Contrasting responses of phytoplankton productivity between coastal and offshore 1 surface waters in the Taiwan Strait and the South China Sea to short-term seawater 2 acidification

Seawater acidification (SA) has been documented to either inhibit or enhance or result in 15 no effect on marine primary productivity (PP). In order to examine effects of SA in 16 changing environments, we investigated the influences of SA (a decrease of 0.4 pH total 17 units with corresponding CO 2 concentrations ranged 22.0–39.7 µM) on PP through 18 deck-incubation experiments at 101 stations in the Taiwan Strait and the South China Sea, 19 including the continental shelf and slope, as well as deep-water basin. The daily primary 20 productivities in surface seawater under incident solar radiation ranged from 17–306 µg 21 C (µg Chl a ) -1 d -1 , with the responses of PP to SA being region-dependent and the 22 SA-induced changes varying from -88% (inhibition) to 57% (enhancement). The 23 SA-treatment stimulated PP in surface waters of coastal, estuarine and shelf waters, but 24 suppressed it in the South China Sea basin. Such SA-induced changes in PP were 25 significantly related to in situ pH and solar radiation in surface seawater, but negatively 26 related to salinity changes. Our results indicate that phytoplankton cells are more 27 vulnerable to a pH drop in oligotrophic waters. Contrasting responses of phytoplankton 28 productivity in different areas suggest that SA impacts on marine primary productivity 29 are region-dependent and regulated by local environments.

the Taiwan Strait is distinguished by its unique location. In addition to riverine inputs, it 68 also receives nutrients from upwelling (Hong et al., 2011). Primary productivity is much 69 higher in coastal waters than that in basin zones due to increased supply of nutrients 70 through river runoff and upwelling (Chen, 2003;Cloern et al., 2014). The South China 71 Sea, located from the equator to 23.8°N, from 99.1 to 121.1°E and encompassing an area 72 of about 3.5 × 10 6 km 2 , is one of the largest marginal seas in the world. As a marginal sea    The incident solar radiation intensity during the cruises was recorded with an     (Fig. 2f).

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The concentration of Chl a ranged from 0.11 to 12.13 µg L -1 in the Taiwan Strait (Fig.   181 3). The highest concentration occurred in the estuary of the Minjiang River. The  (Fig. 4). Vietnam. In basin zones, the surface primary productivity was usually lower than 100 µg  (Table S1). Carbonate chemistry parameters after 24 h of incubation were stable (△pH < 198 0.06, △TA < 53 µmol kg -1 SW) , indicating the successful manipulation (Table S1). It By analyzing the correlations between SA-induced PP changes and regional 216 environmental parameters (Table S2), we found that SA-induced changes in 217 phytoplankton primary productivity was significantly positively related with in situ pH (p 218 < 0.001, r = 0.379), and PAR density (p = 0.002, r = 0.311) (Fig. 6). On the other hand, 219 the influence induced by SA was negatively related to salinity that ranged from 30.00 to 220 34.28 (p < 0.001, r = -0.418).

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In the present study, we found that the elevated pCO 2 and associated pH drop   The first one is the enrichment of CO 2 , which is usually beneficial for photosynthetic

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In the present study, SA increased or did not affect PP in coastal waters but reduced it 250 in offshore waters, which is significantly related to pH, light intensity and salinity (Fig. 6).

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The effect of SA changed from negative to positive with the increase of local pH. The 252 higher pH occurred in coastal zones which may be caused by higher biomass of 253 14 phytoplankton (Fig. 3). Higher pH caused by intensive photosynthesis of phytoplankton 254 is companied with decreased CO 2 levels. In this case, CO 2 is more limiting for 255 photosynthesis of phytoplankton compared to lower pH. Therefore, SA could stimulate 256 primary productivity via supplying more available CO 2 (Hurd et al., 2019). On the other 257 hand, lower pH occurred in deep-water basin. Lower pH represents higher CO 2 258 availability. CO 2 is not limited or less limited in this case. Therefore, more CO 2 brought 259 by SA may not benefit photosynthesis of phytoplankton. Instead, decreased pH 260 accompanied by SA may inhibit photosynthesis or growth of phytoplankton, which is Sea grown under different CO 2 levels (Wulff et al., 2018). In this study, the negative 287 relationship between salinity and SA effects seems to be an autocorrelation between 288 salinity and in situ pH (Fig. S1) because lower salinity occurred in coastal waters where 289 seawater pH was higher while the basin zone usually had higher salinities and lower pH.  Table S3). Although the effects of SA 312 were not related to temperature as shown in this study (Table S2)