This is chapter 5 of the State of Environmental Science in Svalbard (SESS) report 2024 (https://sios-svalbard.org/SESS_Issue7). Arctic marine systems are vital to the Earth’s ecological and climatic balance and harbour a unique biodiversity adapted to extreme conditions. However, they are under unprecedented threat from climate change, in particular from “Atlantification”, i.e., the loss of sea ice and the increasing influence of Atlantic Water (AW) in this region. Long-term observatories are crucial to detect even small changes in these dynamic ecosystems. Svalbard, with its easily accessible infrastructure, is a strategic location for studying these changes. We have identified marine time series that clearly illustrate the signature of Atlantification in the Svalbard region over the last two decades. These time series provide insight into the propagation of AW and the interaction between the continental slope, the shelf and the fjord systems. Seasonal and multi-year temperature and salinity patterns illustrate the varying influence of AW, with long-term variability showing, among others, a freshening phase since 2018, possibly related to North Atlantic freshwater anomalies.