Some factors effecting algal densities in a eutrophic farmland stream

Population dynamics of attached algae in a eutrophic farmland stream (Wellow Brook, England) were correlated through multiple regression analyses with changes in 7 physico-chemical parameters (temperature, light, water velocity, pH, NO₃-N, P₂O₅-P, SiO₂-Si). Samples were taken every 2 weeks for 25 consecutive months between June 1973 and June 1975. The relative significance of the 7 parameters in controlling densities varied widely depending on species. Overall light was most important, accounting for 28 and 17% of density changes in the epipelon and epilithon respectively. While winter flooding caused a sharp reduction in the density of these 2 communities, the concentration of P₂O₅-P and NO₃-N and pH usually had little effect on numbers. The 7 parameters did not exert major control over the epiphyton, normally accounting for <30% of densities variation. Grazing by the protozoan Frontonia acuminata may have significantly reduced the density of the epipelon for 2 months during the spring but otherwise was of no importance to any community.