Changing nutrients, dissolved oxygen and carbonate system in the Bohai and Yellow Seas, China

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The Bohai and Yellow Seas in the Northwest Pacific are semi-enclosed shallow marginal seas of ecological and economic significance. By reviewing and synthesizing literature data, basin-wide decadal changes in nutrients and bottom-water dissolved oxygen and carbonate system parameters in the two coastal oceans were investigated. Results showed that both of the two coastal oceans were subject to basin-wide increases in wintertime nitrate during the past 40 years. The present-day seawater N:P ratios are usually within the algae-favorable range of 14–19. Presumably due to these changes, the Bohai Sea exhibits a 33-year decline in summertime bottom-water dissolved oxygen and the associated suppression of pH and CaCO3 saturation states in summer. The historically lowest bottom-water dissolved oxygen in the Bohai Sea was recorded at 67 μmol/L in early September 2015, which was very close to the threshold value of hypoxia. In the Yellow Sea, periodical suppression of pH and CaCO3 saturation states occurs in its central basin area, where the net community carbonate dissolution was detectable in bottom waters in late summer and autumn, threatening marine calcifiers inhabiting there and with potentially severe consequences for valuable shellfish fisheries.

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