The effect of lake fertilisation on the rotifers of Seathwaite Tarn,an acidified lake in the English Lake District

Seathwaite Tarn, in the English Lake District, was used as an experimental site to test the feasibility of increasing the pH of an acidified waterbody by adding a phosphorus-based fertiliser solution. The impact of this experiment on the planktonic rotifer community was assessed by monitoring for two years before treatment (1990–1991), and for a further two years once treatment had begun (1992–1993). The pH of the water rose by 0.5 pH units over the period of fertilisation, and the lake became very eutrophic. This had little effect on the species composition of the rotifer assemblage, which continued to reflect the acid, oligotrophic nature of the untreated lake. However, there were marked changes in the levels of abundance of most planktonic species. Population densities of Keratella serrulata and Trichocerca longiseta increased in early spring, compared to pre-treatment levels, apparently due to the rises in chlorophyll-a concentrations. In contrast, Polyarthra dolichoptera became very scarce after fertilisation began. The treatment seemed to have little effect on the population dynamics of the dominant openwater rotifer, Polyarthra remata. In 1993, very high densities of Bosmina coregoni (up to 250 × 10³ ind M^–3 ) developed in response to the eutrophication programme. This seemed to suppress the rotifer community in the summer months.