Algal pigment concentrations, High Arctic, August-September 2018

This project was conducted as a part of a United States-Swedish Joint Arctic Research Initiative. The goal of this initiative involved mooring the Icebreaker (IB) Oden to an ice floe in the inner pack ice in the high Arctic Ocean, and monitoring key oceanic-atmospheric parameters as the ice drifts. The cruise timeline (August through September) was chosen to highlight the transitional time period from the summer maximum in microbial biomass to declining stocks as autumn conditions result in lower nutrient and light levels, concomitant with the onset of freezing conditions. Biogenic aerosol production and fluxes are key research parameters in understanding the formation of Cloud Condensation Nuclei (CCN) and their impacts on the radiation budget of the Arctic Ocean. At present, there exists a paucity of data regarding how microbial community composition might change in the high Arctic Ocean, especially with respect to changes in the production of volatile aerosol precursor compounds as pelagic microbial communities replace sympagic communities. Specifically, this project focused on linking microbial community structure with the oceanic-atmosphere fluxes of Volatile Organic Carbon compounds (VOCs) emitted from various oceanic and pack ice ecosystems. The role of diminishing sea ice cover in the Arctic Ocean will significantly impact biogenic aerosol production and fluxes via changes in microbial community structure and the release of VOCs. At present, however, the scarcity of in-situ oceanic VOC measurements available from the high Arctic Ocean prevents the development of robust models correlating phytoplankton biomass with VOCs and their impact on aerosol production. For instance, most current models utilize satellite Chlorophyll a (Chla) imagery for estimating phytoplankton biomass (e.g. Gabric et al. 2014; Becagli et al. 2016). It is also well recognized that high concentrations of sea surface chromophoric dissolved organic matter (CDOM) can significantly bias remotely-sensed Chla concentrations, especially when Chla levels are <0.5 milograms per meter cubed (mg m-3) (Matsuoka et al. 2017). In addition to the bias in estimating in-situ phytoplankton biomass from satellite-derived Chla, the contribution made by oceanic VOC fluxes to the atmospheric aerosol optical depth (e.g. Gabric et al., 2002) is unknown. Moreover, incorrect estimates of the oceanic Mixed Layer Depth (MLD) using climatological datasets and/or subsurface Chla maximum can further compound the errors associated with attempting to correlate phytoplankton integrated water column production with estimates of biomass derived using satellite Chla algorithms (Arrigo et al., 2011). As a result, questions remain regarding the reliability of using Chla estimates as a surrogate to estimate the organic carbon enrichment in submicron marine aerosols (Rinaldi et al. 2013). Hence, models that use satellites over relatively large areal expanses in the Arctic may be biased with regards to estimates of biomass, net primary production and as a result correlations to biogenic aerosol (Arrigo et al., 2011, Becagli et al., 2016). More importantly, however, total Chla biomass is not the only important variable affecting the production of oceanic biogenic VOCs and aerosols. The microbial community composition and physiology will not only affect the cell-specific production rate of precursor biogenic aerosol compounds, but also the secondary transformations of those compounds. Furthermore, determining VOCs or phytoplankton functional groups from space are both fraught with even more difficulty than Chla estimates alone. Consequently, at present, virtually no data exists regarding the suite of VOCs released to the high Arctic atmosphere as a function of the in-situ microbial community composition. The included dataset contains algal pigment concentrations found in samples collected during this expedition. Not only do these data provide estimates of phytoplankton community biomass, they also begin to shed light on the phytoplankton community composition, as taxonomic groups can be identified (or at least estimated) by the presence of a few indicating pigments. Because most of these samples were collected in the same location, but over time, we begin to gain insight into the shift of the phytoplankton community composition from the summer maximal biomass (August) to the declining community as autumn conditions result in lower nutrient and light levels and increased ice cover (September). Over the course of this expedition, CTD casts were made to collect samples from 5-200 meters , to adequately sample the photic zone. Casts were made at the marginal ice zone, as well as on an almost-daily basis at the mooring location in the high Arctic Ocean.

