Dataset (2003-2018) for southeast Bering Sea used for analyses in the manuscript: Phytoplankton communities in the Bering Sea respond to environmental shifts, with implications for consumer species

Phytoplankton community structure changes in response to environmental conditions, potentially impacting zooplankton through altered trophic interactions. In the Bering Sea, recent stanzas of warmer and colder years coincided with changes in community structure of zooplankton and fish. However, attempts to link these trends to primary producers based on patterns in total chlorophyll a concentration (Chl) have not been successful. Using Chl as a proxy for food availability ignores potential consequences of community shifts in phytoplankton, which span a vast range of cell shapes, sizes, and biochemical composition that do not contribute equally to the diet of primary consumers. We analyzed size and taxonomic composition of phytoplankton with environmental variables from 2003 – 2018, and examined relationships to abundance of two copepods (Calanus and Pseudocalanus) important to age-0 pollock diets. Three distinct environmental stanzas were identified: Warm1 (2003-2005) with high temperature and salinity and low nutrients favoring dominance of flagellates, ciliates, and dinoflagellates, and low abundances of Calanus copepods; Cold (2006-2012) with low temperature and salinity and high nutrients favoring the cryptophyte Rhodomonas and small centric diatoms, and high abundance of Calanus copepods; and Warm2 (2014-2018) with high temperature, salinity, and nutrients (including high SiO4), supporting high abundance of elongated rod-like diatoms, and low abundance of Calanus copepods. Calanus abundance was significantly negatively correlated with biomass of large (>10 µm) phytoplankton, and the taxa Leptocylindrus, Thalassionema, Pseudo-nitzschia, and Dictyocha, and positively correlated with Rhodomonas and small disc diatoms (<10 µm), highlighting links between phytoplankton and zooplankton communities in response to stanzas.

浮游植物（Phytoplankton）群落结构随环境条件变化而发生改变，并可通过重塑营养级相互作用对浮游动物（zooplankton）产生潜在影响。在白令海（Bering Sea），近年来的冷暖年份环境特征时段分别对应了浮游动物与鱼类群落结构的变化。然而，基于总叶绿素a浓度（Chl）的变化趋势，尝试将上述现象与初级生产者（primary producers）建立关联的研究均未获得成功。以Chl作为食物可获得性的替代指标（proxy），会忽略浮游植物群落结构转变的潜在影响——浮游植物的细胞形态、尺寸与生化组成跨度极大，且对初级消费者的食源贡献并不均等。本研究结合2003年至2018年的环境变量，分析了浮游植物的尺寸与分类学组成（taxonomic composition），并探究了其与两种桡足类（copepods）——对0龄狭鳕（age-0 pollock）食性至关重要的哲水蚤属（Calanus）和伪哲水蚤属（Pseudocalanus）——丰度的关联关系。研究共识别出三类特征鲜明的环境特征时段：暖期1（2003-2005年）：水温与盐度偏高、营养盐浓度偏低，利于鞭毛虫（flagellates）、纤毛虫（ciliates）和甲藻（dinoflagellates）占据优势，同时哲水蚤属丰度较低；冷期（2006-2012年）：水温与盐度偏低、营养盐浓度偏高，利于隐藻门（cryptophyte）的红胞藻属（Rhodomonas）和小型中心硅藻（centric diatoms）占据优势，同时哲水蚤属丰度较高；暖期2（2014-2018年）：水温、盐度与营养盐（包括高浓度硅酸盐SiO4）均偏高，利于细长杆状硅藻（elongated rod-like diatoms）占据优势，同时哲水蚤属丰度较低。哲水蚤属丰度与大型（>10 μm）浮游植物及细柱藻属（Leptocylindrus）、海链藻属（Thalassionema）、伪菱形藻属（Pseudo-nitzschia）和双鞭藻属（Dictyocha）的生物量呈显著负相关，而与红胞藻属及小型圆盘硅藻（<10 μm）的生物量呈显著正相关，这凸显了浮游植物与浮游动物群落在响应环境特征时段时的关联关系。