IMIS

During most of the year, diatom production in the ice-covered Central Arctic Ocean (CAO) is limited by light availability and nutrient supply. Therefore, biological production is thought to be generally low, with higher biological production at the sea ice edge and over partially ice-free shelf areas. The major surface ocean current in the CAO is the Transpolar Drift (TPD), which transports sea ice and water from the rivers and shelves of the Laptev and the East Siberian Seas across the CAO toward the Fram Strait, carrying high amounts of terrestrial-derived material over long distances. We used Si isotopes (δ³⁰Si) to better understand the difference between lower and higher biological production areas and how the TPD potentially affects the Si cycle in the CAO. Our data show lowdissolved Si concentrations ([DSi]) paired with high values of δ ³⁰Si-DSi in all surface samples indicating fractionation by diatoms. Specifically, outside the TPD influence, all nutrients were depleted and supply was limited due to stratified conditions, thus preventing further phytoplankton growth in the area during the sampling time in late summer-early fall. In contrast, under the TPD influence, diatom primary production was limited by low nitrate and strongly limited by light due to the presence of sea ice, even though [DSi] values were much higher than outside the TPD. Based on δ³⁰Si, we could identify low but measurable DSi utilization in the TPD, potentially highlighting the importance of sea ice-attached diatoms transported to the CAO via the TPD for the Si cycle in this region.