Data_Sheet_1_Responses of a Natural Phytoplankton Community From the Drake Passage to Two Predicted Climate Change Scenarios.docx

Contrasting models predict two different climate change scenarios for the Southern Ocean (SO), forecasting either less or stronger vertical mixing of the water column. To investigate the responses of SO phytoplankton to these future conditions, we sampled a natural diatom dominated (63%) community from today’s relatively moderately mixed Drake Passage waters with both low availabilities of iron (Fe) and light. The phytoplankton community was then incubated at these ambient open ocean conditions (low Fe and low light, moderate mixing treatment), representing a control treatment. In addition, the phytoplankton was grown under two future mixing scenarios based on current climate model predictions. Mixing was simulated by changes in light and Fe availabilities. The two future scenarios consisted of a low mixing scenario (low Fe and higher light) and a strong mixing scenario (high Fe and low light). In addition, communities of each mixing scenario were exposed to ambient and low pH, the latter simulating ocean acidification (OA). The effects of the scenarios on particulate organic carbon (POC) production, trace metal to carbon ratios, photophysiology and the relative numerical contribution of diatoms and nanoflagellates were assessed. During the first growth phase, at ambient pH both future mixing scenarios promoted the numerical abundance of diatoms (∼75%) relative to nanoflagellates. This positive effect, however, vanished in response to OA in the communities of both future mixing scenarios (∼65%), with different effects for their productivity. At the end of the experiment, diatoms remained numerically the most abundant phytoplankton group across all treatments (∼80%). In addition, POC production was increased in the two future mixing scenarios under OA. Overall, this study suggests a continued numerical dominance of diatoms as well as higher carbon fixation in response to both future mixing scenarios under OA, irrespective of different changes in light and Fe availability.

本研究对比模型预测了南大洋（SO）的两种不同的气候变化情景，预测水柱垂直混合作用将减弱或增强。为探究SO浮游植物对这些未来条件的响应，我们从今日相对中等混合的德雷克海峡水域中，采集了以自然硅藻占主导地位的（63%）群落，并在此环境下（低铁和低光，中等混合处理）进行培养，以此作为对照组。此外，基于当前气候模型预测，浮游植物在两种未来混合情景下进行培养。混合作用通过光和铁的可用性变化来模拟。两种未来情景分别为低混合情景（低铁和较高光）和高混合情景（高铁和低光）。此外，每个混合情景的群落均暴露于环境条件和低pH值环境中，后者模拟海洋酸化（OA）。评估了这些情景对颗粒有机碳（POC）生产、痕量金属与碳的比率、光生理学以及硅藻和纳米鞭毛虫的相对数量贡献的影响。在第一生长阶段，在环境pH值条件下，两种未来混合情景均促进了硅藻（约75%）相对于纳米鞭毛虫的数量增长。然而，这种正面效应在两种未来混合情景的群落中（约65%）对海洋酸化的响应中消失，其生产力也表现出不同的影响。实验结束时，硅藻在所有处理中仍然是数量最丰富的浮游植物群体（约80%）。此外，在海洋酸化条件下，两种未来混合情景均增加了POC的生产。总体而言，本研究表明，无论光和铁的可用性如何变化，硅藻将继续保持数量上的主导地位，并且对两种未来混合情景以及海洋酸化条件下的碳固定能力都有所提高。