Oenanthe fluviatilis (Bab.) Coleman, River Water-dropwort

Account Summary

Growth form and preferred habitats

A truly aquatic, wintergreen perennial with stems 30-100 cm long, floating, procumbent or ascending when flowering, O. fluviatilis produces fibrous roots rather than forming tubers and its lower leaves are always submerged and are deeply cut into filiform lobes. Being entirely aquatic and confined to lowland waters, O. fluviatilis has a longer growing season than related terrestrial or semi-aquatic Oenanthe species and thus does not require tuberous storage organs (Walls 1995). As a general rule, as its names suggest, River Water-dropwort is found in constantly running water at depths varying from shallow to 1.5 m, in small rivers and in streams that usually are calcareous and moderately eutrophic (ie meso-eutrophic). However, this is not an absolute situation, since it can occasionally also inhabit still or sluggish waters, eg in fenland man-made ditches and canals. In terms of bottom substrate, it prefers organic-rich or sandy ones poor in calcium and avoids only deep, very fine silty conditions. It seems to be confined to relatively cool, clear water (Cook 1983; Preston & Croft 1997).

In comparison, the very similar, closely related O. aquatic (Fine-leaved Water-dropwort), with which it is easily confused, requires or prefers sluggish or still waters in lakes, ponds and ditches. In addition, O. aquatica often contends with very shallow pools that dry up in summer, forcing that species into a terrestrial mode of growth form not seen in O. fluviatilis (Walls 1995; Preston & Croft 1997; Sell & Murrell 2009).

Fermanagh occurrence

There are only five records for this aquatic in the Fermanagh Flora Database at present, all of which post-date 1985 and are confined to the Upper Lough Erne area and along the River Finn. RHN and the current author (RSF) believe there is a possibility that two of the records might be mis-identifications of the closely related O. aquatica. The two species are very similar in form, behaviour and are ecologically alike, being found in Fermanagh in rivers, streams and ditches near lakeshores.

The original plant list for Ross Lough, for instance, records both O. fluviatilis and O. aquatica and the EHS Habitat Survey Team clearly marked O. fluviatilis on their field card with the code '3', which indicates that they knew the species was noteworthy and rare. Of these two Oenanthe species, however, O. aquatica is much more common and widespread in Fermanagh and elsewhere in B & I.

Both species are definitely aquatic members of this genus, since for most of the year their plants are submerged, only producing aerial or, in the case of O. fluviatilis, aerial and floating stems, in the early summer. They become conspicuous (and fully recognisable) only for a brief period in late summer when they flower and fruit. This is why all five Fermanagh records were determined in the month of August when the fruits are mature and recognisable.

The record details of the other four finds of O. fluviatilis are: marsh to E of Inishroosk, 1 August 1986, EHS Habitat Survey Team; S shore Ross Lough, 5 August 1986, S.A. Wolfe-Murphy & L.W. Austin; Kilturk Lough, Derrymacrow Td, 14 August 1986, P.J.T. Brain & T. Waterman; S shore of Trasna Island, 19 August 1986, P.J.T. Brain & T. Waterman (a rather doubtful determination).

Flowering reproduction

It is well known that some stands of O. fluviatilis only rarely produce flowers and hence they are readily over-looked, especially since they very often associate with aquatic Ranunculus species, submerged Potamogeton species and other aquatic plants with finely dissected leaves like theirs. When plants do manage to flower, it is usually in slow flowing or sluggish waters and it takes place in July and August (Preston & Croft 1997). In comparison with the submerged leaves, the aerial ones that accompany the flowering stem have broader leaflets up to 10 mm long, with blunt tips. The umbels arise from the axils of leaves and there are no bracts. Fruits are longer than in most other Oenanthe species, 5-6.5 mm, ovoid in shape and ripen in August and September (Walls 1995). However, ripe fruits are rarely found and the frequency of successful seed production is unknown (Cook 1983; Preston & Croft 1997).

Vegetative reproduction

In fast-flowing waters, O. fluviatilis stems root at their nodes and often develop large, clonal mats that rarely flower. In these circumstances, the species reproduces by vegetative fragmentation without any specialised structures being involved.

British and Irish occurrence

In Britain, O. fluviatilis is completely absent from Wales and Scotland. In England, apart from a few outliers in W Yorkshire, it is virtually restricted to the area south of a line between the Wash and the Severn estuary. In comparison with O. aquatica, O. fluviatilis is the scarcer of the two species, the New Atlas survey mapping it in just 160 hectads from the 1987-99 date range in the whole of B & I.

The New Atlas map displays quite a difference between the distribution of O. fluviatilis in England and in Ireland. In the latter, the species is thinly but rather widely scattered, mainly in C Ireland, but it extends much further north than it does in England, reaching Mountsandel, near Coleraine in Co Londonderry (H40).

O. fluviatilis appears to be slowly declining throughout these islands, probably due to gradually increasing eutrophication as a result of pollution and agricultural fertiliser run-off, together with excessive disturbance from mechanised stream channel clearance operations and pleasure boating activities (Preston & Croft 1997).

European occurrence

Beyond our shores, O. fluviatilis is endemic to NW Europe – being confined to N France, Germany and W Denmark, where it is nowhere common. In fact a significant proportion of the species total population resides in B & I (Cook 1983; Walls 1995).

Threats

O. fluviatilis is sensitive to pollution, including excessive eutrophication, and also to large-scale disturbance from over-zealous drainage and boating.

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