KOSMOS 2017 Peru mesocosm study: overview data@en

Eastern boundary upwelling systems (EBUS) are among the most productive marine ecosystems on Earth. The production of organic material is fueled by upwelling of nutrient-rich deep waters and high incident light at the sea surface. However, biotic and abiotic factors can mod- ify surface production and related biogeochemical processes. Determining these factors is important because EBUS are considered hotspots of climate change, and reliable predic- tions of their future functioning requires understanding of the mechanisms driving the biogeochemical cycles therein. In this field experiment, we used in situ mesocosms as tools to improve our mechanistic understanding of processes con- trolling organic matter cycling in the coastal Peruvian up- welling system. Eight mesocosms, each with a volume of ∼ 55 m3, were deployed for 50 d ∼ 6 km off Callao (12◦ S) during austral summer 2017, coinciding with a coastal El Niño phase. After mesocosm deployment, we collected sub- surface waters at two different locations in the regional oxy- gen minimum zone (OMZ) and injected these into four meso- cosms (mixing ratio ≈ 1.5 : 1 mesocosm: OMZ water). The focus of this paper is on temporal developments of organic matter production, export, and stoichiometry in the indi- vidual mesocosms. The mesocosm phytoplankton commu- nities were initially dominated by diatoms but shifted to- wards a pronounced dominance of the mixotrophic dinoflag- ellate (Akashiwo sanguinea) when inorganic nitrogen was exhausted in surface layers. The community shift coincided with a short-term increase in production during the A. san- guinea bloom, which left a pronounced imprint on organic matter C : N : P stoichiometry. However, C, N, and P export fluxes did not increase because A. sanguinea persisted in the water column and did not sink out during the experiment. Accordingly, export fluxes during the study were decou- pled from surface production and sustained by the remain- ing plankton community. Overall, biogeochemical pools and fluxes were surprisingly constant for most of the experiment. We explain this constancy by light limitation through self- shading by phytoplankton and by inorganic nitrogen limita- tion which constrained phytoplankton growth. Thus, gain and loss processes remained balanced and there were few oppor- tunities for blooms, which represents an event where the sys- tem becomes unbalanced. […]东边界上升流系统（Eastern boundary upwelling systems, EBUS）是地球上生产力最高的海洋生态系统之一。有机质的生产依赖于富含营养盐的深层海水上升流与海表的高入射光照。然而，生物与非生物因子可改变海表生产过程及其相关的生物地球化学过程。明确此类因子至关重要，因为EBUS被视为气候变化的热点区域，而要可靠预测其未来的功能运转，需理解驱动其内生物地球化学循环的机制。 在本次野外实验中，我们利用原位中型实验生态系统（in situ mesocosms）作为研究工具，以深化对秘鲁沿岸上升流系统中调控有机质循环过程的机制性认知。2017年南半球夏季，我们在卡亚俄外海（12°S，距岸约6公里）部署了8个中型实验生态系统，单个体积约55立方米，实验持续50天，该时段与沿海厄尔尼诺事件阶段重合。 中型实验生态系统部署完成后，我们在区域低氧区（oxygen minimum zone, OMZ）的两个不同站位采集次表层海水，并将其注入4个中型实验生态系统中，混合比例约为1.5:1（中型实验生态系统水体：低氧区海水）。本文的研究重点为单个中型实验生态系统中有机质生产、输出及化学计量比的时间动态变化。 中型实验生态系统内的浮游植物群落最初以硅藻为优势类群，但当表层水体的无机氮耗尽后，群落结构向混合营养型甲藻（Akashiwo sanguinea）的显著优势转变。该群落转变与血红哈卡藻（A. sanguinea）水华期间的短期生产提升相吻合，此次水华对有机质的碳:氮:磷化学计量比留下了显著印记。然而，碳、氮、磷的输出通量并未出现增长，这是因为实验期间血红哈卡藻始终留存于水体中，未发生沉降。 据此，本研究期间的输出通量与海表生产解耦，并由剩余的浮游群落维持。总体而言，实验的大部分阶段中，生物地球化学库与通量均保持了出人意料的稳定性。我们将该稳定性归因于浮游植物自身遮蔽造成的光限制，以及抑制浮游植物生长的无机氮限制。因此，物质的增益与损耗过程始终保持平衡，几乎没有水华发生的契机——而水华正是系统失去平衡的典型事件。[...]