Little is known about the horizontal spatial variation of water quality in lakes and the underlying mechanism causing such a potential variation. This study focused on five research questions: 1. Is there a horizontal spatial variation of water quality in lakes? 2. Can a mid-lake sample be regarded as representative for the whole lake? 3. How large is the horizontal spatial variation of water quality in the littoral zone and between the littoral and the pelagic zone? 4. Which factors affect the potential horizontal spatial variation in Storträsket? 5. How much of the horizontal spatial variation in the bays can be explained by the effects of land use in the surrounding landscape? Pelagic and littoral surface water samples were taken at 14 localities in Storträsket, a boreal lake at the land uplift coast in northern Sweden. Eleven water quality factors were included in this study: chlorophyll concentration, conductivity, colour, pH, Secchi depth, total organic carbon and total contents of nitrogen, phosphorous, iron, magnesium and manganese. Water quality between the littoral and the pelagic zone was significantly different for two of eleven water variables. Differences in water quality between bays were significant for seven of eleven water variables. This indicated, that the bays average might not be a good measure of the individual bays’ water quality. Lake morphology and land use in the local catchment areas of the studied bays was analysed in a geographic information system. The water quality variables of the bays were analysed for correlations with the area of two main land use types (coniferous forest and mire) in the local catchments. The area of coniferous forest correlated significantly with total N concentrations in the bays. The area for mire correlated significantly with chlorophyll, Fe, Mn, and total P concentration. Data on colour, conductivity, pH and temperature from a second sampling occasion at Storträsket (119 localities) was interpolated (kriging) to assess the variation of water quality in the whole lake. Colour, conductivity and temperature were significantly different between the northern and the southern basin of Storträsket. My results indicated that there is a horizontal spatial variation of water quality. Therefore, I would not regard a mid-lake sample as representative for the whole lake. Land use in the vicinity of Storträsket, lake morphology, and inlets entering the lake had an impact on both, the variation of local water quality in the littoral zone and the spatial variation of water quality in the whole lake. The occurrence of mires in the local catchments of the bays had the largest impact on the variation of water quality in the littoral zone. Thus effects of land use in the surrounding landscape should be taken into account when discussing determining factors of spatial variation of water quality in lakes.