Ocean Acidification

Highlights

• Continuous, massive green tides have occurred in the Yellow Sea over the past decade (2007–2018).
• This study integrates remote sensing, field observation, laboratory measurements and indoor cultivation.
• Ulva prolifera blooming is influenced by higher concentrations of Fe(II) and HCO3-, and a lower pH.

Abstract

Over the past decade, massive outbreaks of Ulva prolifera have occurred in the Yellow Sea, China, and caused negative effects to the coastal environments. In response, many scientific investigations have been conducted to ascertain the origins of and reasons for the algal bloom that has resulted in continuous green tides. In this work, we explored the influences of iron and dissolved inorganic carbon (DIC) on the occurrence of green algal blooms. The moderate-resolution imaging spectroradiometer (MODIS) data showed the blooming areas and movement of U. prolifera. Field observation showed that higher Fe(II) concentrations (average 0.145 mg L−1) can be correlated with large Ulva prolifera blooms. Furthermore, lower pH might enhance the accumulation of dissolved carbon into the green algae; a premise that was supported by higher concentrations of CO2(0.037 mmol L−1), HCO3−(2.58 mmol L−1) and the lowest pH value (7.69) being found together at site H11. The indoor iron- and bicarbonate-enrichment experiments further confirmed that higher concentrations of Fe(II) and HCO3− and a lower pH can increase the growth rate of U. prolifera. This study indicates that seawater chemical factors contribute to the long term, ongoing green tides in the Yellow Sea and provides new thoughts for the causes of U. prolifera blooms.