Lakes and dams play crucial roles in the global cycles for nutrients and organic carbon as links from the terrestrial sources to the marine sinks. The degree of retention is sometimes high in these water bodies. Treguér et al. (1995) report that increasing inputs of dissolved inorganic nitrogen and phosphorus into rivers and lakes support enhanced production of diatoms, resulting in decreasing concentrations of dissolved silicate (DSi) in the river loads. The construction of storage lakes in rivers enhances this effect, even in less eutrophicated rivers. This phenomenon is reported for large-scale systems like the Danube and the Black Sea (Humborg et al., 1997), the Mississippi river (Turner and Rabalais, 1994) as well as for more modest rivers like those entering the Baltic Sea (Humborg et al. 2000). A key factor in the retention process of the river system seems to be the residence time. The question is now which physical mechanisms governs the retention process in reservoir vs. dams. A comparative study between Akkajaure and Torne Träsk is under way to resolve the consequences of river regulation.This project is a part of a larger "programme" to investigate the reason for decreasing DSi levels in the Baltic with implications on the World Ocean.