Association between water darkening and hypoxia in a Norwegian fjord

Previous studies have shown that climate change makes Norwegian fjords prone to deoxygenation and water darkening (reduced light penetration) with ecological implications across the food web from phytoplankton to fish. While deoxygenation occurs in deep fjord basins due to reduced water renewal, water darkening has primarily been linked to increased loads of colored dissolved organic matter (CDOM) of terrestrial origin in rivers draining to the Baltic Sea, the North Sea and ultimately to the Norwegian Coastal Current and associated coastal waters. Here, we investigate the hypothesis that water darkening is also linked to deoxygenation of fjord basins. We measure the downwelling irradiance in a water column with hypoxic and anoxic water and compare it with a nearby, well-oxygenated water column. Our data show increased darkening in the hypoxic and anoxic layers, likely due to elevated concentrations of CDOM that is produced locally in these layers. We discuss the ecological implication of this result, which we believe is particularly relevant for the mesopelagic habitat. • Downwelling irradiance was measured in oxygenated, hypoxic and anoxic fjord water. • Light attenuation was higher in low-oxygen water than in well-oxygenated water. • Oxygen-related darkening was likely linked to CDOM from microbial degradation. • Climate change makes fjords prone to a two-sided darkening effect. • The two-sided darkening involves locally produced CDOM in addition to increased supplies of CDOM of terrestrial origin.