Seawater carbonate chemistry and net photosynthesis of seagrass Zostera marina in a brackish water environment

Seagrasses are distributed across the globe and their communities may play key roles in the coastal ecosystems. Seagrass meadows are expected to benefit from the increased carbon availability which might be used in photosynthesis in a future high CO2 world. The main aim of this study was to examine the effect of elevated pCO2 on the net photosynthesis of seagrass Zostera marina in a brackish water environment. The short-term mesocosm experiments were conducted in Kõiguste Bay (northern part of Gulf of Riga, the Baltic Sea) in June-July 2013 and 2014. As the levels of pCO2 naturally range from ca. 150 µatm to well above 1000 µatm under summer conditions in Kõiguste Bay we chose to operate in mesocosms with the pCO2 levels of ca. 2000, ca. 1000, and ca. 200 µatm. Additionally, in 2014 the photosynthesis of Z. marina was measured outside of the mesocosm in the natural conditions. In the shallow coastal Baltic Sea seagrass Z. marina lives in a highly variable environment due to seasonality and rapid changes in meteorological conditions. This was demonstrated by the remarkable differences in water temperatures between experimental years of ca. 8°C. Thus, the current study also investigated the effect of elevated pCO2 in combination with short-term natural fluctuations of environmental factors, i.e., temperature and PAR on the photosynthesis of Z. marina. Our results show that elevated pCO2 alone did not enhance the photosynthesis of the seagrass. The photosynthetic response of Z. marina to CO2 enrichment was affected by changes in water temperature and light availability.海草（Seagrasses）广泛分布于全球各海域，其群落对海岸生态系统可能具有关键支撑作用。海草床（Seagrass meadows）有望从碳供给提升中获益——在未来高二氧化碳（CO₂）环境中，额外的碳可用于光合作用过程。本研究的核心目标为探究咸淡水环境中，升高的pCO₂对大叶藻（Zostera marina）净光合速率的影响。2013年与2014年的6月至7月，研究团队于波罗的海里加湾北部的科伊古斯特湾（Kõiguste Bay）开展了短期中型实验生态系统（mesocosm）试验。科伊古斯特湾夏季自然pCO₂水平大致介于150 µatm至远超1000 µatm之间，因此本试验设置了约2000 µatm、约1000 µatm与约200 µatm三个pCO₂梯度的中型实验生态系统组别。此外，2014年研究人员还在中型实验生态系统外的自然生境中，测定了大叶藻的光合速率。波罗的海浅海区域的大叶藻（Z. marina）生存环境兼具季节波动性与气象条件快速变化的特征，环境异质性极强。2013与2014年实验周期内的水温差异可达约8℃，这一数据也印证了该海域环境的不稳定性。因此，本研究同时考察了升高的pCO₂与温度、光合有效辐射（Photosynthetically Active Radiation, PAR）等环境因子的短期自然波动共同作用下，对大叶藻光合速率的影响。研究结果显示，单独升高pCO₂并未提升该海草的光合速率；大叶藻对二氧化碳富集的光合响应，会受到水温与光照供给条件变化的调控。