THE EFFECT OF LAKE FERTILIZATION 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 experime ntal site to test the feasibility of increasing the pH of an acidified waterbody by adding a phosphorus-based fertiliser solution. The impac t of this experiment on the planktonic rotifer community was assessed by monitoring for two years before treatment (1990-1991), and for a fu rther two years once treatment had begun (1992-1993). The pH of the wa ter rose by 0.5 pH units over the period of fertilisation, and the lak e became very eutrophic. This had little effect on the species composi tion of the rotifer assemblage, which continued to reflect the acid, o ligotrophic nature of the untreated lake. However, there were marked c hanges in the levels of abundance of most planktonic species. Populati on densities of Keratella serrulata and Trichocerca longiseta increase d in early spring, compared to pre-treatment levels, apparently due to the rises in chlorophyll-a concentrations. In contrast, Polyarthra do lichoptera became very scarce after fertilisation began. The treatment seemed to have little effect on the population dynamics of the domina nt openwater rotifer, Polyarthra remata. In 1993, very high densities of Bosmina coregoni (up to 250 x 10(3) ind m(-3)) developed in respons e to the eutrophication programme. This seemed to suppress the rotifer community in the summer months.