Made available in DSpace on 2016-06-02T19:31:36Z (GMT). No. of bitstreams: 1 1260.pdf: 1503714 bytes, checksum: 556c38596d4d4ea47b6d75a7441691bd (MD5) Previous issue date: 2006-08-28 / Universidade Federal de Sao Carlos / Anabaena spiroides and Microcystis aeruginosa are cyanobacteria that frequently bloom in Barra Bonita Reservoir. These floating organisms and the characteristic turbulence found at the reservoir exposure the cells to higher light intensities by lifting them from lower depths in the photic column to surface. The sudden exposure at higher irradiance could cause photoinhibition and photooxidation. This phenomenon could liberate high amounts of dissolved organic matter. This work aimed to investigate the carbon fixation photoinhibition in cells of A. spiroides and M. aeruginosa exposed to high irradiances, the chlorophyll photooxidation and the DOM released by cells of cultures differing in their physiologic state, after exposure to high irradiances through time at laboratory and field investigations. A. spiroides suffered photoinhibition and liberation of dissolved organic matter when exposed to irradiances higher than their EK during four hours as well as when exposed to irradiance of 2000 µmol.m-2.s-1 for eight hours. M. aeruginosa did not present photoinhibition in any case, however, their C14OD liberation was significantly increased. The higher indexes of C14OD excreted by theses cyanobacteria are not apparently due to cell damage. Hence it might represent an adaptation factor to higher irradiances. Similar results were found in experiments incubated in Barra Bonita reservoir. / Anabaena spiroides e Microcystis aeruginosa são cianobactérias que frequentemente formam florescimentos no Reservatório de Barra Bonita. A flutuação desses organismos e a turbulência característica do reservatório fazem com que as células que estavam iniciando o desenvolvimento do florescimento nas camadas inferiores da coluna fótica possam rapidamente ficar expostas a altas intensidades de luz quando na superfície. Esta exposição repentina a irradiâncias maiores pode causar fotoinibição e mesmo fotooxidação, podendo causar a liberar uma grande quantidade de matéria orgânica dissolvida pelas células. O presente trabalho foi realizado com o intuito de verificar se células de diferentes idades de cultivo de A. spiroides e M. aeruginosa sofrem fotoinibição da fixação de carbono, fotooxidação da clorofila-a e aumento na liberação de MOD quando expostas por diferentes períodos a irradiâncias elevadas. A. spiroides apresentou fotoinibição, juntamente com o aumento na excreção de matéria orgânica dissolvida, quando exposta a irradiâncias superiores ao seu EK durante 4 horas e ao ser exposta a irradiância de 2000 µmol.m-2.s-1 durante 8 horas. M. aeruginosa não apresentou fotoinibição quando exposta a irradiâncias superiores a seu EK por 4 horas, entretanto, apresentou um incremento significativo na liberação de C14OD. As concentrações elevadas de C14OD excretado pelas duas cianobactérias, aparentemente, não resultam de danos nas células, podendo representar um fator de adaptação às irradiâncias elevadas a que foram submetidas. Respostas semelhantes foram encontradas nos experimentos realizados no reservatório de Barra Bonita.

