The perspective on water transparency changed since the early days of limnology from being a physical parameter of optical water property to an ecological indicator tracking algal turbidity due to eutrophication or an overall success of sustained lake restoration in the late 60ies to 80ies. In modern cities, where ecosystems are commonly deteriorated by man-made modifications, water transparency offers a great opportunity to the public to raise socio-ecological consciousness concerning urban green-blue spaces. We thus re-emphasize water transparency as a key indicator of multi-functional value when assessing an oxbow lake of the riverine floodplain in Vienna, the Alte Donau. Our study covers the eutrophication from 1987 to 1994 due to the inclusion of the riverine landscape in the urban area, the following lake restoration with an ecosystem shift from a nutrient-rich, algal-turbid water body to a nutrient-poor, clear-water macrophyte controlled system and the impact of global warming in recent decades. We used light attenuation profiles to identify depth layers of specific ambient light requirements for photosynthetic domains (phytoplankton and submerged macrophytes), and to interpret Secchi measurements. Here, we calculated the depth at 1% (minimum light requirements for phytoplankton growth as euphotic depth), 3% (minimum light requirements for macrophytes as maximum macrophyte colonization depth), and 12% (preferred light requirements for phytoplankton development) of surface ambient light. A Secchi disk water transparency of 1.5 m (“lake bottom view”), judged as good water quality by human perception, refers to mesotrophic conditions with a maximum colonization depth for macrophytes exceeding the mean lake depth in Alte Donau. Water clarity required for sustained macrophyte growth, in particular for favoring bottom-dwelling Chara meadows instead of tall-growing Myriophyllum spicatum, is 3.5 m Secchi depth and thus exceeds by far water clarity requested due to bathing aesthetics. Global warming, mirrored by an advanced warming in spring seems to favor significantly a higher yield of macrophytes mainly built up by Myriophyllum at the expense of the yield of algae. The prolongation of the summer period above 21°C, however, coincides with lowered Secchi transparency. Water visibility during the hot season thus seems to be slightly hampered against lake restoration efforts by global warming.

中文翻译：

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新强调水透明度作为城市水的社会生态指标：连接生态系统服务供应和可持续生态系统健康

自湖泊学早期以来，水透明度的观点就从作为光学水属性的物理参数转变为跟踪由于富营养化引起的藻类浑浊度或 60 年代后期至 80 年代持续湖泊恢复的整体成功的生态指标。在生态系统通常因人为改造而恶化的现代城市中，水的透明度为公众提高对城市绿-蓝空间的社会生态意识提供了一个很好的机会。因此，在评估维也纳河流泛滥平原的牛轭湖 Alte Donau 时，我们再次强调水透明度是多功能价值的关键指标。我们的研究涵盖了 1987 年至 1994 年由于城市地区的河流景观而导致的富营养化，随后的湖泊恢复，生态系统从营养丰富、藻类混浊的水体转变为营养贫乏、清澈的大型植物控制系统，以及近几十年来全球变暖的影响。我们使用光衰减剖面来确定光合域（浮游植物和沉水植物）的特定环境光要求的深度层，并解释 Secchi 测量值。在这里，我们计算了表面环境的 1%（浮游植物生长的最低光要求为透光深度）、3%（大型植物的最低光要求为最大大型植物定植深度）和 12%（浮游植物发育的首选光要求）的深度光。1.5 m 的 Secchi 圆盘水透明度（“湖底视图”），人类感知判断为良好的水质，指中营养条件下，大型植物的最大定植深度超过了 Alte Donau 的平均湖泊深度。大型植物持续生长所需的水透明度为 3.5 m Secchi 深度，尤其是有利于底栖 Chara 草甸而不是高大生长的 Chara 草甸，因此远远超过了由于沐浴美学而要求的水透明度。全球变暖，由春季的提前变暖反映出来，似乎有利于显着提高主要由狐尾藻建立的大型植物的产量，但牺牲了藻类的产量。然而，夏季延长至 21°C 以上与 Secchi 透明度降低相吻合。因此，由于全球变暖，炎热季节的水能见度似乎略微阻碍了湖泊恢复工作。