Myriophyllum spicatum L., Spiked Water-milfoil
Account Summary
Native, locally frequent. Eurasian temperate, but naturalised in N America and now circumpolar.
1900; Praeger, R.Ll.; around Enniskillen.
May to December.
Growth form and preferred habitats
This submerged aquatic rhizomatous perennial has whorled, feather-like leaves and branched shoots that vary from 0.5-7.0 m long. It often forms extensive mats at the water surface. M. spicatum usually grows on sand or gravel bottoms with an admixture of organic silt in lowland lakes and flowing waters, which can be up to 4 m or more in depth, but are often shallower, and which can be mildly acidic, near-neutral, or more usually calcareous and moderately to richly productive (ie mesotrophic to eutrophic) (Preston & Croft 1997). M. spicatum is absent from acidic waters and almost so from pure sandy bottoms (Reed 1977). In highly calcareous waters, plants precipitate lime and become white and marl encrusted. M. spicatum can grow and survive in water down to 17 m deep, although this is very exceptional, the normal maximum depth being closer to 5 m. The success of the plant depends much more on light, temperature and nutrient levels than the actual water pressure at depths below about 5 m (Dale 1981). Most plants are found attached in water 65-150 cm in depth (Reed 1977).
M. spicatum can also tolerate brackish water in estuarine conditions and thrives in water with a salinity up to 10 parts per thousand, but grows more slowly at a salinity of 15 parts per thousand (Beaven 1960). This level of salt tolerance explains how the species might be transported around the world in ship's ballast, as appears might have been the case at Chesapeake Bay in its original introduction to N America. Propagules might have been either seed or vegetative stem fragments (Aiken et al. 1979).
The light compensation point in M. spicatum is approximately 1-2% of surface light (Grace & Wetzel 1978), but it has nearly optimal configuration of photosynthetic tissue because it concentrates its leaf biomass near the water surface (Adams et al. 1974). M. spicatum displays a very wide tolerance of water reaction, indeed ranging from pH 5.4-11.1, and in wide lakes like Lower Lough Erne and in river estuaries, it can withstand considerable wave action, although it really grows best in more sheltered coves.
M. spicatum grows vigorously and can often shade out other aquatics, including Potamogeton species like P. crispus (Curled Pondweed) (Nichols & Shaw 1986). However, submerged aquatics are relatively unproductive when compared to the productivity of terrestrial and emergent plants. One reason for the low productivity is believed to be the limited carbon availability in the aquatic environment, which is explained by the slower rate of diffusion of carbon dioxide in water compared to in air. The anatomy of M. spicatum, however, provides a large internal lacunal system that acts as a gas reservoir that is capable of retaining respired CO2, allowing diffusive gas exchange to take place between roots and shoots (Grace & Wetzel 1978).
While M. spicatum can also grow on a wide variety of sediment types, like other aquatics it grows best on fine sediments where organic matter ranges from 10-25% (Pearsall 1920). Coarse substrates such as larger gravels probably do not offer good anchorage and they may be nutrient poor. However, fine bottom sediments can become too soft and flocculent to support plant growth, plants then being unable to root and anchor sufficiently in soft, moving bottom substrates (Nichols & Shaw 1986).
Emergent plants
As in M. verticillatum, emergent aerial shoots develop in M. spicatum if the water body temporarily dries out or water level drops slowly and sufficiently to expose a shoreline. The leaves of the land plant are smaller, stiffer and they are less divided. If re-submerged, new growth with typically divided aquatic leaves redevelops in around 7-10 days (Aiken et al. 1979).
Flowering reproduction
Except in deep or turbulent water, Spiked Water-milfoil populations flower and set seed regularly. The inflorescence is a terminal spike, 5-20 cm long, of reduced flowers in whorls of four, often pink in colour. The stem, 5-20 nodes below the spike, is double the rest of the stem in width, very rigid and distinctly curved so that this inflated portion lies parallel to the water surface. The stigmas ripen well before the anthers do, favouring cross-pollination. Cook (1987) notes that M. spicatum is self-compatible, but says that there is a good chance of cross-fertilization. The anthers are linear, 1.8-2.2 mm long producing large amounts of dry pollen. The spike is erect when flowering takes place in June and July and is held above the water surface. After wind pollination takes place, it lies parallel to the water surface during fruit set in August and September. The fruit is a 4-lobed, 3 mm sub-orbicular schizocarp. The ripe fruit eventually breaks up into 1-seeded mericarps and the 'seeds' float for a short time, perhaps for 24 hours or so, and may disperse in any available water flow. The actual true seed has a stony endocarp and shows prolonged dormancy which can be broken by chilling or by abrasion (Aiken et al. 1979).
