Plant management as an integrated part of Ireland's aquatic resources

Abstract

Aquatic plants are fundamental to the structure and functioning of many freshwater habitats. In undisturbed aquatic habitats plants serve a multitude of important functions including the provision of substrates for colonisation by periphyton and macro-invertebrates, a direct and indirect food source for macro-invertebrates, fish and wildfowl, spawning substrates for coarse fish species, shelter for young and adult fish oxygen as a by-product of photosynthesis. In disturbed habitats, however, the natural balance among aquatic plants is altered and an excessive, often monospecific, vegetation commonly develops. This can adversely affect the beneficial use of the watercourse by diminishing its amenity and recreational potential, exacerbating water supply and flooding problems and resulting in excessive siltation in river or canal habitats. This paper outlines the nature and extent of the more important weed problems in lrish aquatic situations and describes a range of control strategies that may be adopted to rehabilitate these habitats and to enable their exploitation by a diversity of user groups. In certain instances paucity of aquatic vegetation presents problems, including bankside destabilisation and inchannel siltation. Reed transplantation trials have been conducted in denuded lrish canals in an effort to rehabilitate these habitats and the results from these are presented. Weed control procedures generally embrace four broad categories. These are mechanical, chemical, environmental and biological control. Of these only biological control is not widely adopted in Ireland. The effects that these procedures have on specific weeds were quantitatively investigated at a number of aquatic situations throughout the country. Results from experiments among a diversity of aquatic plant species and involving the use of mechanical weed cutting apparatus show that, unless the plant is cut to a depth at which light penetration is significantly diminished, rapid regrowth will occur and higher standing crops than might otherwise be expected may be recorded. Experimentation with a small number of herbicides has revealed that, when used with discrimination, these provide effective weed control, with little or no detrimental effect to the aquatic ecosystem. Results from trials with dichlobenil and glyphosate on aquatic and riparian species are presented. The effect that shading has on macrophyte biomass and the effect that rotted barley straw has on algal growths in watercourses is also described.