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En raison des forçages anthropiques exercés sur les écosystèmes aquatiques et des effets des changements globaux présents et à venir, on s’attend à une augmentation de la fréquence et de l’intensité des proliférations cyanobactériennes lacustres. Une meilleure connaissance des facteurs biotiques, et notamment du parasitisme, impliqués dans le control des dynamiques cyanobactériennes semble essentielle. Il est désormais établi que les Chytridiomycota (chytrides) constituent les principaux pathogènes fongiques du phytoplancton. Ainsi, les travaux de recherche menés au cours de cette thèse ont eu pour objectif d’étudier le parasitisme fongique associé aux efflorescences cyanobactériennes, et plus précisément l’écologie des chytrides parasites de la cyanobactérie Anabaena macrospora, espèce filamenteuse présentant des proliférations massives et récurrentes dans le lac d’Aydat (France). Par la mise en place d’un schéma d’échantillonnage temporel et spatial à haute fréquence et prenant en compte deux années consécutives (2010 et 2011), nous avons pu (i) étudier les cycles de vie des deux espèces de parasites fongiques associées à A. macrospora, (ii) évaluer l’impact de ces parasites sur la dynamique de la population cyanobactérienne, et (iii) caractériser des facteurs contrôlant la dynamique des systèmes hôtes-parasites d’intérêt. De plus, (iv) afin de mettre en exergue les relations étroites existantes entre l’hôte et la production fongique associée, nous avons mis au point des protocoles méthodologiques permettant une analyse microscopique fine de l’hôte, des sporanges et de leur contenu en zoospores (fécondité des chytrides). Les résultats acquis mettent en évidence la coexistence de deux espèces de chytrides, Rhizosiphon crassum et R. akinetum, associées à A. macrospora. La reconstruction des cycles de vie par analyse d’images prises à partir d’échantillons naturels nous a permis de montrer que les deux chytrides présentaient une durée de leur cycle de vie similaire, et estimée à environ 3j. pour R. crassum. Cependant, ces deux espèces se différencient d’un point de vue fonctionnel en parasitant des types cellulaires distincts : R. crassum influence directement la biomasse active en parasitant les cellules végétatives, alors que R. akinetum parasite les cellules de résistances (akinètes) de A. macrospora. Cette dernière espèce peut donc avoir un impact sur le recrutement, la structure génétique et la variabilité interannuelle de la population hôte. Par ailleurs, outre leur impact direct, nous montrons que l’action des chytrides parasites peut aboutir à une fragmentation mécanique des filaments de A. macrospora, augmentant potentiellement la perte de biomasse cyanobactérienne par broutage. De plus, nous avons pu mettre en évidence que la production zoosporique des chytrides dépendait des ressources nutritives disponibles tant d’un point vue quantitatif (taille de l’hôte) que qualitatif (groupe phytoplanctonique parasité, métabolisme de l’hôte...). La réduction de la biomasse active de cyanobactéries, la fragmentation mécanique, ainsi que la production de zoospores dont la qualité nutritive pour le zooplancton a été démontrée, établissent les chytrides comme lien trophique entre les proliférations cyanobactériennes filamenteuses « inedible », considérée comme impasses trophiques, et les niveaux trophiques supérieurs. Enfin, l’ensemble des résultats acquis montre l’intérêt de prendre en compte, désormais, le rôle fonctionnel des champignons microscopiques parasites, notamment comme agents régulateurs direct et indirect du développement phytoplanctonique. Cette prise en compte permettrait, sans aucun doute, d’améliorer les modèles de transferts de matière et d’énergie qui transitent dans les écosystèmes aquatiques, dans le contexte général d’optimiser la gestion des plans d’eau. / Face to both the important anthropogenic input in nutrients and the global change, numerous authors predict that the cyanobacterial blooms will increase in relative abundance in aquatic ecosystems. An exhaustive knowledge of the driving biotic factors of the cyanobacterial dynamic is essential. In lakes, the most common fungal parasites of phytoplankton belong to the phylum Chytridyomycota (i.e. chytrids). The aim of the thesis was to investigate the fungal parasitism associated to the cyanobacterial blooms, particularly the ecology of chytrids parasitizing the filamentous cyanobacterial species Anabaena macrospora, in Lake Aydat (France). During two successive years (2010-2011), investigations on (i) the chytrid cycle of life of two chytrid species parasitizing A. macrospora, (ii) the impact of the fungal parasitism on the cyanobacterial bloom dynamic and (iii) driving factors of the host-parasite pairings dynamics have been led during two spatio-temporal surveys using high resolution sampling strategies. Moreover (iv) a double staining method based on a combination of CFW and SYTOX green for counting, identifying, and investigating the fecundity of phytoplankton fungal parasites and the putative