Observed changes in oxygen and nutrient concentrations at 50 m to 500 m depth in the eastern North Pacific Subtropical Gyre

Abstract

As the ocean increases in temperature due to climate change, it is expected to also 2 experience shifts in its chemistry due to increased stratification and decreased solubility. 3 Changes in the subsurface ocean, like decreases in oxygen as well as increases in nutrients like 4 nitrate and phosphate, have been suggested as possible effects of this phenomenon, and could 5 have significant impacts on biogeochemical cycles. For this study, the concentrations of oxygen, 6 nitrate, and phosphate were measured between the depths of 50 m and 500 m depth in the eastern 7 portion of the North Pacific Subtropical Gyre during late December of 2021. These present-day 8 data were then compared to corresponding data from hydrographic cruises conducted in this 9 region in 1985 and 2005, as well as long-term climatological averages. Between 1985 and 2021, 10 oxygen concentrations were shown to decrease by 7.24 μmol/kg (+/- 16.52 μmol/kg), and nitrate 11 and phosphate concentrations were shown to increase by 2.43 μmol/kg (+/- 3.45 μmol/kg) and 12 0.16 μmol/kg (+/- 0.22 μmol/kg), respectively. The nitrate and phosphate concentrations showed 13 increases of 1.07 μmol/kg (+/- 2.32 μmol/kg) and 0.05 μmol/kg (+/- 0.15 μmol/kg), respectively, 14 when measured against climatological averages. In general, these data were shown to be 15 statistically significant and consistent with the long-term changes predicted by climate models, 16 suggesting that the impact of ocean warming on oxygen and nutrient concentrations will only 17 become more relevant as time passes and the effects of climate change intensify.