SummaryIn this study of the Lough Neagh catchment a relationship has been sought between the loads of dissolved and total silica and the flow of each of the six inflowing rivers. Two of these rivers‐the Main and Six Mile Water‐carry a higher ratio of dissolved to total silica than do the remaining four rivers due to the geology of the catchment. The multiple regressions of loads in tonnes against river flows in cubic metres show a linear relationship for dissolved but not for total silica. The explanation for this non‐linearity has been sought in the river proaies, where the fall for the last 30 m varies from 5 to 25 km depending on the river. The rivers Main and Six Mile Water are the steepest, and linear relationships were obtained in their regressions while the shallowest profile was found in the Blackwater where non‐linearity in the multiple regression was obtained.In Lough Neagh the dissolved silica content was gradually reduced from early winter through to springtime by diatom growth and a balance has been drawn up for this utilization. This reduction also compares with the quantity in the diatom crop at maximum based on cell volume measurements. The summer build up of the dissolved silica content of the lough has been shown to be greater than the quantity contributed by the inflowing rivers from late spring to autumn and the data show that the recycling of silica from some 65% of the frustules from the spring crop after its collapse would account for the summer i