relationships established between hosts and their fungal parasites has been developed. Results underlined the coexistence of two chytrids, Rhizosiphon crassum and R. akinetum, which have similar life cycles but differed in their infective regimes depending on the cellular niches offered by their host. R. crassum infected both vegetative cells and akinetes while R. akinetum infected only akinetes. A reconstruction of the developmental stages suggested that the life cycle of R.crassum was completed in about 3 days. By infecting akinetes, R. akinetum could reduce or modify the genetic structure of the cyanobacterial bloom of the following year. Furthermore, chytrids may reduce the length of filaments of Anabaena macrospora significantly by ‘‘mechanistic fragmentation’’ following infection. All these results suggest that chytrid parasitism is one of the driving factors involved in the decline of cyanobacterial blooms, by direct mortality of parasitized cells and indirectly by the mechanistic fragmentation, which could weaken the resistance of A. macrospora to grazing. Moreover, we underlined that the production of zoospore depends on the nutritional host quantity (host size) and quality (host phytoplanktonic group, host metabolism...). The decrease of the cyanobacterial active biomass, mechanistic fragmentation, and production of zoospores which exhibit a high nutritional quality for the zooplankton, established the chytrids as a real link between the inedible filamentous cyanobacteria, considered as trophic dead ends, and the higher trophic levels. Overall, we consider that the acquisition of our data places the chytrid parasitism as an important driving factor of the phytoplankton dynamic, allowing the inclusion of fungi and their main function (parasitism) in the energy and matter fluxes in the pelagic ecosystems.

O impacto causado pelo aumento da quantidade de substâncias químicas descartadas no meio ambiente está presente na maioria dos ecossistemas. Poluentes industriais contendo metais são frequentemente transportados para a água, o solo e o ar, podendo-se acumular nas cadeias tróficas e apresentar toxicidade para a biota. Em ambientes aquáticos, a biodisponibilidade e destino dessas substâncias xenobióticas podem ser influenciadas por vários fatores, entre eles a matéria orgânica dissolvida e outros compostos quelantes, os quais tem capacidade de aprisionar ou liberar íons para o ambiente. Os exopolissacarídeos, substâncias excretadas pelo fitoplâncton, podem interagir com diversas substâncias, interferindo na toxicidade dos compostos para os organismos ou comunidades biológicas aquáticas e, conseqüentemente, subestimando o verdadeiro valor tóxico das substâncias. Este trabalho foi conduzido para determinar a influência dos exopolissacarídeos da Clorofícea Pseudokirchniriella subcapitata, e da Cianofícea Anabaena spiroides na toxicidade dos elementos traço cádmio e cromo em Daphnia similis (Cladocera). Os metais foram escolhidos pela afinidade por quelantes orgânicos. Foram realizados testes ecotoxicológicos agudos para verificar a sensibilidade do cladócero Daphnia similis em diferentes concentrações dos metais cádmio (cloreto de cádmio) e cromo (dicromato de potássio), adicionando algas e exopolissacarídeos. Nos testes com os exopolissacarídeos foram utilizadas diferentes frações (excretado total, 10000D). Para obtenção das frações de exopolissacarídeos foram realizadas filtrações tangenciais em cartucho oco de celulose com bomba peristáltica. Nos testes com a adição das algas foram usados números conhecidos de células obtidas do concentrado de algas. Observou-se redução da toxicidade de 20 a 30% nos testes com a adição de excretado total, e reduções menores para as frações 10KD para as clorofícea e cianofícea. Com os resultados deste trabalho, foi possível avaliar a capacidade dessas substâncias em quelar e indisponibilizar compostos tóxicos e avaliar a toxicidade das substâncias quando testadas nos organismos planctônicos. / Chemical substances have been exerting increasing impact on ecosystems. Industrial pollutants containing metals frequently reach water bodies, soil, and air, wherein they may accumulate on the trophic chain, resulting in toxicity to the biota. In aquatic environments, the bioavailability and the destination of these xenobiotics are influence by several factors, such as the amount of solved organic matter and other chelating compounds, since these substances can either bind or liberate ions to the environment. Exopolysaccharides, in particular, are excreted by phytoplankton and, once in water, can interact with several substances altering the toxicity of compounds to aquatic organisms and biological communities. As a consequence, the real toxic potential of these xenobiotics is underestimated. The aim of this work was to determine the influence of the exopolysaccharides produced by the Chloroficeae Pseudokirchniriella subcapitata and by the Cyanophiceae Anabaena spiroides on the toxicity of the trace-elements cadmium and chromo over the primary consumer Daphnia similis. These metals were