The Arctic Ocean is extremely susceptible to climate change, which has led to warmer air temperatures, accelerated sea ice loss, and an intensified inflow of Atlantic water masses into the Arctic. Furthermore, the reduction in sea ice has been linked to a prolonged phytoplankton growth season. Fewer days of sea ice cover and thinner sea ice increases the light availability for ice algae and expands the habitat for pelagic phytoplankton. An extension of the growth season prolongs the release of phytoplankton-derived organic matter with consequences for the seasonal carbon cycle. However, the seasonal carbon stock and microbial processes are largely unexplored because many parameters require in situ sampling. A systematic strategy to enhance microbial observations is through time series stations. The long-term ecological research observatory HAUSGARTEN was established in 1999 and is the only pelagic microbial observatory in the Arctic to date. The observatory is sampled annually during the summer months, which limits its explanatory power to assess the seasonal carbon cycling and microbial dynamics. To close this gap, the goals of this doctoral project were to (a) investigate the seasonal variability of organic matter, (b) explore seasonal organic matter turnover and the microbial community, and (c) evaluate the inter-annual variability in the Fram Strait
