Sedimentology, geochemistry and ostracodes from Lake Ohrid

Ancient Lake Ohrid, located in the southern Balkan Peninsula in Macedonia and Albania is characterized by a high degree of endemism and it is considered to be the oldest lake in Europe. But its exact age (between one and ten million years) and also its origin are so far not known. To unravel these uncertainties an ICDP (International Continental Scientific Drilling Program) drilling project (Scientific Collaboration On Past Speciation Conditions in Ohrid (SCOPSCO)), started in April 2013. In addition to the investigations about the age and origin, other paleolimnological studies, e.g., the reconstruction of past climate and of past lake level changes, should be performed with the drilled cores. Used proxies in such paleolimnological studies are, e.g., ostracodes because they respond sensitively to environmental changes but an accurate knowledge of their preferences and tolerances to specific environmental conditions is necessary for this purpose. So far, this knowledge about the, mostly endemic, Ohrid ostracodes was limited. Thus, within the framework of this thesis, ostracodes and a multiplicity of environmental data were collected in Lake Ohrid and its adjacent waters during four field campaigns.
In a total of 47 ostracode species could be detected in the entire study area and 32 of them were found alive in Lake Ohrid. Multivariate statistic identified that water depth, salinity, conductivity, pH, and dissolved oxygen were the main determining factors for ostracode distribution in the entire study area. In Lake Ohrid, the distribution was mainly controlled by water depth, water temperature, and pH. Some ostracodes were identified as strong indicator species for important environmental variables, e.g., water temperature and water depth. A distinctive feature of Lake Ohrid was the finding of the ostracode genus Amnicythere whose species normally inhabit oligo-(meso-)haline waters and this could point to a marine origin of the lake. So far, the specialized endemic ostracodes show the highest abundances and the greatest spatial distribution in Lake Ohrid but during the sampling eight widespread species were found for the first time in the lake. They inhabited mainly the northern part of the lake, where two cities are located and industry and agriculture play a major role, and they were limited to water depths above 50 m and this could be an evidence for an increasing anthropogenic pressure because widespread ostracode species often replace endemic species. To unravel the human impact on Lake Ohrid during the last decades short sediment cores were taken and the multi-proxy study indicated that the lake productivity between the early 1920s and the late 1980s was relatively low. Diatom assemblages indicate a rising productivity in the southern part of Lake Ohrid since the mid 1970s and geochemical proxies and ostracodes point to an increasing productivity since the late 1980s in the southern and in the northern part. A slight increase in the productivity continued until 2009. Noticeable is the fact that since the early 1990s, the increasing productivity and the increasing concentrations of heavy metals correspond to a decreasing number of ostracodes in the northern part of Lake Ohrid. Perhaps, this indicates that living conditions in this lake part became less favorable for the mostly endemic ostracode species. Furthermore, the sediment samples from the cores show relatively high concentrations of arsenic, iron, and nickel. Fluctuations in ostracode assemblages from three longer sediment cores, the longest spans approximately 136 ka, taken in Lake Ohrid, correspond to fluctuations in the productivity, in the carbonate content, of the lake level, and of climate changes. Between the marine isotope stage (MIS) 6 and MIS 2 the number of ostracode valves is very low or the valves were completely absent. This corresponds to a low lake productivity, a low carbonate content, and a low lake level. At the onset of the Holocene, the number of valves increased markedly and this correlates with an increased productivity and carbonate content and a warmer climate. But during the Little Ice Age (LIA), the number of valves dropped again and species which prefer warmer waters disappeared completely. This drop corresponds also to a low productivity. After the LIA, the number of species increased again but since 1895 AD a strong and abrupt decrease is visible. A reason for this could be an increase in the heavy metal concentrations.