chosen due to their high affinity towards organic chelators. Tests for acute ecotoxicity were performed to verify the sensitivity of the Cladocera Daphnia similis exposed to different cadmium (cadmium chloride) and chromo (potassium dichromate) concentrations, with and without algae and/or exopolysaccharides. Three fractions of exopolysaccharides were tested (total excreted, < 1000D, and > 1000D), which were obtained by tangencial filtration through a cellulosis membrane using a peristaltical pump. On tests using algae, a fixed number of cells were obtained from an algae concentrate. The toxicity of Daphnia similis to the metals was 20-30% reduced when the total excreted from algae was added, while the reduction in toxicity was lower to the fraction than 10KD. The results demonstrated that exopolysaccharides chelate toxic compounds rendering them unavailable for exerting their effects on planktonic organisms.

O impacto causado pelo aumento da quantidade de substâncias químicas descartadas no meio ambiente está presente na maioria dos ecossistemas. Poluentes industriais contendo metais são frequentemente transportados para a água, o solo e o ar, podendo-se acumular nas cadeias tróficas e apresentar toxicidade para a biota. Em ambientes aquáticos, a biodisponibilidade e destino dessas substâncias xenobióticas podem ser influenciadas por vários fatores, entre eles a matéria orgânica dissolvida e outros compostos quelantes, os quais tem capacidade de aprisionar ou liberar íons para o ambiente. Os exopolissacarídeos, substâncias excretadas pelo fitoplâncton, podem interagir com diversas substâncias, interferindo na toxicidade dos compostos para os organismos ou comunidades biológicas aquáticas e, conseqüentemente, subestimando o verdadeiro valor tóxico das substâncias. Este trabalho foi conduzido para determinar a influência dos exopolissacarídeos da Clorofícea Pseudokirchniriella subcapitata, e da Cianofícea Anabaena spiroides na toxicidade dos elementos traço cádmio e cromo em Daphnia similis (Cladocera). Os metais foram escolhidos pela afinidade por quelantes orgânicos. Foram realizados testes ecotoxicológicos agudos para verificar a sensibilidade do cladócero Daphnia similis em diferentes concentrações dos metais cádmio (cloreto de cádmio) e cromo (dicromato de potássio), adicionando algas e exopolissacarídeos. Nos testes com os exopolissacarídeos foram utilizadas diferentes frações (excretado total, 10000D). Para obtenção das frações de exopolissacarídeos foram realizadas filtrações tangenciais em cartucho oco de celulose com bomba peristáltica. Nos testes com a adição das algas foram usados números conhecidos de células obtidas do concentrado de algas. Observou-se redução da toxicidade de 20 a 30% nos testes com a adição de excretado total, e reduções menores para as frações 10KD para as clorofícea e cianofícea. Com os resultados deste trabalho, foi possível avaliar a capacidade dessas substâncias em quelar e indisponibilizar compostos tóxicos e avaliar a toxicidade das substâncias quando testadas nos organismos planctônicos. / Chemical substances have been exerting increasing impact on ecosystems. Industrial pollutants containing metals frequently reach water bodies, soil, and air, wherein they may accumulate on the trophic chain, resulting in toxicity to the biota. In aquatic environments, the bioavailability and the destination of these xenobiotics are influence by several factors, such as the amount of solved organic matter and other chelating compounds, since these substances can either bind or liberate ions to the environment. Exopolysaccharides, in particular, are excreted by phytoplankton and, once in water, can interact with several substances altering the toxicity of compounds to aquatic organisms and biological communities. As a consequence, the real toxic potential of these xenobiotics is underestimated. The aim of this work was to determine the influence of the exopolysaccharides produced by the Chloroficeae Pseudokirchniriella subcapitata and by the Cyanophiceae Anabaena spiroides on the toxicity of the trace-elements cadmium and chromo over the primary consumer Daphnia similis. These metals were chosen due to their high affinity towards organic chelators. Tests for acute ecotoxicity were performed to verify the sensitivity of the Cladocera Daphnia similis exposed to different cadmium (cadmium chloride) and chromo (potassium dichromate) concentrations, with and without algae and/or exopolysaccharides. Three fractions of exopolysaccharides were tested (total excreted, < 1000D, and > 1000D), which were obtained by tangencial filtration through a cellulosis membrane using a peristaltical pump. On tests using algae, a fixed number of cells were obtained from an algae concentrate. The toxicity of Daphnia similis to the metals was 20-30% reduced when the total excreted from algae was added, while the reduction in toxicity was lower to the fraction than 10KD. The results demonstrated that exopolysaccharides chelate toxic compounds rendering them unavailable for exerting their effects on planktonic organisms.

