Department of Aquatic Ecosystems Analysis and Management, Helmholtz Centre for Environmental Research - UFZ, Magdeburg, Germany.
Water Res. 2017 May 15;115:162-171. doi: 10.1016/j.watres.2017.02.062. Epub 2017 Feb 27.
Ecological theory predicts that the relative importance of benthic to planktonic primary production usually changes along the rivers' continuum from a predomination of benthic algae in lower stream orders to a predomination of planktonic algae at higher orders. Underlying mechanisms driving the interaction between algae in these habitats, its controlling factors and consequences for riverine ecosystems are, however, only partly understood. We present a mechanistic analysis of the governing ecological processes using a simplified, numerical model and examine how abiotic factors and biotic interactions influence benthic and planktonic algae by changing resource competition. We compare the outcome of the model with the results of a factorial mesocosm experiment mimicking the parameter spaces of the model. The results show a remarkable similarity with regard to the temporal development of benthic and pelagic algal biomass and shifting dominance patterns. In particular we analyse the effects of the pathways of nutrient supply (upwelling from the hyporheic zone, direct supply to the surface water, or via both pathways) and grazing in a gradient of river depths. Our results show that detachment of benthic algae, sinking of planktonic algae and the pathway of nutrient supply are key processes determining the respective algal biomass distributions particularly in shallow and intermediate deep systems. Increasing nutrient supply increases algal biomasses, but does not change the general pattern of the interactions. Decreasing light supply decreases the dominance of planktonic algae, but increases dissolved nutrients. At intermediate to high grazing rates algal biomass can be controlled by grazers, but however, at high grazing rates, dissolved nutrients accumulate in the surface water. Our results indicate that nutrient pathways, resource competition and internal control by grazing need to be considered explicitly for the understanding and explanation of eutrophication phenomena in riverine ecosystems. As a consequence, ecologically effective eutrophication management of running water systems has to go beyond the control of nutrient emissions or the achievement of limiting threshold values in the receiving waters, but requires the consideration of the nutrient pathways (surface water versus groundwater) and the shifting biological controls from lower to higher order stream ecosystems.
生态理论预测，从河流下游到上游，底栖生物与浮游生物初级生产力的相对重要性通常会发生变化，在下游河流中底栖藻类占主导地位，而在上游河流中浮游藻类占主导地位。然而，驱动这些栖息地藻类之间相互作用的潜在机制、其控制因素以及对河流生态系统的影响，人们只是部分了解。我们使用简化的数值模型对控制生态过程进行了机制分析，并研究了非生物因素和生物相互作用如何通过改变资源竞争来影响底栖和浮游藻类。我们将模型的结果与模拟模型参数空间的析因中观实验的结果进行了比较。结果表明，在底栖和浮游藻类生物量的时间发展以及优势模式的转变方面，模型的结果与实验结果非常相似。特别是，我们分析了营养物质供应途径（从地下河带上升、直接供应地表水、或通过这两种途径）和在河流深度梯度中的摄食作用的影响。我们的结果表明，底栖藻类的脱落、浮游藻类的下沉以及营养物质的供应途径是决定各自藻类生物量分布的关键过程，特别是在浅水区和中等深度系统中。增加营养物质供应会增加藻类生物量，但不会改变相互作用的一般模式。减少光照供应会降低浮游藻类的优势，但会增加溶解的营养物质。在中等至高的摄食率下，藻类生物量可以被食草动物控制，但在高摄食率下，溶解的营养物质会在地表水中积累。我们的研究结果表明，在理解和解释河流生态系统中的富营养化现象时，需要明确考虑营养物质途径、资源竞争和食草动物的内部控制。因此，对于流水系统的富营养化管理，需要超越对营养物质排放的控制或对受纳水体中限制阈值的实现，而是需要考虑营养物质途径（地表水与地下水）以及从下游到上游河流生态系统中生物控制的转变。
