Salto Grande is a large South American reservoir on the border between Uruguay and Argentina that was impounded in 1979 and experiences recurrent, massive cyanobacteria blooms. A water-monitoring program was initiated 20 years after the dam was built, hence the causes and onset of cyanobacteria blooms remain poorly known. We collected two sediment cores from the old river channel in the reservoir (z = 17 m) and used physical, chemical and biological variables in the sediments, along with existing limnological data, to explore the history of cyanobacteria in the sub-tropical water body. Cyanobacteria fossil pigments were present at low concentrations during the first 24 years after impoundment, but more than doubled thereafter. Phytoplankton abundance tracked shifts in cyanobacteria pigment concentration, indicating an overall increase in all primary producers. Several sediment variables indicate a decline in water quality after 2003, such as increases in the number of photosynthetic sulfur bacteria and a reduction in sediment magnetic susceptibility. Akinetes (dormant cyanobacteria cells, Order Nostocales) in recent reservoir deposits were abundant and five species germinated under laboratory conditions, underscoring the ability of akinetes to initiate cyanobacteria blooms. The germinated assemblage reflected closely the composition of cyanobacteria blooms in the reservoir. Recorded increases in air temperature and decreases in wind speed, together with other variables (e.g. nutrients), can promote the large, recurrent cyanobacteria blooms. Invasion of the bivalve Limnoperna fortunei apparently promoted cyanobacteria blooms by preferential feeding on other phytoplankton taxa, and perhaps by altering nutrient concentrations and ratios. This work highlights the potential for using multiple variables in sediment cores from large reservoirs to better understand the responses of biota to multiple environmental stressors.

中文翻译：

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从沉积岩心分析推断出的大型亚热带水库中近期蓝藻的丰度

Salto Grande 是位于乌拉圭和阿根廷边界的南美大型水库，于 1979 年被蓄水，并经历了反复出现的大规模蓝藻水华。在大坝建成 20 年后启动了水监测计划，因此蓝藻水华的原因和发生仍然知之甚少。我们从水库的旧河道 (z = 17 m) 中收集了两个沉积物核心，并利用沉积物中的物理、化学和生物变量以及现有的湖沼学数据来探索亚热带水体中蓝藻的历史. 蓝藻化石色素在蓄水后的前 24 年中以低浓度存在，但此后增加了一倍多。浮游植物丰度跟踪蓝藻色素浓度的变化，表明所有初级生产者的总体增长。几个沉积物变量表明 2003 年后水质下降，例如光合硫细菌数量增加和沉积物磁化率降低。近期水库沉积物中的 Akinetes（休眠蓝藻细胞，Order Nostocales）非常丰富，并且在实验室条件下有 5 个物种发芽，强调了 Akinetes 引发蓝藻大量繁殖的能力。发芽的组合密切反映了水库中蓝藻水华的组成。记录到的气温升高和风速降低，以及其他变量（例如营养物质），可以促进蓝藻大规模、反复的繁殖。双壳类 Limnoperna Fortunei 的入侵显然通过优先取食其他浮游植物类群，并可能通过改变营养浓度和比例来促进蓝藻的大量繁殖。这项工作强调了在大型水库沉积岩芯中使用多个变量以更好地了解生物群对多种环境压力因素的反应的潜力。