| Abstract: | 1. The first recorded effects of human activity on the aquatic flora of Llangorse Lake, South Wales, were the result of introductions of Elodea canadensis in the early 1900s and of Nymphoides peltata in 1936, altering the species balance of both submerged and floating plant communities. 2. The most significant impact on the lake in recent times has been effluent from the sewage treatment works, which discharged into the lake between the early 1950s and 1982, and the impact of a concurrent intensification of agriculture within the catchment, both leading to accelerated eutrophication. 3. The eutrophication of the lake resulted in a significant change in the submerged flora, which declined in diversity and extent to only a few stands of two species in 1982. 4. Following the diversion of the sewage effluent, the submerged flora recovered to a situation similar to that existing before the effects of accelerated eutrophication. 5. The emergent and floating plant communities did not reflect the eutrophication in the same way as the submerged flora, although Equisetum fluviatile has declined noticeably. 6. The floating and emergent communities, notably Phragmites australis reed swamp, have been significantly damaged in the immediate vicinity of mooring developments, but elsewhere the introduction of water‐based recreation cannot be demonstrated to have had a widespread detrimental effect. 7. The recovery of the submerged plant community is considered to be in part due to residual populations of the various species ‘lost’ from the main body of the lake existing in refugia in and around the lake. 8. The history of the lake is notable in terms of the recovery of the submerged flora and in the absence of dominants new to the system invading during the period of accelerated eutrophication. Copyright © 1999 John Wiley & Sons, Ltd. |
