The Arctic Ocean is changing severely since one decade. Extremely low ice coverage in late summer has been recorded since 2007 with a new minimum in 2011. The zone of the receding ice edge is known for its high primary productivity in polar waters, but depending on light and nutrient availability productivity in the ice can also be high. We don’t know yet what the influence of less ice will be for the future Arctic Ocean ecosystem. Several scenarios are under discussion. Chlorophyll a is a measure of biomass standing stock of phytoplankton and can give information on surface as well as depth distribution of autotrophic biomass in the ocean. Chlorophyll a measurements also serve as ground truth data used to validate productivity estimates by remote sensing from space. Here we present a data set obtained since 1991 during several cruises carried out on RVs Polarstern, Lance & Maria S Merian to the Fram Strait, Greenland Sea and to the central Arctic Ocean including Laptev and Kara Seas. Almost every year samples have been taken from at least six different depth horizons within the upper 100 meters of the water column in the Fram Strait and Greenland Sea. During five expeditions sampling has been carried out to the central Arctic Ocean between the years 1993 and 2011. The plankton of distinct depth samples has been filtered onto glass-fibre filters and chlorophyll a concentrations were analyzed with a fluorometer and/or a photometer as a measure of phytoplankton standing stock. Here we will focus on the seasonality and interannual variability in the chlorophyll a distribution. A comparison of spring, summer and fall in-situ data show highest values in April and May decreasing towards summer. In fall, only a small fraction of the chlorophyll a concentrations measured reach values above 1 µg L-1, most concentrations were below 0,1 µg L-1 in Greenland Sea and Fram Strait. Whereas highest values above 10 µg L-1 have been recorded in the Kara Sea in the vicinity of Ob and Yenisei, lowest chlorophyll a concentrations were found in the ice-covered Arctic Ocean (< 0,5 µg L-1). In the future, data obtained in-situ will be compared with data retrieved by satellites to gain a broader view of the biomass distribution of phytoplankton in the entire Arctic Ocean. Some first examples will be shown. Shifts in chlorophyll a concentration patterns can partly be attributed to variations in sea-ice distribution and eventually to climate changes. Long-term trends in chlorophyll concentration patterns reveal slight alterations in biomass in the investigated regions of the Arctic Ocean. This can be the result of differences in productivity possibly leading to changes in trophic interactions, and in sequestration efficiency of CO2 in the ocean.