KOSMOS Finland 2012 mesocosm study: Phosphorus pool sizes and uptake

Studies investigating the effect of increasing CO2 levels on the phosphorus cycle in natural waters are lacking although phosphorus often controls phytoplankton development in many aquatic systems. The aim of our study was to analyse effects of elevated CO2 levels on phosphorus pool sizes and uptake. The phosphorus dynamic was followed in a CO2-manipulation mesocosm experiment in the Storfjärden (western Gulf of Finland, Baltic Sea) in summer 2012 and was also studied in the surrounding fjord water. In all mesocosms as well as in surface waters of Storfjärden, dissolved organic phosphorus (DOP) concentrations of 0.26 ± 0.03 and 0.23 ± 0.04 µmol/l, respectively, formed the main fraction of the total P-pool (TP), whereas phosphate (PO4) constituted the lowest fraction with mean concentration of 0.15 ± 0.02 in the mesocosms and 0.17 ± 0.07 µmol/l in the fjord. Transformation of PO4 into DOP appeared to be the main pathway of PO4 turnover. About 82% of PO4 was converted into DOP whereby only 18% of PO4 was transformed into particulate phosphorus (PP). PO4 uptake rates measured in the mesocosms ranged between 0.6 and 3.9 nmol/l/h. About 86% of them was realized by the size fraction < 3 µm. Adenosine triphosphate (ATP) uptake revealed that additional P was supplied from organic compounds accounting for 25-27% of P provided by PO4 only. CO2 additions did not cause significant changes in phosphorus (P) pool sizes, DOP composition, and uptake of PO4 and ATP when the whole study period was taken into account. However, significant short-term effects were observed for PO4 and PP pool sizes in CO2 treatments > 1000 µatm during periods when phytoplankton biomass increased. In addition, we found significant relationships (e.g., between PP and Chl a) in the untreated mesocosms which were not observed under high fCO2 conditions. Consequently, it can be hypothesized that the relationship between PP formation and phytoplankton growth changed with CO2 elevation. It can be deduced from the results, that visible effects of CO2 on P pools are coupled to phytoplankton growth when the transformation of PO4 into POP was stimulated. The transformation of PO4 into DOP on the other hand does not seem to be affected. Additionally, there were some indications that cellular mechanisms of P regulation might be modified under CO2 elevation changing the relationship between cellular constituents.

尽管在众多水生生态系统中，磷通常是浮游植物生长发育的主控因子，但目前针对大气CO₂浓度升高对天然水体磷循环影响的研究仍较为匮乏。本研究旨在探讨CO₂浓度升高对磷库规模及磷摄取过程的影响。2012年夏季，研究团队在波罗的海芬兰湾西部的斯托尔菲厄登（Storfjärden）开展了CO₂调控中型实验生态系（mesocosm）实验，追踪了其中的磷动态变化，并同时对周边峡湾水体的磷动态进行了观测。在所有中型实验生态系及斯托尔菲厄登表层水体中，溶解态有机磷（dissolved organic phosphorus, DOP）浓度分别为0.26±0.03 μmol/L与0.23±0.04 μmol/L，是总磷库（total P-pool, TP）的主要组成组分；而正磷酸盐（phosphate, PO₄）占比最低，中型实验生态系中正磷酸盐平均浓度为0.15±0.02 μmol/L，峡湾水体中为0.17±0.07 μmol/L。正磷酸盐向溶解态有机磷的转化被认为是正磷酸盐周转的主要途径：约82%的正磷酸盐被转化为溶解态有机磷，仅18%转化为颗粒态磷（particulate phosphorus, PP）。中型实验生态系中测得的正磷酸盐摄取速率介于0.6~3.9 nmol/(L·h)，其中约86%的摄取量由粒径<3 μm的生物组分完成。三磷酸腺苷（adenosine triphosphate, ATP）摄取实验结果显示，有机化合物可额外提供磷源，其贡献量约为仅由正磷酸盐提供磷源的25%~27%。若以整个研究周期为考察范围，CO₂添加并未对总磷库规模、溶解态有机磷组成以及正磷酸盐与三磷酸腺苷的摄取过程造成显著影响。但在浮游植物生物量升高的阶段，当CO₂处理浓度高于1000 μatm时，观测到正磷酸盐与颗粒态磷库规模出现了显著的短期变化。此外，在未施加CO₂处理的中型实验生态系中，观测到颗粒态磷与叶绿素a（Chl a）等显著相关关系，但在高CO₂分压（fCO₂）条件下并未出现此类关联。据此可提出假说：颗粒态磷的形成与浮游植物生长之间的关联会随CO₂浓度升高发生改变。由此可推导得出，当正磷酸盐向颗粒态有机磷（particulate organic phosphorus, POP）的转化过程被促进时，CO₂对磷库的显著影响与浮游植物生长过程相耦合；而正磷酸盐向溶解态有机磷的转化过程则似乎不受CO₂浓度升高的影响。此外，有迹象表明，CO₂浓度升高可能会改变细胞内的磷调控机制，进而改变细胞组分间的关联关系。