Identification of a Seasonal Subsurface Oxygen Minimum in Rivers Inlet, British Columbia

A subsurface oxygen minimum layer (OML) in intermediate water is identified and characterized in Rivers Inlet, a fjord on British Columbia’s central coast, using data from 1998 to 2018. The OML was observed in most years from May to September and was most persistent at the middle and head of the fjord. The Rivers Inlet OML develops in three stages: (i) in early spring, the cessation of winter storms from downwelling-favourable winds stops the ventilation of the water column; (ii) throughout spring and summer, the remineralization of organic matter, likely primarily phytoplankton, consumes oxygen in the intermediate waters; (iii) in late spring, deep-water renewal by oxygenated offshore water forms the base of the OML inside the inlet. The strength and persistence of the OML vary interannually, mainly due to variability in hemispheric-scale winds and primary production. In some years, the OML was hypoxic, which could influence the local marine ecosystem. Changes to downwelling, upwelling, or primary production in Rivers Inlet could affect the OML in the future.