Expansion of a localized population of the introduced Stratiotes aloides (Hydrocharitaceae) in Lough Derg, Ireland

Stratiotes expansion in an Irish lake

Abstract

An expansion of an introduced female clone of Stratiotes aloides L. (Water-soldier) was located in a delta region on the western side of Lough Derg, Co Galway (v.c.H15), Ireland in 2007. This population was followed over a thirteen-year period. It was initially located at three adjacent sheltered localities, within sweepback bays on either side of an emerging river and within an adjacent canal. The study involved surface observations later supplemented with aerial images. The shallow water conditions, shelter and the presence of Phragmites australis appear to have supported the early establishment by retaining small S. aloides clusters amongst its stems. These clusters later merged to produce a mainly surface expanding monoculture. This enlarged from less than 1 ha to approximately 3.3 ha to occupy much of the sheltered Rossmore Bay area during this study. Flowers were first noticed in 2008, and during subsequent visits, producing infertile seed-pods.  Expansion took place with the production of daughter plants. A small nearby population, 1 km to the east, within an unused harbour, did not produce an emergent phase. A small group in a shallow cut, between the two locations, disappeared during the study. Aerial images from different sources were useful to identify the expansion of the emergent stage due to the distinctive bright green coloration of surface leaves. It is unclear how this plant arrived in Lough Derg, but might have been a garden plant release. Small drifting plants, seen during wintertime, may yet colonize other regions within this lake.