MEPS 605:73-86 (2018)  -  DOI: https://doi.org/10.3354/meps12739

ABSTRACT: This study investigated the effects of a bloom of the macroalga Ulva prolifera on the carbonate system in the Yellow Sea. Two cruises were carried out in the coastal waters of Qingdao, one during U. prolifera late bloom stages in 2015 and one after the bloom. In parallel, an 8 d incubation experiment was conducted to determine the variation in the seawater carbonate system during the late bloom period. During the late bloom period, pH was decreased by 0.06 units, while the concentrations of dissolved inorganic carbon (DIC), total alkalinity (TA) and the values of partial pressure of CO₂ ( pCO₂) were increased by 27 µmol kg^-1, 57 µmol kg^-1 and 77 µatm in surface waters compared with those after the bloom, respectively. The mean (±SD) air-water CO₂ flux during the first cruise was estimated to be 0.82 ± 0.88 mmol m^-2 d^-1, while it declined to -1.31 ± 2.41 mmol m^-2 d^-1 during the second cruise. During both cruises, the pH decreased with depth, and this trend was reversed for DIC, whereas small variations in TA were detected throughout the water column. Comparing final data of the incubation experiment with initial values showed that pH was reduced by 0.23 ± 0.01 units, while DIC and TA increased on average by 71 ± 37 and 37 ± 18 µmol kg^-1, respectively. The release of CO₂ from U. prolifera occurred rapidly during the late bloom period, which resulted in conversion of Qingdao coastal waters from a sink to a weak CO₂ source.

KEY WORDS: Ulva prolifera · Carbonate system · Southern Yellow Sea · Late bloom period