本项目隶属于美-瑞典联合北极研究计划（United States-Swedish Joint Arctic Research Initiative）。该计划的目标是将破冰船（Icebreaker, IB）奥登号（Oden）锚定在北极高纬度海域密集浮冰区内的浮冰上，并在浮冰漂移过程中监测关键海-气参数。本次科考航次的时间窗口选定为8月至9月，旨在覆盖微生物生物量从夏季峰值逐步下降的过渡阶段——此时秋季来临，营养盐与光照水平降低，同时海冰开始冻结。生物源气溶胶的生成与通量是研究云凝结核（Cloud Condensation Nuclei, CCN）形成及其对北极高纬度海域辐射收支影响的核心参数。目前，针对北极高纬度海域微生物群落组成的变化规律，尤其是当浮游微生物群落取代冰栖微生物群落时，挥发性气溶胶前体物生成量的变化，相关数据仍十分匮乏。具体而言，本项目旨在将微生物群落结构与各类海洋及浮冰生态系统排放的挥发性有机碳（Volatile Organic Carbon, VOCs）海-气通量建立关联。北冰洋海冰覆盖面积缩减将通过改变微生物群落结构以及VOCs的释放过程，显著影响生物源气溶胶的生成与通量。然而，目前北极高纬度海域缺乏原位海洋VOC实测数据，这阻碍了能够关联浮游植物生物量与VOCs及其对气溶胶生成影响的可靠模型的构建。例如，当前多数模型借助卫星叶绿素a（Chlorophyll a, Chla）遥感影像估算浮游植物生物量（如Gabric等人2014年、Becagli等人2016年的研究）。同时学界已达成共识：海表面有色溶解有机物（chromophoric dissolved organic matter, CDOM）浓度过高会显著干扰遥感反演的Chla浓度，尤其当Chla浓度低于0.5毫克每立方米（mg·m⁻³）时（Matsuoka等人2017年）。除了通过卫星反演Chla估算原位浮游植物生物量存在偏差之外，海洋VOC通量对大气气溶胶光学厚度的贡献（如Gabric等人2002年的研究）目前仍不明确。此外，若使用气候学数据集或次表层Chla峰值估算海洋混合层深度（Mixed Layer Depth, MLD）出现偏差，会进一步加剧将浮游植物水柱综合生产量与基于卫星Chla反演算法得到的生物量估算值相关联时产生的误差（Arrigo等人2011年）。因此，使用Chla估算值作为替代指标来评估亚微米海洋气溶胶中的有机碳富集程度，其可靠性仍存在争议（Rinaldi等人2013年）。因此，在北极较大范围海域使用卫星数据的模型，其生物量、净初级生产力估算结果以及由此得到的与生物源气溶胶的关联关系可能存在偏差（Arrigo等人2011年、Becagli等人2016年）。但更关键的是，Chla总生物量并非影响海洋生物源VOCs和气溶胶生成的唯一重要变量。微生物群落组成与生理特性不仅会调控前体生物源气溶胶化合物的细胞特异性生成速率，还会影响这些化合物的次级转化过程。此外，从太空遥感识别VOCs或浮游植物功能群，其难度远高于单独估算Chla。因此，目前几乎没有关于北极高纬度海域释放的VOCs组成随原位微生物群落组成变化规律的实测数据。本数据集包含本次科考采集样本中的藻类色素浓度数据。此类数据不仅可用于估算浮游植物群落生物量，还能通过部分特征色素的存在情况识别（至少是估算）浮游植物的分类类群，从而揭示浮游植物群落组成。由于多数样本采集自同一站位且覆盖了完整航次周期，我们得以解析浮游植物群落组成的演变过程：从8月夏季生物量峰值状态，逐步过渡到9月因秋季营养盐与光照水平下降、海冰覆盖增加而出现的群落衰退状态。本航次期间，为充分覆盖真光层，我们通过温盐深仪（Conductivity-Temperature-Depth, CTD）采水获取了5至200米水深的样本。采水站位覆盖了边缘冰区，且在北极高纬度锚定站位几乎每日开展采水作业。