In common with other aquatic species, despite vast numbers of seed being produced (eg four million per hectare in one Canadian study), seed germination is extremely erratic and field records of seedlings are either non-existent (Aiken et al. 1979; Preston & Croft 1997) or rare (Hartleb et al. 1993).
Unlike reports from elsewhere in Britain, none of the Fermanagh records of M. spicatum are from newly created water bodies, such as reservoirs, flooded quarries, gravel or sand-pits, and thus there is no evidence of jump-dispersal which requires birds or other vectors to transport the seed either internally, or more likely, externally, in mud on their feet or plumage (Ridley 1930, p. 546). Nor are there any examples of dumping of unwanted, excess, aquarium or pond plant material which, apart from shipping ballast, is another method by which this species might have been transported beyond its native Eurasian distribution (Reed 1977; Preston & Croft 1997).
Overwintering perennation, vegetative reproduction and dispersal
The plant is a rhizomatous perennial that dies back to the roots in winter. Unlike M. verticillatum, it does not develop specialised turion overwintering buds, but it does regenerate from axillary buds borne on the previous year's stem base that are easily detached (Preston & Croft 1997). It can also spread locally by the lateral growth of the rhizome. Thus its effective reproduction, as with all aquatics, is largely asexual and highly dependent on detached vegetative buds, stem fragmentation and rhizome spread, all of which means of increase are highly developed in M. spicatum, allowing its rapid dispersal and expansion in the water bodies it colonises. Auto-fragmentation of M. spicatum plants typically occurs after its flowering period in the autumn, each upper stem segment being capable of forming roots and starting a new colony the following summer (Reed 1977; Nichols & Shaw 1986).
Strong waves and water currents, plus human activities such as motor boating and mechanical weed harvesting, all help to produce and distribute stem fragments (Aiken et al. 1979; Preston & Croft 1997). Colonisation success of M. spicatum propagules has been shown to be best in late summer in shallow (0.5 m) water, on rich organic sediments; mortality was highest in deep water with calcareous, nutrient poor sediments in early autumn (Kimbel 1982).
Relationship with algae
The relationship between aquatic macrophytes and algae appears to depend on whether the algae are free living or epiphytic on the macrophyte and the nutrient status of the water body they both inhabit. Macrophytes can alter local water chemistry around them and their presence provides a substrate for periphyton growth (ie on and around them). By the same token, planktonic and filamentous algae may shade macrophytes, but they also provide easily available food for grazing invertebrates that make use of the shelter provided by macrophytes (Nichols & Shaw 1986).
Dense growths of large aquatic plants may have an inhibitory effect upon phytoplankton and rotifer plankton in small water bodies and, in larger lakes, aquatic weeds may display antagonistic activity towards phytoplankton. Fitzgerald (1969) observed that areas of Lake Winga, Wisconsin with an unusually extensive weed bed of mainly Myriophyllum sp., were quite plankton free. The current author (RSF) cannot discover whether more than one Milfoil species was involved (M. spicatum certainly was the dominant species of the genus present), as after all these years, Fitzgerald's 1969 paper is still hiding behind the publisher's paywall. Milfoil species form an excellent host for periphyton because their finely dissected leaves provide a large surface area for colonisation; the epiphytes also benefit from the organic nutrients that are excreted by macrophytes (Wetzel 1975). It has been suggested that epiphytes may provide an easily grazeable food supply, thus protecting the macrophytes they grow upon from invertebrate predation (Hutchinson 1975). However, Milfoil plants from declining populations are often coated with algae or a brown slime of periphyton (Nichols & Shaw 1986). Only one species (sp.)? or more than one (spp.)?
Relationship with waterfowl
The primary importance of macrophytes like M. spicatum to waterfowl is as food. However, information on the topic of food uses is sparse. For instance, Tamisier (1971) found that Myriophyllum and Potamogeton seeds formed less than 2% of the diet of Teal ducks (Anas crecca). Invertebrates associated with macrophytes are important duck foods, especially when young ducklings need a protein-rich food source (Krull 1970). Even hydrophytes that are themselves considered poor waterfowl food plants are believed to be of indirect importance to birds because they harbour large quantities of macroinvertebrates (Nichols & Shaw 1986).
