Habitat forming large brown seaweeds (kelps) are dominating many Arctic rocky shore coastlines, structuring benthic and pelagic species communities. On Svalbard, global warming is occurring at a rate far beyond the global average. The increasing terrestrial run-off leads to a darkening of the water column, which will affect kelps. This project aims to model the spectrally resolved underwater light climate of different sites in Billefjorden, being influenced by 1) high terrestrial run-off, 2) high glacial influence and 3) mostly clear water. These environmental differences will be correlated to kelp physiology, biochemical composition and associated microbial community to be able to model future Arctic fjord ecosystem transitions.