Myriophyllum alterniflorum DC., Alternate Water-milfoil
Account Summary
Native, occasional. Suboceanic boreo-temperate, but also in N America.
1882; Stewart, S.A.; Carrick Lough, Dresternan Td.
March to November.
Growth form and preferred habitats
This submerged, aquatic perennial with whorls of three or four much dissected leaves, grows in a wide range of still to swift-flowing waters, usually in the more acidic, soft-water, upland parts of the county although it can also occur in waters of high pH and low and high concentrations of HCO3- (Spence 1967). It is a rhizomatous species with no specialised overwintering turion buds, but is perfectly capable of vegetative regeneration and colonisation involving short lengths of fragmented shoot. Since it prefers less productive, base-poor waters, in Fermanagh this water-milfoil species is much less common around both parts of Lough Erne than is M. spicatum (Spiked Water-milfoil). Compensating for this, however, it is very widely scattered around the county, particularly in fairly remote lakes and pools on the western plateau uplands.
As its species epithet indicates, M. alterniflorum bears its upper flowers alternately or opposite, not in whorls like M. spicatum does. The mid-stem leaves are also more divided than in M. spicatum, having 13-38 segments, rather than 6-18 in the latter (Crawley 2005).
The evidence of its widely scattered local distribution clearly illustrates that M. alterniflorum is more capable than M. spicatum of dispersing itself and colonising new water-bodes around its existing sites and at higher elevations above them. It has, for instance, been recorded from a pool in the quarry beside Keenaghan Lough and also from a distinctly peaty pool in a cut-over bog in the Whitehouse Cave area. The latter example indicates the extremely wide ecological range of this species, which also tolerates highly calcareous waters and quite often accompanies M. spicatum in less eutrophic situations (Preston & Croft 1997).
M. alterniflorum typically grows in water 30-300 cm deep, generally somewhat shallower than M. spicatum, although there is considerable overlap between their habitat requirements and they can occur together.
Flowering reproduction and genetic variation
Alternate Water-milfoil flowers profusely in shallow water in mid-summer, producing wind-pollinated flowers on a short, 1-2 cm, emergent spike that is female at the bottom and male at the top. The tip of the spike is drooping when in bud. Very occasionally, hermaphrodite (ie bisexual, perfect) flowers may be produced between the unisexual flowers. The fruit is a four-lobed, 1.5-2.0 mm schizocarp, splitting when mature into four single-seeded mericarps (Sell & Murrell 2009); the seeds are viable. A study of enzyme variation found M. alterniflorum displayed a large degree of variability within and between populations (Cook 1968; Harris et al. 1992).
Evidence from the enzyme study showed sexual reproduction via random mating was common in the M. alterniflorum populations and that therefore outbreeding is frequent. Sexual reproduction does not necessarily lead to genetic diversity, but the extent of outbreeding will also be important. The fact that M. alterniflorum grows in sites with a great range of water chemistry, helps explain the high degree of genetic variation found (Spence 1967). "For such a species, genetic diversity may provide one means of exploiting the different environmental conditions." (Harris et al. 1992).
The species is native, fossil pollen evidence from sediments in Scottish lochs indicating its presence from immediately after the last glaciation around 12,000 years BP (Pennington et al. 1972). Populations at different sites have therefore had a long time to accumulate genetic diversity and there are a very large number of freshwater sites where the species occurs, meaning there is a substantial pool of genetic variation. If gene flow can occur between sites by transfer of vegetative shoots (eg by birds), or by pollen transfer on the wind, this leads to increased genetic variation within a site (Sculthorpe 1967; Harris et al. 1992). The latter authors concluded, "Although a large amount of the morphological response and the biochemical and physiological response to the environment by freshwater macrophytes is phenotypic, the results presented here suggest that there is also the potential for genotypic differences, and hence for the existence of ecotypes within a given species adapted to different conditions."
An alien American variety
Slender, worm-like, prostrate plants of M. alterniflorum with very short leaves (only 3-8 mm as compared to normal 8-26 mm length), and with equally short internodes, only 2-8 mm rather than 8-17 mm, were first noticed growing in shallow water on sandy substrates in clear water in Lough Beg (H40) and Lough Neagh (H36-H40), and again in Lough Ree along the R Shannon, just N of Athlone (H23, H24 & H25) by S.A. Stewart back in 1867. This unusual dwarf variety grew in these Irish sites in the apparent absence of typical M. alterniflorum (Praeger 1938b). At the same date (and in the same Journal of Botany issue), Pugsley drew attention to the fact that these dwarf plants were very similar to the normal form of the species in N America, so he then named them var. americanum Pugsley. The same or a similar form was recorded elsewhere in Ireland on the larger lakes on the R Shannon, on smaller lakes in W Donegal (H35) and in the Scottish Hebrides (VCs 103, 110 & 111) (Harron 1986; Preston & Croft 1997; C.D. Preston, in: Rich & Jermy 1998). However, due to subsequent habitat changes, including eutrophication, some of these populations appear to have latterly declined or disappeared. There is a definite need for further investigation of the true nature of the American form of the species (Preston & Croft 1997).
Fermanagh occurrence
M. alterniflorum is thinly and widely scattered in Fermanagh, but is more frequent in the west of the county at all altitudes.
There are records of it from 61 Fermanagh tetrads, 11.6% of those in the VC, but as the distribution map indicates, eight of them are pre-1975 only, suggesting a possible slight decline of the species over the period from the mid-1940s. The local habitats range across lakes, tarns, rivers, drains and pools in quarries and on cut-over peat bogs.
Fossil occurrence
Fossil pollen of M. alterniflorum has been found in a full-glacial freshwater deposit of Middle Midlandian age found at Derryvree, near Maguiresbridge in Fermanagh, and radiocarbon dated to 30,500 BP (Colhoun et al. 1972). The flora and fauna of the deposit indicate that open tundra vegetation and a periglacial climate prevailed at that period. Godwin (1975) reports similar evidence of long persistence of this aquatic in Britain, since at least the middle of the last glacial period and possibly a lot longer.
British and Irish occurrence
The New Atlas hectad map shows M. alterniflorum is frequent and widespread in both acidic and calcareous waters throughout N & W parts of B & I across the whole range of latitude. However, in common with other calcifuge species, in S & E England it is very much more scarce, scattered or absent and is confined to acidic, mesotrophic or oligotrophic waters. Suitably acidic, low productive habitats are infrequent in these parts of England since many sites have been destroyed in the last century or so through drainage and agricultural intensification operations (C.D. Preston, in: Preston et al. 2002).
European and world occurrence
M. alterniflorum is mainly restricted to boreal and temperate zones in N & W Europe and mid-E and mid-C parts of N America, making it an amphi-Atlantic species (Hultén 1958; Hultén & Fries 1986, Map 1375). As mentioned above, the American form of M. alterniflorum is very dwarf in comparison with the European plant.
Threats
None.