Effects of Pleasure-Boat Traffic on Macrophyte Growth in Canals

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Abstract
Quantitative surveys of plant growth in British Cruising and Remainder canals [UK] show significant associations between community composition, abundance of aquatic macrophytes and pleasure-boat traffic. Cluster analysis revealed 4 principal groupings of sites, which had significantly different mean boat traffic densities and markedly different macrophyte community compositions and abundances. Maximum-likelihood and regression analyses indicated significant inverse relationships between total, emergent and submerged macrophyte community abundance and boat traffic. Evidence was found for a critical traffic range, .apprx. 2000-4000 movements ha-1 m depth-1 yr-1 (my), reducing submerged macrophyte standing crop, perhaps by maintaining high daytime water turbidity during the summer momnths as there were significant associations between boat traffic density, water turbidity and summer submerged crop. The seasonal distribution of pleasure-boat traffic appeared to be an important influence on submerged macrophyte community abundance. The course of macrophyte community development may be largely determined by the stage in the growth season at which traffic in the range 300-600 movements ha-1 m depth-1 (4 wk)-1 is attained. Relationships between the abundance of emergent plants and boat traffic were weaker and there was no significant association with water turbidity. Differences in the response of various macrophyte species to boat traffic are noted and discussed. In 1977 .apprx. 50% of the canal system carried low pleasure-boat traffic density (< 2000 my), 24% had traffic within the critical range (2000-4000 my), and 26% had heavy traffic (> 4000 my). Reasons for the absence of weed problems from some canals with low traffic are discussed. Implications for the efficient management of recreational canal ecosystems are considered.

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