Collaborative Research: Potential Climate Change Impacts on Algal Dynamics and Biogeochemistry in the Bering Sea

海洋学家认识到浮游植物光合作用在影响大气二氧化碳浓度和全球气候变化方面的重要性。然而，几乎没有人注意到相反的问题：不断上升的人为二氧化碳水平和相关的气候变化将如何影响藻类群落结构和生物地球化学？与气候有关的浮游植物组合的变化可能对海洋对大气的反馈以及人类对海洋资源的利用产生深远的影响。这是因为特定的藻类群在很大程度上负责垂直碳输出、生物钙化、产生二甲基硫化物(DMS)等气候活性气体等关键过程，以及支持导致经济价值更高的营养水平的食物网。二氧化碳和温度的增加，以及浅水化混合层和更强的温跃层提供的光和养分的变化，可能会导致藻类多样性的主要变化。这些优势变化包括硅质浮游植物、钙质浮游植物和非矿化浮游植物之间的戏剧性变化，对生物泵有主要或次要贡献的类群之间，以及产生微不足道或大量DMS的类群之间。PIS正在调查二氧化碳和气候变化对白令海藻类群落结构和生物地球化学的潜在影响，该地区已经显示出气候制度迅速变化的迹象。正在使用两种独立的实验方法：半连续孵化法，用于模拟风暴等事件引起的非平衡条件，以及独特的船上恒化器系统，用于模拟准稳态分层条件。实验处理包括：(1)环境二氧化碳和温度条件，(2)升高二氧化碳(750ppm)和温度(+30℃)。每个处理被细分为两个混合层深度模拟：(1)环境营养和光照条件和(2)高光/低营养处理，以模拟未来MLD下降。所选择的二氧化碳水平和物理变量是根据目前对2100年表层海洋的模型预测得出的。在这些温室海洋模拟中，浮游植物多样性以及碳和养分循环受到密切关注，特别关注硅藻、球藻和棕囊藻等生物地球化学关键类群之间的变化。

Oceanographers recognize the importance of phytoplankton photosynthesis in affecting atmospheric pCO2 and global climate change. Little attention has been given, however, to the opposite question: How will rising anthropogenic pCO2 levels and associated climate changes impact algal community structure and biogeochemistry? Climate-related shifts in phytoplankton assemblages may have profound implications for oceanic feedbacks on the atmosphere, and for human use of marine resources. This is because particular algal groups are largely responsible for crucial processes like vertical carbon export, biogenic calcification, production of climatically active gases like dimethylsulfide (DMS), and for supporting food webs that lead to economically valuable higher trophic levels. Increasing pCO2 and temperature, as well as changes in light and nutrient supplies from a shoaling mixed layer and a stronger thermocline, can drive major switches in algal diversity. These dominance changes include dramatic shifts between siliceous, calcareous, and non-mineralizing phytoplankton, between groups that are major or minor contributors to the biological pump, and between taxa that produce negligible or copious amounts of DMS. The PIs are investigating the potential impacts of pCO2 and climate change on algal community structure and biogeochemistry in the Bering Sea, an area that already exhibits signs of rapid climate regime shifts. Two independent experimental methods are being used: Semi-continuous incubations, to simulate non-equilibrium conditions due to episodic events such as storms, and a unique shipboard chemostat system to simulate quasi-steady state stratified conditions. Experimental treatments include: (1) Ambient pCO2 and temperature conditions, and (2) Elevated pCO2 (750 ppm) and temperature (+ 3o C). Each treatment is subdivided into two mixed layer depth simulations: (1) Ambient nutrient and light conditions and (2) A higher light/lower nutrient treatment to simulate a future decreased MLD. Levels of pCO2 and physical variables chosen were based on current model predictions for the surface ocean by the year 2100. Phytoplankton diversity and carbon and nutrient cycling are being closely followed in these greenhouse ocean simulations, with particular attention to shifts between biogeochemically critical groups like diatoms, coccolithophorids, and Phaeocystis.