坐落于巴尔干半岛南部、马其顿与阿尔巴尼亚交界处的奥赫里德古湖（Ancient Lake Ohrid）以极高的特有性（endemism）为显著特征，被认为是欧洲现存最古老的湖泊。但其确切形成年代（介于100万至1000万年之间）与起源至今仍未明确。为解开这些科学谜团，国际大陆科学钻探计划（International Continental Scientific Drilling Program, ICDP）于2013年4月启动了奥赫里德古湖过去物种形成条件科学合作钻探项目（Scientific Collaboration On Past Speciation Conditions in Ohrid, SCOPSCO）。除针对湖泊年代与起源开展研究外，科研团队还计划通过钻探获取的岩芯开展一系列古湖沼学（paleolimnological）研究，例如重建古气候与古湖水位变化历史。此类古湖沼学研究中常用的代用指标（proxy）包括介形类（ostracodes），因其对环境变化响应敏感，但要利用其开展环境重建，需精准掌握其对特定环境条件的偏好与耐受范围。然而截至目前，学界对奥赫里德古湖（多数为特有种）的介形类相关认知仍十分有限。因此，本论文研究期间，团队通过四次野外考察，在奥赫里德古湖及其周边水域收集了介形类样本与多组环境数据。
在整个研究区域中共鉴定出47种介形类，其中32种在奥赫里德古湖内被活体采集。多元统计分析（multivariate statistic）结果显示，水深、盐度（salinity）、电导率（conductivity）、pH值与溶解氧（dissolved oxygen）是控制整个研究区域介形类分布的核心环境因子；而在奥赫里德古湖内部，介形类分布主要受水深、水温与pH值调控。部分介形类被确定为关键环境变量的强指示物种，例如水温与水深。奥赫里德古湖的一项显著发现是介形类Amnicythere属的存在，该属物种通常栖息于寡（中）盐水域，这一发现或许暗示奥赫里德古湖具有海洋起源。截至目前，特化的地方性介形类在奥赫里德古湖中展现出最高的丰度与最广的空间分布，但本次采样中首次在该湖内发现了8种广布性介形类。这些广布种主要栖息于奥赫里德古湖北部区域（该区域坐落两座城市，工业与农业活动较为活跃），且仅分布于水深50米以上的水域，这一现象或可作为人为压力不断加剧的佐证——因为广布性介形类通常会取代地方性特有物种。为解析近数十年人类活动对奥赫里德古湖的影响，研究团队采集了短尺寸沉积物岩芯（sediment cores），多代用指标研究结果表明，1920年代初至1980年代末期间，该湖的生产力水平相对较低。硅藻组合（diatom assemblages）数据显示，自1970年代中期以来，奥赫里德古湖南部的生产力持续上升；地球化学代用指标（geochemical proxies）与介形类数据则表明，自1980年代末以来，该湖南部与北部区域的生产力均呈上升趋势，这一增长态势一直持续至2009年。值得注意的是，自1990年代初以来，奥赫里德古湖北部的介形类数量随生产力提升与重金属（heavy metals）浓度升高而持续下降。这一现象或许表明，该湖区的生境条件对以特有种为主的介形类愈发不利。此外，岩芯沉积物样本中检测到较高浓度的砷、铁与镍。对奥赫里德古湖内三根较长沉积物岩芯的介形类组合分析显示，其群落波动与湖泊生产力、碳酸盐含量（carbonate content）、湖水位变化及气候变迁均存在对应关系。在海洋同位素阶段（marine isotope stage, MIS）6至MIS 2期间，介形类介壳的数量极少甚至完全缺失，这与当时较低的湖泊生产力、碳酸盐含量与湖水位相吻合。进入全新世（Holocene）初期，介形类介壳数量显著增加，这与生产力提升、碳酸盐含量升高以及气候变暖的趋势高度相关。但在小冰期（Little Ice Age, LIA）期间，介形类介壳数量再次下降，偏好温暖水域的物种完全消失，这一变化同样对应较低的湖泊生产力。小冰期结束后，介形类物种数量再度回升，但自公元1895年以来，物种数量出现了显著且突然的下降，这一现象的成因可能与重金属浓度升高有关。