Contrasting responses of phytoplankton productivity between coastal and offshore surface waters in the Taiwan Strait and the South China Sea to future CO₂-induced acidification

Future CO2-induced ocean acidification (OA) has been documented to either inhibit or enhance or result in no effect on marine primary productivity (PP). In order to examine effects of OA under multiple drivers, we investigated the influences of OA (a decrease of 0.4 pHtotal units with corresponding CO2 concentrations ranged 22.0–39.7 µM) on PP through deck-incubation experiments at 101 stations in the Taiwan Strait and the South China Sea (SCS), including the coastal zone, the continental shelf and slope, as well as deep-water basin. The daily net 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 OA being region-dependent and the OA-induced changes varying from −88.03 % (inhibition) to 56.87 % (enhancement). The OA-treatment stimulated PP in surface waters of coastal, estuarine and shelf waters, but suppressed it in the South China Sea basin. Such OA-induced changes in PP were significantly related to NOX (the sum of NO3− and NO2−) availability, 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 pH drop in oligotrophic waters. Considering high nutrient and low salinity in coastal waters and reduced nutrient availability in pelagic zones with the progressive stratification associated with ocean warming, our results imply that future OA will enhance PP in coastal waters but decrease it in pelagic oligotrophic zones.