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

今天波罗的海的特点是大量的固氮异晶蓝细菌大量繁殖，这对水生生态系统的健康构成了严重威胁，因为它们的出现被认为促进了低氧条件的传播(溶解氧和2毫克/L)。虽然人们普遍认为，人类对波罗的海的营养物质(特别是磷)的增加刺激了蓝藻水华的强度和频率，但关于除营养物质可获得性以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出口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 347：波罗的海古环境考察期间从波罗的海获取的地下沉积物，提供了环境和气候变化的连续档案，它们首次能够重建蓝藻水华形成在低氧条件传播及其对波罗的海生态系统的影响方面所起的作用，从最后一次冰盛期到现在。在项目的第一个资助期内，我们调查了在IODP探险347期间钻探的Landsor型深海(M0063)、博恩霍尔姆盆地(M0065)和小带(M0059)的沉积物序列，以获得其大宗地球化学和脂肪生物标志物清单，并建立了蓝藻活动和海表面温度变化的高分辨率记录。我们的分析结果表明，至少在大约7000年前半咸水条件建立以来，蓝藻水华在波罗的海反复发生，蓝藻水华的发生和强度恰好与初级生产力提高和底层水域缺氧蔓延的时期相吻合。有趣的是，所有以大量和广泛的蓝藻水华为特征的时间间隔都表明，TEXL86重建的海表面温度超过了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。在该项目第二阶段内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。这决定了蓝藻水华对波罗的海水化学和水生群落结构构成的影响程度，最终目的是建立模型，准确预测未来全球变暖下蓝藻水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.