Contrasting responses of phytoplankton productivity between coastal and offshore surface waters in the Taiwan Strait and the South China Sea to short-term seawater acidification

Abstract. Seawater acidification (SA) has been documented to either inhibit, enhance, or result in no effect on marine primary productivity (PP). In order to examine the effects of SA in changing environments, we investigated the influences of SA (a decrease of 0.4 pHtotal units with corresponding CO2 concentrations in the range of 22.0–39.7 µM) on PP through deck-incubation experiments at 101 stations in the Taiwan Strait and the South China Sea, including the continental shelf and slope, as well as the deep-water basin. The daily primary productivities in surface seawater under incident solar radiation ranged from 17–306 µg C (µg Chl a)−1 d−1, with the responses of PP to SA being region-dependent and the SA-induced changes varying from −88 % (inhibition) to 57 % (enhancement). The SA treatment stimulated PP in surface waters of coastal, estuarine, and shelf waters but suppressed it in the South China Sea basin. Such SA-induced changes in PP were significantly related to in situ pH and solar radiation in surface seawater but negatively related to salinity changes. Our results indicate that phytoplankton cells are more vulnerable to a pH drop in oligotrophic waters. Contrasting responses of phytoplankton productivity in different areas suggest that SA impacts on marine primary productivity are region-dependent and regulated by local environments.