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

当今波罗的海的特点是大量固氮异囊蓝细菌大量繁殖，这对水生生态系统的健康构成严重威胁，因为它们的出现被认为会促进缺氧条件的传播（溶解氧<2毫克/升）。尽管人们普遍认为，（特别是磷）对波罗的海的影响在最近的过去刺激了蓝藻水华的强度和频率，只有很少有人知道除了营养物质的可用性之外的环境因素如何促进水华的形成，以及蓝藻生物量的大量积累和输出如何影响水化学，波罗的海的营养循环和生态系统健康。次表层沉积物，从波罗的海在IODP远征347：波罗的海古环境，提供了环境和气候变化的连续档案，他们，第一次，允许重建蓝藻水华形成的缺氧条件的传播及其后果的作用波罗的海的生态系统从末次冰期到现在。在该项目的第一个资助期内，我们调查了来自Landsort Deep（M0063），Bornholm Basin（M0065）和Little Belt（M0059）的沉积物序列，这些沉积物序列是在IODP Expedition 347期间钻探的，用于其散装地球化学和脂质生物标志物清单，并建立了蓝藻活动和海面温度变化的高分辨率记录。我们的分析结果表明，蓝藻水华反复发生在波罗的海至少自成立的半咸水条件下约7,000年前，蓝藻水华的发生和强度与初级生产力的增加和底层水缺氧的蔓延的时期相吻合。有趣的是，所有以丰富和广泛的蓝藻水华为特征的时间间隔都显示TEXL 86重建的海面温度超过了16°C的阈值，因此温度变化似乎是促进波罗的海蓝藻水华形成的主要驱动因素。这一点特别值得注意，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严重增加，从而导致未来波罗的海缺氧状况的蔓延。在该项目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水华的丰度、组成和时空分布的问题。这决定了在何种程度上蓝藻水华影响水化学和组成的水生生物群落结构在波罗的海的最终目的是开发模型，准确预测蓝藻水华的影响，对生态系统健康的波罗的海在未来的全球变暖。

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.