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The Hudson Bay system (HBS) is a shallow inland sea in the Arctic, composed of Hudson Strait, Foxe Basin/Channel, James Bay, and Hudson Bay. Dissolved inorganic carbon (DIC) and total alkalinity (TA) measurements were used to investigate the state of ocean acidification, specifically calcium carbonate saturation states (X) and pH. The freshwater sources were identified from the relationship between oxygen isotope composition (d 18 O) and salinity to understand the role of freshwater in ocean acidification. The saturation state of seawater with respect to calcium carbonate (X) in surface water (<10 m) of the HBS was strongly influenced by river runoff. Aragonite under-saturation (X arg < 1) was observed in the surface water of the south-eastern Hudson Bay, where the river runoff fraction was high (>10%). The watershed characteristics, however, influenced the alkalinity of river runoff in different parts of Hudson Bay, which contributed to X variation in the coastal region. In southwestern Hudson Bay where the watershed is dominated by limestone, X was higher compared to eastern Hudson Bay, where the watershed consists of an igneous rock formation. In deeper waters, low X is caused by remineralization of organic matter. The highest DIC concentrations (>2300 mmol/kg) were observed in the depths of central Hudson Bay with a pH total of 7.49 and X arg of 0.37. Over 67% and 22% of the bottom water of Hudson Bay was undersaturated with respect to aragonite and calcite respectively, despite Hudson Bay being very shallow (less than 250 m deep). The aragonite saturation horizon in the central Hudson Bay was around 50 m.

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