Particle size spectra and plankton community size structure: Response to elevated CO2 during an in situ mesocosm experiment

In this study, we report from a long-term in situ mesocosm experiment, where we investigated the response of natural plankton communities in temperate waters (Gullmarfjord, Sweden) to elevated CO2 concentrations and OA as expected for the end of the century (~760 µatm pCO2). Based on a plankton-imaging approach, we examined size structure, community composition and food web characteristics of the whole plankton assemblage, ranging from picoplankton to mesozooplankton, during an entire winter-to-summer succession. The plankton imaging system revealed pronounced temporal changes in the size structure of the copepod community over the course of the plankton bloom. These observed shift towards smaller individuals resulted in an overall decrease of copepod biomass by 25%, despite increasing numerical abundances. Furthermore, we observed distinct effects of elevated CO2 on biomass and size structure of the entire plankton community. Notably, the biomass of copepods, dominated by Pseudocalanus acuspes, displayed a tendency towards elevated biomass by up to 30-40% under simulated ocean acidification. This effect was significant for certain copepod size classes and was most likely driven by CO2-stimulated responses of primary producers and a complex interplay of trophic interactions that allowed this CO2 effect to propagate up the food web.

本研究报道了一项长期原位（in situ）中型实验生态系统（mesocosm）实验：针对瑞典古尔马尔峡湾（Gullmarfjord）温带海域的天然浮游生物群落，探究其对本世纪末预计升高的二氧化碳浓度与海洋酸化（Ocean Acidification，OA）的响应，实验设置的二氧化碳分压约为760 µatm pCO₂。
本研究采用浮游生物成像技术，对从超微型浮游生物（picoplankton）到中型浮游动物（mesozooplankton）的全尺度浮游生物集合体的体型结构、群落组成与食物网特征展开分析，观测周期覆盖完整的冬-夏群落演替过程。
浮游生物成像系统观测到，在浮游生物旺发期间，桡足类（copepod）群落的体型结构出现显著的时间动态变化。尽管桡足类的个体数量丰度有所上升，但观测到的体型向小型个体的转变，使得桡足类总生物量整体下降了25%。
此外，我们还观测到二氧化碳浓度升高对整个浮游生物群落的生物量与体型结构存在显著影响。值得注意的是，在模拟海洋酸化条件下，以尖伪镖水蚤（Pseudocalanus acuspes）为优势类群的桡足类生物量呈现出最高提升30%~40%的增长趋势。该效应在特定桡足类体型组中表现显著，其驱动机制大概率为：二氧化碳浓度升高对初级生产者的刺激作用，以及复杂的营养级相互作用共同促使该二氧化碳效应沿食物网向上传递。