoetes-like aquatic species), the relative recruitment rates from seed and from vegetative propagules for Water Lobelia remains a grey area of ignorance, which would repay further study in the particular range of lake environments found in Fermanagh.

When Lobelia dortmanna flowers it becomes a much more conspicuous plant than either Littorella uniflora or Isoetes lacustris, and since it is so similar to I. lacustris in its ecological requirements and tolerances, it is a very good indicator of the likely presence of Quillwort at a site.

Hybrids

No hybrids are known to occur (Farmer 1989).

Fermanagh occurrence

While L. dortmanna is typically found in lakes on high ground, locally, in Fermanagh, it descends to an altitude of only 25 m on the shores of larger lakes such as Lough Melvin and Lough MacNean. As the tetrad distribution map shows, the occurrence at Lough Corry (225 m) is a very isolated eastern upland site, while overall this perennial is really quite frequent, appearing in 26 Fermanagh tetrads, 4.9% of the total in the VC. It is particularly well represented in the blanket bog lakelets in the west of the county.

Irish occurrence

The New Atlas hectad map shows that L. dortmanna has a very pronounced western distribution on the island of Ireland, with relatively few inland sites scattered across the country, mainly concentrated in the northern counties. There have been gradual losses across these inland stations for many years and, for example, the FNEI 3 has described a marked decline during the last century in the number of sites in the three NE counties of Ulster, particularly in lowland lake sites. John Harron in his 1986 Flora of Lough Neagh likewise chronicled the decline of Water Lobelia to extinction around Lough Neagh, where the last recorded occurrence was on the Co Armagh shore in 1939.

British occurrence

L. dortmanna is locally common in Britain, again with a very marked N & W distribution. It is especially well distributed and frequent to common in N & W Scotland, the English Lake District and NW Wales, although it declined in the eastern margins of its range, largely before 1930, principally on account of habitat loss and destruction involving eutrophication, intensification of agriculture and pressures of human population density and associated urban development (Preston & Croft 1997; T.D. Dines, in: Preston et al. 2002).

European and world occurrence

The species has a European boreal-temperate distribution but is also native in N America and so is amphi-atlantic. In Europe, it extends as far north as the Lofoten Islands (68^oN) and western parts of Russia and the Baltic States, Scandinavia, Poland, N Germany and the Low Countries, but it is rare in W France, although it does extend south, very locally, in SW France just north of the Pyrenees. In N America, it is common in a narrow belt extending across the continent from Newfoundland westwards to outliers in British Columbia (Hultén & Fries 1986, Map 1763; Farmer 1989; Preston & Croft 1997).

Names

The genus name 'Lobelia' commemorates the 16th century Flemish botanist and herbal writer Matthias de l'Obel, who was botanist to James I of England (Grigson 1974). The specific epithet 'dortmanna' again is given for a famous botanist of old, the Dutch apothecary Dortmann from Groningen (the genus previously and variously known as Dortmanna, Dortmannia or Dortmania is now our modern Lobelia) (Melderis & Bangerter 1955; Gilbert-Carter 1964; Willis 1973).

There are only a few English common names listed by Grigson (1955, 1987) and he gives no attention to 'Water Lobelia'. One name given is 'Water Gladiole' from Cumberland, a name shared with Butomus umbellatus (Flowering Rush) (Prior 1879; Britten & Holland 1886), which sometimes is simply listed as 'Gladiole'. The Latin 'gladiolus' means 'a small sword', which implies sword-shaped leaves, which hardly applies to either plant species anyway, and certainly not to Lobelia dortmanna! In N Wales, L. dortmanna is referred to as 'bidawglys', 'dagger plant', which is not much better in terms of a meaningful connection with the plant's structure (Grigson 1955, 1987).

Threats

Eutrophication from agricultural or forestry applied chemical runoff, or drainage, appear to be the most obvious threats, but in many of the existing upland sites these would not be likely to happen.

References

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