Search

Compte tenu de la nature des lacs, et des petites tailles et des grandes populations du plancton, le phytoplancton lacustre est un modèle végétal optimal pour l’étude du rôle du filtrage environnemental dans l'assemblage des communautés. L'eutrophisation étant le gradient le plus important. Le Réseau du CRSNG sur l’état des lacs du Canada (Lake Pulse ci-dessous) a échantillonné 664 lacs pour plus de 100 variables environnementales, biologiques et optiques, avec l'évaluation nationale des lacs de l'US-EPA, ce qui crée un ensemble de données continentales continues permettant d'évaluer l'écologie du plancton à l'échelle continentale. L'échantillonnage optique obtenue par Lake Pulse permet d'étudier les effets de l'éclairement et des spectres sur les communautés. Cette thèse vise à clarifier le rôle de l'eutrophisation et de la lumière sur la structure des communautés phytoplanctoniques d'un point de vue de niche tout en résolvant les obstacles méthodologiques nécessaires à la modélisation de la photosynthèse aquatique dans les lacs. Elle est divisée en quatre parties. La première partie est une étude de l'effet des variables environnementales sur la structure des niches, la coexistence et la biodiversité. Nous avons constaté que les niches dans les lacs sont comblées selon un modèle multimodal et que la coexistence est plus faible parmi les espèces présentant des différences de niche intermédiaires. Nous avons constaté que l’eutrophisation entraîne une perte de biodiversité en réduisant l’espace viable des niches, en perdant la multimodalité des niches et en augmentant le chevauchement des niches. Dans la deuxième partie, nous avons développé et validé une méthode de mesure de l'absorption des particules basée sur une extraction séquentielle, un blanchiment et un post-traitement, optimisée pour les eaux douces, qui permet de séparer avec précision les composantes phytoplanctoniques des composantes non algales. Nous avons utilisé cette méthode pour mesurer les absorpti, Given the nature of lakes, the small size and large populations of plankton, lake phytoplankton provides a model vegetation process that is optimal to study of the role of environmental filtering in community assembly, with eutrophication as the most impactful gradient. The NSERC Canadian Lake Pulse survey sampled 664 lakes for more than 100 environmental, biological, and optical variables, and together with the US-EPA National Lakes Assessment, they constitute a continuous continental dataset allowing the study of plankton ecology at a continental scale. In addition, optical measurements taken during the Lake Pulse survey allow effects of light irradiance and spectra on the communities to be examined. This thesis aims to clarify the role of eutrophication and light in structuring phytoplankton communities from a niche perspective while addressing methodological hurdles required for modelling underwater photosynthesis in lakes. It is divided into four parts. The first part is an investigation of the effect of environmental variables on niche structure, coexistence, and biodiversity. We found that niches in lakes are filled in a multimodal pattern, and that coexistence is lower among species with intermediate niche differences. We found that eutrophication drives biodiversity loss by a narrowing of the viable niche space, reducing niche multimodality, and increasing niche overlap. In the second part, we developed and validated a method to measure particulate absorption based on sequential extraction, bleaching, and post-processing, optimized for inland waters. This allows accurate separation of phytoplankton and from non-algal components, and it was used to process the particulate absorption in the Lake Pulse sampled lakes. In the third part, we studied the relations between carotenoid structure, function, and their role in phytoplankton ecological strategies, proposing a set of functional groups based on molecular structure. We used this classification to

Cyanobacterial blooms have serious adverse effects on human and environmental health. In Latin America, one of the main world's freshwater reserves, information on this phenomenon remains sparse. To assess the current situation, we gathered reports of cyanobacterial blooms and associated cyanotoxins in freshwater bodies from South America and the Caribbean (Latitude 22 degrees N to 45 degrees S) and compiled the regulation and monitoring procedures implemented in each country. As the operational definition of what is a cyanobacterial bloom remains controversial, we also analyzed the criteria used to determine the phenomena in the region. From 2000 to 2019, blooms were reported in 295 water bodies distributed in 14 countries, including shallow and deep lakes, reservoirs, and rivers. Cyanotoxins were found in nine countries and high concentrations of microcystins were reported in all types of water bodies. Blooms were defined according to different, and sometimes arbitrary criteria including qualitative (changes in water color, scum presence), quantitative (abundance), or both. We found 13 different cell abundance thresholds defining bloom events, from 2 x 10(3) to 1 x 10(7) cells mL(-1). The use of different criteria hampers the estimation of bloom occurrence, and consequently the associated risks and economic impacts. The large differences between countries in terms of number of studies, monitoring efforts, public access to the data and regulations regarding cyanobacteria and cyanotoxins highlights the need to rethink cyanobacterial bloom monitoring, seeking common criteria. General policies leading to solid frameworks based on defined criteria are needed to improve the assessment of cyanobacterial blooms in Latin America. This review represents a starting point toward common approaches for cyanobacterial monitoring and risk assessment, needed to improve regional environmental policies.

Cyanobacterial blooms imperil the use of freshwater around the globe and present challenges for water man-agement. Studies have suggested that blooms are trigged by high temperatures and nutrient concentrations. While the roles of nitrogen and phosphorus have long been debated, cyanobacterial dominance in phytoplankton has widely been associated with climate warming. However, studies at large geographical scales, covering diverse climate regions and lake depths, are still needed to clarify the drivers of cyanobacterial success. Here, we analyzed data from 464 lakes covering a 14,000 km north-south gradient in the Americas and three lake depth categories. We show that there were no clear trends in cyanobacterial biomass (as biovolume) along latitude or climate gradients, with the exception of lower biomass in polar climates. Phosphorus was the primary resource explaining cyanobacterial biomass in the Americas, while nitrogen was also significant but particularly relevant in very shallow lakes (< 3 m depth). Despite the assessed climatic gradient water temperature was only weakly related to cyanobacterial biomass, suggesting it is overemphasized in current discussions. Depth was critical for predicting cyanobacterial biomass, and shallow lakes proved more vulnerable to eutrophication. Among other variables analyzed, only pH was significantly related to cyanobacteria biomass, likely due to a biologically mediated positive feedback under high nutrient conditions. Solutions toward managing harmful cyanobacteria should thus consider lake morphometric characteristics and emphasize nutrient control, independently of tem-perature gradients, since local factors are more critical - and more amenable to controls - than global external forces.