Importance of ecological boundaries in aquatic ecosystem processes

近几十年来，有害藻华在世界范围内的水生生态系统中有所增加，而不仅仅是在富营养化生态系统中。在大陆水域，这些水华主要是由于潜在有毒的蓝绿藻（蓝藻）。蓝绿色水华与湖泊营养物（尤其是磷）之间的关系早在70年代初就已为人所知。今天的问题是，为什么水华也发生在地表水营养浓度低的湖泊中。在过去20-30年中，其中一些湖泊的外部磷负荷有所减少，另一些湖泊的外部人为来源尚不清楚。在这些低地表营养物湖泊中，水华的发生和强度随年份而变化。最近，有毒蓝绿植物的增加被归因于全球变暖，这一假说的作者主张对气候变化和开花蓝绿植物的种群动态进行更详细的研究。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的藻类群落结构和蓝绿华。特别是，我们想测量湖底（底栖边界层）的风致物理强迫在营养物和生物从沿海和深海生境到水柱（上层生境）的通量中的重要性；以及在物理上受限的相邻区域之间（例如分层湖泊中的温跃层）。我们的假设是：i)沉积物中积累的P允许藻类在低外部P负荷的湖泊中生长，ii)通过边界连接不同栖息地（从底部到水柱并通过水柱）的营养物质和生物体的物理强迫强度，并在藻类的补充中产生正反馈或负反馈。我们的研究为解释分层湖泊藻类群落结构的年际变化提供了新的途径，并可能解释蓝藻华。我们对分层小湖泊生态过程中物理强迫重要性的理解，应该为有毒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.