The transport and retention of dissolved silicate by rivers in Sweden and Finland

Here we report on the role of natural (lakes) and artificial barriers (dams) in regulating dissolved silicate (DSi) concentrations in rivers from Sweden and Finland. Concentrations of DSi and total phosphorus in rivers were strongly affected by the presence of lakes and reservoirs dong the aquatic continuum, with riverine DSi concentrations decreasing as the percentage of area occupied by lakes and reservoirs increased. Rivers with <2% lake and reservoir area in their watershed averaged 164 M, whereas DSI in rivers containing greater than 100/o lake and reservoir area averaged 46 muM. The relationship between percentage lake area and DSi concentration was best fit by an exponential function (r(2) = 0.79, P < 0.001), because once lake and reservoir area exceeds 100/o, minimum DSi concentrations are reached. Because lakes and reservoirs act to reduce watershed DSi concentrations through diatom growth and sedimentation, our results support the hypothesis that further declines in the delivery of DSi to the coastal ocean should be expected as new dams are constructed on rivers. These reductions in DSi transport to the coastal zone with dam construction will have important repercussions on diatom growth and coastal food webs.