St. Croix Watershed Research Station, Science Museum of Minnesota, Marine on St. Croix, Minnesota 55047, USA.
Ecol Appl. 2012 Jul;22(5):1483-96. doi: 10.1890/11-1485.1.
Multiple stressors to a shallow lake ecosystem have the ability to control the relative stability of alternative states (clear, macrophyte-dominated or turbid, algal-dominated). As a consequence, the use of remedial biomanipulations to induce trophic cascades and shift a turbid lake to a clear state is often only a temporary solution. Here we show the instability of short-term manipulations in the shallow Lake Christina (Minnesota, USA) is governed by the long-term state following a regime shift in the lake. During the modern, managed period of the lake, three top-down manipulations (fish kills) were undertaken inducing temporary (5-10 years) unstable clear-water states. Paleoecological remains of diatoms, along with proxies of primary production (total chlorophyll a and total organic carbon accumulation rate) and trophic state (total P) from sediment records clearly show a single regime shift in the lake during the early 1950s; following this shift, the functioning of the lake ecosystem is dominated by a persistent turbid state. We find that multiple stressors contributed to the regime shift. First, the lake began to eutrophy (from agricultural land use and/or increased waterfowl populations), leading to a dramatic increase in primary production. Soon after, the construction of a dam in 1936 effectively doubled the depth of the lake, compounded by increases in regional humidity; this resulted in an increase in planktivorous and benthivorous fish reducing phytoplankton grazers. These factors further conspired to increase the stability of a turbid regime during the modern managed period, such that switches to a clear-water state were inherently unstable and the lake consistently returned to a turbid state. We conclude that while top-down manipulations have had measurable impacts on the lake state, they have not been effective in providing a return to an ecosystem similar to the stable historical period. Our work offers an example of a well-studied ecosystem forced by multiple stressors into a new long-term managed period, where manipulated clear-water states are temporary, managed features.
多个浅水湖泊生态系统的胁迫因素能够控制替代状态（清澈、大型植物为主或浑浊、藻类为主）的相对稳定性。因此，利用补救性生物操纵来诱导营养级联并将浑浊湖泊转变为清澈状态通常只是一种临时解决方案。在这里，我们展示了明尼苏达州克里斯蒂娜浅湖（Lake Christina）的短期操纵不稳定，这是由湖泊状态发生转变后的长期状态所决定的。在湖泊的现代管理时期，进行了三次自上而下的操纵（鱼类死亡），导致了暂时（5-10 年）不稳定的清澈水状态。从沉积物记录中可以清楚地看到，硅藻的古生态学遗迹以及初级生产力的替代物（总叶绿素 a 和总有机碳积累率）和营养状态（总磷）表明，湖泊在 20 世纪 50 年代初期发生了单一的状态转变；在此转变之后，湖泊生态系统的功能主要由持久的浑浊状态主导。我们发现，多种胁迫因素促成了这种状态转变。首先，湖泊开始富营养化（来自农业土地利用和/或水禽数量的增加），导致初级生产力的急剧增加。不久之后，1936 年建造的水坝使湖泊的深度增加了一倍，再加上区域湿度的增加；这导致了浮游动物和底栖动物鱼类的增加，减少了浮游植物的食草动物。这些因素进一步合谋增加了浑浊状态的稳定性，使得清澈水状态的转变本质上不稳定，湖泊始终恢复到浑浊状态。我们的结论是，虽然自上而下的操纵对湖泊状态产生了可衡量的影响，但它们并没有有效地使湖泊恢复到类似于稳定历史时期的生态系统。我们的工作提供了一个经过充分研究的生态系统的例子，该系统受到多种胁迫因素的影响，进入了一个新的长期管理时期，其中操纵的清澈水状态是暂时的、管理特征。
