Oenanthe aquatica (L.) Poir., Fine-leaved Water-dropwort

Account Summary

Growth form and preferred habitats

An erect to sprawling floating marginal aquatic plant, O. aquatica differs from other members of the genus Oenanthe in being a short-lived annual or biennial. Young plants initially develop tubers on their fleshy roots, but these storage organs are re-absorbed and disappear whenever the plant matures and produces a flowering stem. This tuber behaviour is also the pattern in the closely related perennial species, O. fluviatilis (River Water-dropwort).

In the summer months, the shallow, still or sluggish, marginal waters of the ecologically open, but sheltered shores of lowland lakes, ponds, rivers, drains or swampy fens that this normally aquatic species occupies, frequently dry out or have markedly reduced water levels in summer. When this happens, the previously submerged or floating plant produces a terrestrial form with feathery, finely divided, 3-4 pinnate aerial leaves that differ considerably from its submerged filiform leaves (ie it is heterophyllous) (Jonsell & Karlsson 2010). Under these drier conditions, the terrestrial plant then produces erect or sprawling, stout, flowering stems, 60-150 cm tall (Walls 1995).

O. aquatica tends to prefer shallow ponds and ditches in damp or marshy pastures, where it grows on deep, silty, clay- or mud-based, often eutrophic soils. This type of site is kept open by grazing and trampling cattle (but see Toxicity section below) (Preston & Croft 1997).

Although it prefers fully illuminated situations, the species can tolerate moderate levels of shade from nearby trees. However, it avoids closed vegetation in fen carr, tall fens or reed swamps.

Fermanagh occurrence

The typically tall, hollow, striate, 1 cm wide flowering stems of O. aquatica make it a distinctive feature of many of the nutrient-rich, muddy, organic soils of drainage channels that connect the multitude of lakelets around Upper Lough Erne to one another and to the shores of the eutrophic large lake itself. Elsewhere in Fermanagh, O. aquatica features along the slow flowing lower reaches of rivers and around suitable shallow ponds in pastures and water meadows, where water levels fluctuate considerably with prevailing precipitation. Currently, the Fermanagh Flora Database contains records of O. aquatica from 56 tetrads (10.6%), 50 of them with post-1975 dates.

Flowering reproduction

Flowering usually occurs from June to September. The inflorescence is a series of small compound umbels, 2-4 cm diameter, all leaf-opposed, plus a terminal one. Each umbel consists of 4-16 rays and peduncles are usually shorter than the rays. Most umbels contain both hermaphrodite and male flowers, the bisexual ones being in the majority (Jonsell & Karlsson 2010). The flowers are white and the outer petals scarcely radiating. The leaflets of the aerial leaves are smaller than those of the closely related, but very much rarer O. fluviatilis – only about 5 mm long and they have acute tips, making the aerial leaves of O. aquatica look daintier than those of O. fluviatilis. Pollination is carried out by a range of insect visitors, including beetles, flies and bees (Fitter 1987).

In August, the mature fruits of O. aquatica are also smaller, being only 3-4.5 mm long, compared to 5-6.5 mm in O. fluviatilis. These features, plus the more terrestrial habitat, are the most reliable characters for separating these two Water-dropwort species (Tutin 1980; Walls 1995). The fruit mericarps have five prominent but low, greenish to yellow-brown, rather thick ridges and the pedicels are not thickened (Sell & Murrell 2009). The mericarp ridges contain cells with large air spaces to assist flotation and water dispersal (Jonsell & Karlsson 2010).

The seeds of O. aquatica germinate soon after they are shed and 'swarms' of seedlings can be found on ground from which water has receded. Seedlings emerging in autumn behave as winter annuals, flowering the next summer, while those that spring germinate will complete their life-cycle the same year and thus are summer annuals. The longer-lived winter annuals accumulate more photosynthetic reserves in their temporary tubers and produce larger aerial plants and more seeds (Preston & Croft 1997).

Overwintering plants and temporary tubers

Both O. aquatica and O. fluviatilis are wintergreen and they have fibrous rather than tuberous roots. Since the main bulk of the plants spend most of the year submerged in water, they have a much longer growing period than the more clearly terrestrial or semi-aquatic species of the genus. This explains why they do not require or form any specialized overwintering organ, such as root tubers. Young plants, however, do develop temporary root tubers before they reach flowering condition and the photosynthate energy stored in them becomes completely consumed powering the sexual reproduction process of the mature plant (Tutin 1980).

Vegetative reproduction

The plant has a procumbent stem which roots at the nodes acting like stolons and producing vegetative offsets. The individual flower stems are annual or biennial, ie they are monocarpic. Overwintering survival is achieved by the annual populations of vegetative offsets and seed (Clapham et al. 1987).

Toxicity

Like all Oenanthe species, O. aquatica is considered poisonous, the active principle being oenanthotoxin, a polyunsaturated higher alcohol, chemically similar to cicutoxin found in Cicuta virosa (Cowbane). It is a convulsant poison that is unaffected by drying and storage. There have been reports of cattle deaths caused by it in Poland and Sweden (Cooper & Johnson 1998).

British and Irish occurrence

Despite its hectad frequency in the New Atlas map, this is an uncommon species throughout most of its B & I range. The British distribution shown in the New Atlas is rather odd, the plant being mainly southern in England plus in E Wales, but more or less absent from a broad tract down the centre of England. Presumably, this vacant area represents the outcome of a combination of factors that make habitats unsuitable for the species, including higher ground and both dense urban and widespread farmland development. The distribution only just reaches the extreme SE of Scotland, so that temperature limits must clearly also apply (New Atlas). In fact, O. aquatica populations have declined in England in recent decades and have become rather scattered (Rich & Woodruff 1996).

In NI, O. aquatica is chiefly found in Lough Neagh, the River Bann and Lough Erne. In the RoI, it is much more widespread, although again it is locally frequent in the centre and much rarer elsewhere (An Irish Flora 1996).

European and world occurrence

The species is widespread throughout middle latitudes in Europe and also into W Asia plus a few scattered areas in the SC Asiatic mountains. On the Mediterranean islands, it is only present on Corsica and Sicily. It has been accidently introduced to New Zealand, probably as a seed contaminant (Hultén & Fries 1986, Map 1401).

Threats

Drainage operations and excessive disturbance are potential, but not really actual threats in the Fermanagh area.

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