THE DISTRIBUTION AND EXPANSION OF ORNAMENTAL PLANTS ON THE SHANNON NAVIGATION

Abstract

Seven alien aquatic plants occur on the Shannon Navigation. Six ornamental plants have been recorded in Lough Derg. Two, Elodea canadensis and Lemna minuta, are widely distributed in the lough, locally abundant within the Navigation and the only alien plants found in Lough Ree. The species with the greatest impact, Elodea nuttallii, was first recognised in 2004 in harbours and cuts about the southern region of Lough Derg. It spread northwards by drifting fragments and almost certainly by leisure craft. Likewise, the fertile water violet, Hottonia palustris, originally found on the eastern side of the lake, has become abundant in sheltered shallows, reedbeds, canals and drains on the western side of the lake and continues to spread northwards and southwards. Confined populations of Stratiotes aloides occur only on the western side of Lough Derg and may have been there for several years. An isolated population of a pink cultivar of Nymphaea alba, present for at least four decades, occurs in Lough Derg beside a lakeside domaine. Since the introduction of the zebra mussel, Dreissena polymorpha, in c. 1993/4 there have been increases in water transparency that have presumably enabled some macrophytes to extend their ranges into deeper water. Given the likelihood of further range expansions or new introductions, regulation of the sale and distribution of alien aquatic species is needed to prevent further exotic plants from entering Irish waterways.