Importance of ecological boundaries in aquatic ecosystem processes

在过去的几十年里，有害的藻华在世界范围内的水生生态系统中不断增加，而且不仅仅是在富营养化的生态系统中。在大陆水域，这些水华主要是由潜在有毒的蓝藻(蓝藻)造成的。自70年代初以来，人们就知道蓝绿色水华与湖泊营养物质，特别是磷(P)的肥力之间的关系。今天的问题是，为什么水华也发生在表层水体营养物质浓度较低的湖泊中。在这些湖泊中，一些湖泊的外部磷负荷在过去20-30年里有所减少，另一些湖泊的外部人为来源未知。在这些表面营养含量较低的湖泊中，水华的发生和强度在不同年份之间有所不同。近年来，有毒蓝绿色植物的增加被归因于全球变暖，这一假说的作者主张更详细地研究气候变化和形成花期的蓝绿色植物的种群动态。在我们为期5年的研究中，我们希望验证一些与气候有关的机制，这些机制可以控制小湖泊中的藻类群落结构和蓝绿色水华。特别是，我们想衡量在湖底(底栖生物边界层)由风引起的物理强迫在营养物质和生物从沿海和深层生境到水柱(中上层生境)的通量中的重要性；以及在物理限制的相邻区域之间的重要性(例如分层湖泊中的温跃层)。我们的假设是：1)在低磷负荷的湖泊中，底栖期藻类生长的是沉积物中积累的磷；2)营养物质和生物通过边界的物理强迫的强度将不同的生境(从底部到水柱和通过水柱)联系在一起，并在藻类补充过程中产生正反馈或负反馈。我们的研究为解释层状湖泊中藻类群落结构的年际变化提供了一种新的方法，并可能解释蓝藻水华。我们对物理强迫对分层小湖泊生态过程的重要性的理解，应该会为有害水华的管理和控制提供更好的工具。

During recent past decades, harmful algal blooms have increased world wide in aquatic ecosystems, and not only in eutrophic ecosystems. In continental waters these blooms are mainly due to potentially toxic blue-green algae (cyanobacteria). The relationship between blue-green blooms and lake fertilisation by nutrients, particularly phosphorus (P), is known since the early 70s. The question today is why the blooms also occur in lakes with low nutrient concentrations in their surface waters. In some of these lakes external P-loading has been decreased in the last 20-30 years, in others external anthropogenic sources are unknown. In these low surface nutrient lakes the occurrence and intensity of the blooms varies between years. Recently, the increase of noxious blue-greens has been attributed to global warming, and the author of this hypothesis advocates for more detailed studies of climate change and population dynamics of bloom-forming blue-greens. In our 5-yr study we wish to verify some of the mechanisms related to climate that could control algal community structure and blue-green blooms in small lakes. In particular, we want to measure the importance of wind-induced physical forcing at the bottom of lakes (benthic boundary layer) in the fluxes of nutrients and organisms from littoral and deep habitats to the water column (pelagic habitats); and between physically constrained adjacent areas (e.g. thermocline in stratified lakes). Our hypotheses are that: i) it is the P-accumulated in the sediments that allows the growth of the bottom resting stages of algae in lakes with low external P loads and, ii) the intensity of the physical forcing of nutrients and organisms through boundaries links the different habitats (from the bottom to the water column and through the water column) and creates a positive or negative feedback in algae recruitment. Our study constitutes a novel approach to explaining the inter-annual variability in algae community structure in stratified lakes and may explain cyanobacteria blooms. Our understanding of the importance of the physical forcing on ecological processes in stratified small lakes should produce better tools for noxious bloom management and control.