This project will focus on the effect river runoff has on the seasonal plankton bloom of an arctic fjord, using Adventfjorden, Svalbard as a model system. Water column sampling will be conducted every two weeks in the timeperiod April 2023-September 2023 at two stations; one marine (IsA time series) and one heavily influenced by river runoff (A2). The samples will be used to identify the composition of the phytoplankton, using microscopy and DNA metabarcoding. Algal biomass will be measured as chlorophyll a, and rates of production by primary production. CTD measurements, including light, turbidity, fluorescence and oxygen will also be taken, and nutrient samples collected. This will provide information about the seasonality of the phytoplankton as well as the bottom-up control. A grazing experiment will be performed 2-3 times during the productive season, to study top-down control of phytoplankton. Samples from the ice-free Adventfjorden will also be compared to samples taken from an ice-covered fjord that is not affected by river runoff.