The role of diazotrophic cyanobacterial blooms in the expansion of bottom water hypoxia in the Holocene and late Weichselian Baltic Sea

如今的波罗的海以大量的固氮异囊蓝藻为特征，它们的出现被认为会促进缺氧条件（溶解氧<2 mg/L）的传播，对水生生态系统的健康构成严重威胁。尽管人们普遍认为，波罗的海的营养物（特别是磷）的人为负荷增加刺激了最近蓝藻华的强度和频率，但对于除营养物可用性外的环境因素如何促进水华形成以及蓝藻生物量的大量积累和输出如何影响波罗的海的水化学、营养循环和生态系统健康，人们知之甚少。在IODP远征347：波罗的海古环境期间，从波罗的海获得的地下沉积物提供了环境和气候变化的连续档案，并且首次允许重建蓝藻水华形成在缺氧条件蔓延中的作用及其对波罗的海生态系统的影响，从最后一次冰川极大期到现在。在项目的第一个资助期内，我们研究了在IODP远征347期间钻探的Landsort Deep (M0063), Bornholm Basin （M0065）和Little Belt （M0059）的沉积物序列，以获取其体积地球化学和脂质生物标志物清单，并建立了蓝藻活性和海面温度变化的高分辨率记录。我们的分析结果表明，至少从大约7000年前微咸环境建立以来，蓝藻大量繁殖在波罗的海反复发生，而且蓝藻大量繁殖的发生和强度与初级生产力提高和底水缺氧扩散的时期相吻合。有趣的是，所有以大量和广泛的蓝藻华为特征的时间间隔都表明，texl86重建的海面温度超过16°C的阈值，因此温度变化似乎是促进波罗的海蓝藻华形成的主要驱动因素。这一点尤其值得注意，因为根据我们的初步结果，目前的全球变暖可能导致蓝藻活动的严重增加，从而导致波罗的海未来缺氧条件的蔓延。在该项目的第二阶段进行的研究将具体解决气候变化如何控制波罗的海蓝藻繁殖的丰度、组成和时空分布的问题。这决定了蓝藻华在多大程度上影响了波罗的海的水化学和水生群落结构的组成，最终目的是建立模型，准确预测未来全球变暖下蓝藻华对波罗的海生态系统健康的影响。

The present-day Baltic Sea is characterized by massive blooms of N2-fixing heterocystous cyanobacteria, which pose a severe threat for the health of the aquatic ecosystem as their occurrence is considered to promote the spread of hypoxic conditions (dissolved oxygen <2 mg/L). Although there is common consensus that the increased anthropogenic loading of nutrients (in particular phosphorous) to the Baltic Sea has stimulated the intensity and frequency of cyanobacterial blooms in the recent past, only little is known about how environmental factors other than nutrient availability facilitate bloom formation and how the massive accumulation and export of cyanobacterial biomass affects the water chemistry, nutrient cycling and ecosystem health of the Baltic Sea. Subsurface sediments, obtained from the Baltic Sea during IODP Expedition 347: Baltic Sea Paleoenvironment, provide continuous archives of environmental and climate changes and they, for the first time, allow reconstructing the role of cyanobacterial bloom formation on the spread of hypoxic condition and its consequences for the ecosystem of the Baltic Sea from the last glacial maximum to the present-day. Within the first funding period of the project, we have investigated sediment sequences from the Landsort Deep (M0063), Bornholm Basin (M0065) and Little Belt (M0059) that were drilled during IODP Expedition 347 for their bulk-geochemical as well as lipid biomarker inventory and have established high-resolution records of cyanobacterial activity and sea surface temperature change. Results of our analyses demonstrate that cyanobacterial blooms occurred repeatedly in the Baltic at least since the establishment of brackish conditions about 7,000 yrs ago and that the occurrence and intensity of cyanobacterial blooms coincides with periods of increased primary productivity and the spread of bottom-water anoxia. Interestingly, all of the time intervals characterized by abundant and widespread cyanobacterial blooms show TEXL86-reconstructed sea surface temperatures that exceed a threshold of 16°C and temperature variation thus seems to be a major driver for promoting cyanobacterial bloom formation in the Baltic Sea. This is particularly noteworthy as, based on our initial results, the current global warming may result in a severe increase in cyanobacterial activity and consequently the spread of hypoxic conditions in the Baltic Sea in the future. The research conducted within the second phase of the project will specifically address the questions how climate variation controls the abundance, composition and spatiotemporal distribution of cyanobacterial blooms in the Baltic Sea. This determines to which extent cyanobacterial blooms affect the water chemistry and the composition of the aquatic community structure in the Baltic Sea with the ultimate aim to develop models that accurately predict the impact of cyanobacterial blooms on the ecosystem health of the Baltic Sea under future global warming.