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Studies of silicon (Si) isotope fractionation during diatom growth in open ocean systems have documented lower Si isotopic values (δ³⁰Si) in the biogenic silica of diatom frustules compared to dissolved silicon. Recent findings also indicate that Si isotope fractionation occurs during dissolution of diatom frustules, producing higher δ³⁰Si values in the remaining biogenic silica. This study focuses on diatoms from high production areas in estuarine and coastal areas that represent approximately 30–50% of the global marine primary production. Two species of diatoms, Thalassiosira baltica and Skeletonema marinoi, were isolated from the brackish Baltic Sea, one of the largest estuarine systems in the world. These species were used for laboratory investigations of Si isotope fractionation during diatom growth and the subsequent dissolution of the diatom frustules. Both species of diatoms give an identical Si isotope …