1. The total area of standing fresh water in Great Britain has been estimated at approximately 2400 km2. The aim of the work described in this paper was to estimate the extent of the dystrophic, oligotrophic, mesotrophic and eutrophic habitat types making up this total.
2. Botanical survey data collected by the statutory nature conservation agencies provided habitat type and surface area for 3500 water bodies, mostly in Scotland. Because survey in England and Wales had been less thorough than in Scotland, it was not possible to estimate the areas of habitat types simply by direct extrapolation from the survey sample to the total resource of standing water in Great Britain.
3. A system of Trophic Ranking Scores (TRS) has been developed for aquatic plant species. Using plant records from the Biological Records Centre, a mean TRS for each 10×10 km square was produced, and Great Britain was divided into TRS bands. For each of these bands, the percentage by surveyed area of each freshwater habitat type was calculated. These percentages were applied to the total area of standing water in each TRS band, obtained from Ordnance Survey summary statistics, in order to predict the proportions of the different habitat types likely to occur in each band. The extent of these habitats in Great Britain as a whole could then be calculated.
4. Estimates produced by this method of the extent of the four standing freshwater habitat types in Great Britain are: dystrophic — 11 km2 (0.5%); oligotrophic — 1445 km2 (60%); mesotrophic — 267 km2 (11%); eutrophic — 679 km2 (28%).
5. The application of this work to nature conservation is discussed. Copyright © 2001 John Wiley & Sons, Ltd.