Seawater carbonate chemistry and species composition, sinking rate of coastal phytoplankton assemblage@en

In addition to ocean acidification, a significant recent warming trend in Chinese coastal waters has received much attention. However, studies of the combined effects of warming and acidification on natural coastal phytoplankton assemblages here are scarce. We conducted a continuous incubation experiment with a natural spring phytoplankton assemblage collected from the Bohai Sea near Tianjin. Experimental treatments used a full factorial combination of temperature (7 and 11°C) and pCO2 (400 and 800 ppm) treatments. Results suggest that changes in pCO2 and temperature had both individual and interactive effects on phytoplankton species composition and elemental stoichiometry. Warming mainly favored the accumulation of picoplankton and dinoflagellate biomass. Increased pCO2 significantly increased particulate organic carbon to particulate organic phosphorus (C:P) and particulate organic carbon to biogenic silica (C:BSi) ratios, and decreased total diatom abundance; in the meanwhile, higher pCO2 significantly increased the ratio of centric to pennate diatom abundance. Warming and increased pCO2 both greatly decreased the proportion of diatoms to dinoflagellates. The highest chlorophyll a biomass was observed in the high pCO2, high temperature phytoplankton assemblage, which also had the slowest sinking rate of all treatments. Overall, there were significant interactive effects of increased pCO2 and warming on dinoflagellate abundance, pennate diatom abundance, diatom vs. dinoflagellates ratio and the centric vs. pennate ratio. These findings suggest that future ocean acidification and warming trends may individually and cumulatively affect coastal biogeochemistry and carbon fluxes through shifts in phytoplankton species composition and sinking rates.除海洋酸化外，中国近海水域近期出现的显著增温趋势也受到广泛关注。然而，针对增温与酸化对中国近海自然浮游植物群落的联合影响的研究仍较为匮乏。本研究对采自天津附近渤海的春季自然浮游植物群落开展了连续培养实验。实验采用温度（7℃与11℃）与二氧化碳分压（pCO₂）400ppm和800ppm的全因子交叉处理设计。研究结果显示，pCO₂与温度变化对浮游植物物种组成及元素化学计量比均存在单独影响与交互影响。增温主要促进了超微型浮游植物（picoplankton）和甲藻（dinoflagellate）生物量的积累。升高的pCO₂显著提升了颗粒有机碳-颗粒有机磷（C:P）以及颗粒有机碳-生物硅（C:BSi）比值，同时降低了硅藻总丰度；此外，更高的pCO₂显著提高了中心硅藻（centric diatom）与羽纹硅藻（pennate diatom）的丰度比值。增温与升高的pCO₂均大幅降低了硅藻相对于甲藻的占比。在高pCO₂、高温的浮游植物群落中，叶绿素a（chlorophyll a）生物量达到最高，同时其沉降速率为所有处理组中最低。总体而言，升高的pCO₂与增温对甲藻丰度、羽纹硅藻丰度、硅藻与甲藻的丰度比值以及中心硅藻与羽纹硅藻的丰度比值均存在显著的交互影响。本研究结果表明，未来海洋酸化与增温趋势可能通过改变浮游植物物种组成及沉降速率，单独或协同影响近海生物地球化学循环与碳通量。