AbstractRiver inputs of dissolved silica into a shallow eutrophic lake (Lough Neagh, N Ireland) are quantified. Exports of silica are high (3.6 to 9.6 tonnes dissolved SiO2km−2y−1), but the spring concentration of dissolved silica in the lake depends on release of silica from the sediments. Two planktonic diatomsMelosira italica subarcticaO. MÜLL, andStephanodiscus astraea(EHR.) GRUN. dominate the spring flora. Their periodicity is described over ten years and their usage of silica and phosphorus is examined. Data are presented on the resuspension of cells from the sediment, the growth rates of the diatoms in the lake, the incidence of parasites and changes in the filament length ofM. italica subarc