A Multivariate Assessment of Plant Management Impacts on Macrophyte Communities in a Scottish Canal

Abstract

SUMMARY (1) A series of aquatic weed control trials was conducted in the Union Canal, Scotland (boat traffic < 500 boat movement ha- ' year-') during 1977-84. The data were used to test the suitability of a multivariate approach for assessing the impacts of plant management on canal macrophyte communities. (2) The procedures detrended correspondence analysis (DCA), and two-way indicator species analysis (TWINSPAN), revealed some fairly subtle trends of plant community change. These were related to the environmental impacts of weed control by herbicides and cutting, and (in a few cases) to shading of submerged macrophytes by accumulations of surface-floating plants. (3) Dissolved oxygen status was significantly negatively correlated with one major axis of the DCA ordination. This axis appeared to reflect a trend of plant community change associated with efficient weed control operations. A second major axis was probably associated with a hydroseral gradient within the (open-water) early stages of vegetational change in the canal plant community. (4) The timing and type of control operations appeared to play an important role in determining the hydroseral age of the plant community present. Sites recently subject to effective weed control generally showed increased dominance by rapid-growing filamentous algae or other opportunists. Untreated sites, sites sampled at long time intervals after control operations, and those where weed control measures had been inefficient, had a lower incidence of opportunist species in the long term. (5) Floating-leaved rooted vegetation (e.g. Potamogeton natans), appeared to be naturally resistant to the weed control regimes used. Such species also compete well with Lemnaceae mats. Because of this relative immunity to the main ecological pressures influencing the submerged community this component of the vegetation is increasing in dominance, and is a cause of major weed problems in the canal. (6) There were significant differences in the efficacy of herbicide and cutting treatments in trials where near-optimal herbicide availance was achieved. Repeated cutting, and early season diquat-alginate application gave acceptable weed control without altering macrophyte community structure to an extent unacceptable in conservation terms. Suboptimal diquat-alginate or terbutryne treatments, and late-season weed-cutting increased the abundance of opportunist algae and Lemna spp., and reduced the diversity of the canal macrophyte community.