Responses of the marine carbonate system to a green tide: A case study of an Ulva prolifera bloom in Qingdao coastal waters

•The carbonate system was affected intensely during the Ulva prolifera bloom.•The calcification in the seawater affected by the Ulva prolifera bloom was enhanced.•The release and distribution of organic acids were controlled by the bloom.•Qingdao coastal waters changed from a sink of atmospheric CO2 to a strong source.