The silicon isotope composition (delta Si-30) of dissolved silicon (DSi) and biogenic silica (BSi) provides information about the silicon cycle and its role in oceanic carbon uptake in the modern ocean and in the past. However, there are still questions outstanding regarding the impact of processes such as oceanic mixing, export and dissolution on the isotopic signature of seawater, and the impacts on sedimentary BSi. This study reports the delta Si-30 of DSi from surface waters at the Rothera Time Series (RaTS) site, Ryder Bay, in a coastal region of the West Antarctic Peninsula (WAP). The samples were collected at the end of austral spring through the end of austral summer/beginning of autumn over two field seasons, 2004/5 and 2005/6. Broadly, for both field seasons, DSi diminished and delta Si-30 of DSi increased through the summer, but this was accomplished during only a few short periods of net nutrient drawdown. During these periods, the delta Si-30 of DSi was negatively correlated with DSi concentrations. The Si isotope fractionation factor determined for the net nutrient drawdown periods, epsilon(uptake), was in the range of -2.26 to -1.80 parts per thousand when calculated using an open system model and -1.93 to -1.33 parts per thousand when using a closed system model. These estimates of epsilon are somewhat higher than previous studies that relied on snapshots in time rather than following changes in delta Si-30 and DSi over time, which therefore were more likely to include the effects of mixing of dissolved silicon up into the mixed layer. Results highlight also that, even at the same station and within a single growing season, the apparent fractionation factor may exhibit significant temporal variability because of changes in the extent of biological removal of DSi, nutrient source, siliceous species, and mixing events. Paleoceanographic studies using silicon isotopes need careful consideration in the light of our new results.