AbstractThe Lough Erne System (Northwest Ireland) consists of two linked eutrophic lakes. Phosphorus input budgets suggested mean catchment exports of 30–62 kg total P km−2a−1, resulting in a heavy P loading to the small upper lake. The export of soluble P is related to the human population density in the catchment. Although complex in outline, the upper lake has similar phytoplankton populations throughout, but the populations in the simpler shaped lower lake differ quantitatively and qualitatively from place to place. This is probably due to differences in mixed depth. A chlorophyllamap of the surface water of the lower lake, obtained by fluorometry, showed the relationship between algal biomass and depth of