Fermanagh occurrence
M. spicatum is by a considerable margin the most common of the three native Water-milfoil species in Fermanagh, being recorded in 91 tetrads, 17.2% of those in the VC. It persists not only in both parts of Lough Erne, the major water bodies of Fermanagh, but also in managed ditches and riverbeds draining the land around these lakes or lake systems, since the shoreline of Upper Lough Erne, in particular, is extremely heavily dissected. As the Fermanagh tetrad map shows, M. spicatum is very much confined to the Lough Erne area and along the marl lakes on the River Finn. The feeding waters in these areas derive from catchments of limestone geology.
The observed linkage between the distribution of M. spicatum and calcium levels might be partially explained by the correlation with water colour which may limit photosynthesis in submerged species. Gibson (1988) found all calcium-rich waters in Fermanagh had clear water and all highly coloured waters were low in calcium. The patterns of human settlement and of farming mean that both parts of Lough Erne are much more subject to eutrophication than the huge number of smaller lakes and ponds widely scattered across the VC (Gibson et al. 1980, 2003; Gibson 1988).
In the summer of 1948, Meikle and colleagues recorded M. spicatum in Lough Navar on the western plateau, a site where it has not been seen since, although M. alterniflorum, with which it can coexist, is still present there. The creation of the coniferous Forest Park around Lough Navar has undoubtedly led to acidification of the waters and possibly substantial phosphorus enrichment (Gibson 1976), but M. spicatum tolerates a wide range of both these factors (Aiken et al. 1979), so the current author (RSF) cannot easily explain the apparent demise of the species here.
Irish occurrence
M. spicatum is well, but very unevenly distributed throughout Ireland. The majority of more recent Irish records are from the north of the island – the outcome of several large-scale lake surveys carried out in NI by the DOENI from 1985 onwards.
British occurrence
Spiked Water-milfoil is widespread throughout all of Britain, from the Channel Isles, thinning northwards, but still managing to reach the southern tip of Shetland (VC 112). In the SE of England the species is much more frequent than elsewhere, and especially so to the east of a line drawn between Swansea and Hull (New Atlas). Like the majority of aquatic plants, M. spicatum is now better recorded than in the first BSBI Atlas (Perring & Walters 1962), and it is the most common Myriophyllum species colonising the eutrophic lowlands of the country (C.D. Preston, in: Preston et al. 2002). It has probably increased in abundance across Britain in the last 150 years through a combination of eutrophication of existing grazing marshes and the expanded availability of other suitable man-made habitats including sand- and gravel-pits and disused quarry pools (Preston & Croft 1997).
European and world occurrence
M. spicatum is widely distributed (almost ubiquitous) in Eurasia and extends north-westwards to Iceland and Greenland. It includes subsp. spicatum in Eurasia and subsp. exalbescens (Fern.) Hult. in N America. Variation between these two subspecies certainly overlaps and there is little cause to regard them as separate species (Hultén & Fries 1986). The species sens. lat. is widespread in boreal and temperate regions of the N Hemisphere and belongs to the Circumpolar plants (Hultén 1974, Map 171). From its centre of origin in Eurasia it extends southwards into N, C & S Africa (Swaziland, Transvaal, Natal, the Cape) and eastwards to S Asia, including the Himalaya, Sumatra, the Philippines and Japan. In some of these more eastern areas, it is probably introduced and this is certainly also the case in all of N & S America (Hultén & Fries 1986, Map 1374).
M. spicatum is a serious aquatic alien nuisance weed in many regions of the world where it has been deliberately or accidentally introduced. This is especially the case in NE North America where it poses a danger to native species in established vegetation (Aiken et al. 1979; Nichols & Shaw 1986). Reed (1977) described M. spicatum as, "an economically important and ecologically dangerous waterweed". Spiked Water-milfoil has been present in N America since 1848 and it has spread from the E coast to the W coast of both the US and S Canada. Having said this, there have been problems in the past distinguishing M. spicatum from a native American species referred to as M. exalbescens Fern., although Reed (1977) is clear that both species occur and that it was M. spicatum that became a problem in large water bodies in N America in the late 1950s and 1960s.
M. spicatum was present in Canadian waters for around 50 years before it was recognised as a significant aquatic weed. The population there then exploded in the late 1950s, probably in part due to the occurrence of dramatic weather events (Nichols & Shaw 1986). Some long distance dispersal of M. spicatum in N America is known to be related to the aquarium and aquatic nursery trade (Reed 1977). Shorter distance dispersal is attributed both to transport of plant fragments on boats and trailers moving from lake to lake and also to natural water movements (Nichols & Shaw (1986).
Threats
None. This species copes well with highly eutrophic conditions and, indeed, it is often a good indicator of them.