罗斯海浮游生物群落生产力的季节性研究

The Ross Sea plays a critical role in Antarctic bottom water formation, marine living resources and carbon sequestration. Studies and understandings of the seasonal patterns of phytoplankton biomass and productivity have been largely generated through the application of satellite data, that is, the primary phytoplankton blooms occur in the austral spring dominated by Phaeocystis antarctica and crash in the summer due to iron limitation. Most of P. antarctica biomass is recycled within the mixed layer without being transferred into higher trophic levels or settling into depth. However, recent data collected by autonomous vehicles at high spatial and temporal resolutions over a long duration discovered a repeated accumulation of phytoplankton biomass in summer and late summer that is undetected by satellites because of increased carbon:chlorophyll ratios. This discovery is confirmed by the chlorophyll and particulate organic carbon (POC) data compiled from 40 cruises to the Ross Sea. From the seasonal composite (climatology) generated by these data, the derived POC:Chl ratio also showed a strong increase in summer that was largely independent of chlorophyll. We hypothesize that the late summer POC increase was due to growth and accumulation of diatoms which exhibited enhanced POC:Chl ratios due to iron limitation. We propose to conduct experiments in the Ross Sea to test the hypothesis that diatoms grow under iron-limited conditions and reach substantial accumulations in late summer exhibiting elevated POC:Chl ratios onboard RV Xuelong. We will also investigate the physiological mechanisms that lead to this rapid growth of diatom under iron limitation. We suggest that late summer blooms may be a significant contributor to seasonal productivity, and that estimates of productivity based on satellite pigments have substantial uncertainties in the Ross Sea because these summer diatom blooms are largely undetected by ocean color studies. We will reassess bio-optical productivity models using our experimentally derived POC:Chl ratios to provide a greatly improved estimate of seasonal and annual productivity in the Ross Sea. Understanding the productivity is essential to understanding the overall productivity of the Ross Sea ecosystem, vertical flux of carbon from the surface layer, as well as the biogeochemistry of carbon in this critical region of the Southern Ocean.

罗斯海是南极底层水形成、生物资源利用保护和碳埋葬的关键区域。对生物量和初级生产力的季节性变化主要依赖卫星遥感资料。传统认为春季藻华以棕囊藻为主，并随铁耗尽而在夏天消亡，它们多在混合层分解，极少向食物链传递或向海底沉积。近年来，无人水下滑翔机的高频率测定揭示了： 夏季硅藻出现时，常伴随很高的颗粒有机碳（POC）/叶绿素（Chl）比值。历史数据经气候态分析也发现该现象在11月-1月频频发生。但遥感方法难以精确观测硅藻，故可能低估了硅藻的生产力和对生态系统评估的不确定性。我们假设：在夏季缺铁的条件下，硅藻的有机碳合成率不变或微变，但叶绿素合成率被抑制，导致POC:Chl增加。拟利用雪龙号极地科考船在罗斯海进行培养实验，研究铁限制下，硅藻对铁响应的理化机制；植入实验所得POC:Chl以改进生物-光学模型，重新估算生产力。正确估计初级生产力是认知罗斯海生态系统碳通量及南大洋的地球生物化学过程的基础。

众所周知，南极罗斯海区域在夏季海水层化剧烈时存在着严重的铁限制现象。另有大量研究证据显示，在南极棕囊藻（Phaeocystis antarctica）春季爆发后，硅藻会逐步增长并成为优势浮游植物类群。硅藻在增长过程中会显著提高自身碳和叶绿素的比值（C：Chl）并最终达到较高的生物量。.有如下证据支持上述结论：.1）长期的航次观测表明，在1-2月，罗斯海海域的C：Chl呈明显的季节性增长趋势(Smith and Kaufman, 2018)；.2）高空间分辨率的滑翔机采样数据显示出与上述结果相似的C：Chl季节性增长模式(Jones and Smith, 2017)；.3）通过改进算法提高基于遥感数据反演叶绿素和颗粒碳的精度，并在此基础上证实了C:Chl的显著季节性增长(Chen et al., 2021)；.4）季节性的滑翔机长断面数据也证实C：Chl随时间的增加(Kaufman et al., 2014; Ryan-Keogh and Smith, 2021)。.这些研究结果表明罗斯海（南极初级生产力最高的海域）的初级生产力被严重低估，平均可达78%。.尽管强有力的证据表明C:Chl呈现出周期性的变化，目前依然缺乏数据来评估该比值在时间和空间上的变异性。本研究计划在罗斯海范围进行大规模采样并测算叶绿素和颗粒有机碳的浓度。如果条件允许，还将设计实验进一步探究该海域处于铁限制条件下的浮游植物对铁加富的响应。但是疫情影响，目前在罗斯海的采样工作尚未进行。.为了解决这一问题，我们开始探究实验室培养下的浮游植物对铁限制的潜在适应机制。在此实验计划的进行中依然出现了一些问题：如实验设备（如低温培养箱）没有及时送达；极地浮游植物藻株难以获取等。尽管如此，我们现有的实验数据依然表明，在培养的极地浮游植物中，C：Chl的比值增加非常明显（Cao et al., unpublished）。在此项目结束后，相关实验还将继续进行下去。我们也分析了一些与该项目相关的数据,并在国际期刊上发表论文五篇。在国内外的相关学术会议上也基于该项研究对NSFC提出致谢。在该项目结束后，相关的论文仍将继续发表。

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