Ecological Drivers of Freshwater Macroalgal Blooms across Trophic Gradients

淡水藻华的发生率在整个北美一直在增加，包括没有发生过水华的历史记录的地区。考虑到藻类大量繁殖对社会和经济的负面影响，这是一个令人不安的趋势。富营养化和气候变化被认为是微藻的主要驱动因素（即，浮游植物）大量繁殖。相比之下，大型藻类（即，可见的垫状藻类）大量繁殖并不能用营养来解释，对气候变化的作用也知之甚少。形态多样的大型藻类物种在一系列栖息地和营养条件下（即，营养状态），即使在营养物低的系统中。这种“营养物-生物量”的矛盾现象可以用淡水大型藻类的共同特征来解释：（1）群体基质;（2）底栖或近床生境。跨营养梯度的大型藻类大量繁殖的一个可能驱动因素涉及生活在菌落基质上/内的微生物群落（即，“外生物群系”），当外部营养物可变或低时，其可促进或补充生长。另一个可能的生态驱动因素是底栖区的养分生物利用率，由此开花物种可以利用替代形式的养分（例如，溶解的有机磷）与竞争生物相比。一个可能的大型藻类水华的过度驱动因素是气候变化，它可以直接或间接地影响水华的发展，无论营养状态如何。因此，有三个主要的研究目标，这项拟议的研究计划：（1）确定气候变化如何直接/间接地影响大型藻类水华;（2）评估有机营养供应在大型藻类水华发展中的作用;（3）评估外生物群系在促进水华形成大型藻类生长中的作用。三个不同的，但具有代表性的大型藻类物种（Didymosphenia geminata，Nitellopsis obtusa和Cladophora glomerata）与记录滋扰增长的系统代表了广泛的营养梯度将是主要的研究类群。总的来说，这项拟议的研究计划将提供新的见解滋扰大型藻类生长的原因，包括在类群和生态系统的相似性和差异。预计这一信息将证明是有用的水管理人员从各级政府寻求控制在其管辖范围内的滋扰大型藻类爆发。

The incidence of freshwater algal blooms has been increasing across North America, including regions with no historical record of bloom occurrence. This is a disconcerting trend when considering the negative social and economic impacts of algal blooms. Eutrophication and climate change have been implicated as major drivers of microalgal (i.e., phytoplankton) blooms. In contrast, macroalgal (i.e., visible mat-forming algae) blooms are not consistently explained by nutrients, nor is much known about the role of climate change. Morphologically diverse species of macroalgae bloom across a range of habitats and nutrient conditions (i.e., trophic states), even in systems where nutrients are low. This “nutrient-biomass” paradox may be explained by the shared characteristics of freshwater macroalgae: (1) a colony-matrix; and (2) a benthic or near-bed habitat. One possible driver of macroalgal blooms across trophic gradients involves the microbial community that lives on/within the colony-matrix (i.e., the “epibiome”), which may facilitate or supplement growth when external nutrients are variable or low. Another possible ecological driver is nutrient bioavailability in the benthic zone, whereby blooming species can exploit alternative forms of nutrients (e.g., dissolved organic phosphorus) compared to competing organisms. A likely over-arching driver of macroalgal blooms is climate change, which can either directly or indirectly influence bloom development irrespective of trophic state. As such, there are three main research objectives for this proposed program of study: (1) Determine how climate change directly/indirectly influences macroalgal blooms; (2) Assess the role of organic nutrient supply in macroalgal bloom development; and (3) Evaluate the role of the epibiome in facilitating growth in bloom-forming macroalgae. Three diverse, but representative macroalgal species (Didymosphenia geminata, Nitellopsis obtusa and Cladophora glomerata) with documented nuisance growth in systems representing a broad trophic gradient will be the main study taxa. Overall, this proposed research program will offer new insight on causes of nuisance macroalgal growth, including similarities and differences across taxa and ecosystems. It is anticipated that this information will prove useful to water managers from various levels of government seeking to control nuisance macroalgae outbreaks in their jurisdictions.