L’azote est un des éléments les plus importants dans la nature. Sa disponibilité limite la productivité d’un grand nombre d’écosystèmes naturels, et influencera sans doute de manière importante leurs réponses aux changements climatiques globaux. La première source d’azote dans les écosystèmes non anthropisés est la fixation biologique de l’azote. Ce processus repose sur un groupe de métallo-enzymes spécifiques, les nitrogénases, dont le cofacteur métallique contient soit du fer et un atome de molybdène, soit du fer et un atome de vanadium, soit uniquement du fer. A ce jour, seule la nitrogénase au molybdène est prise en considération dans la dynamique de l’azote dans les écosystèmes, et ce malgré de nombreux indices indiquant que la nitrogénase au vanadium pourrait avoir un rôle important. Est-ce que la nitrogénase au vanadium est utilisée dans les écosystèmes naturels et quelles sont les conditions favorisant son utilisation ? Nous avons cherché à répondre à ces questions à l’aide d’un modèle symbiotique tripartite, un lichen, association entre une algue, un champignon et une cyanobactérie fixatrice d’azote. Nous avons tout d’abord développé une méthode d’étude des contenus en métaux des différents symbiontes, puis nous avons étudié la répartition et la régulation du vanadium au sein des différents symbiontes dans différentes conditions environnementales. Nous avons pu démontrer que dans ce modèle, le vanadium possède toutes les caractéristiques d’un micronutriment essentiel à la fixation d’azote. Nous avons également démontré que la disponibilité du molybdène ainsi que les températures, telles que rencontrées en milieux boréaux, seraient deux facteurs importants contrôlant l’utilisation de la V-Nase. Les résultats présentés dans cette étude apportent une meilleure compréhension de la gestion des métaux cofacteurs de la nitrogénase au sein de la symbiose lichénique. Mais ils permettent surtout de remettre en question le paradigme de l’hégémonie du molybdène sur la fixation biologique de l’azote. Ainsi, la fixation d’azote en milieu continental repose sur un ensemble hétérogène d’enzymes, ce qui autorise aux organismes fixateurs d’azote une grande flexibilité vis-à-vis des paramètres environnementaux comme les basses températures. Cela leurs permet également une meilleure adaptation au stress métallique résultant de carences en micronutriments, notamment celle en molybdène. Ces résultats invitent également à réévaluer les modèles biogéochimiques liant les cycles des micronutriments aux cycles des macronutriments, particulièrement celui de l’azote.

La fixation biologique d’azote réalisée par les cyanobactéries est un processus important dans les écosystèmes où la productivité est limitée par la disponibilité de l’azote. Dans des conditions de changements globaux, l’augmentation du CO2 atmosphérique pourrait engendrer une augmentation des taux de fixation d’azote permettant de combler la demande croissante d’azote nécessaire à la croissance. Cependant cette augmentation des taux de fixation d’azote pourrait dépendre des disponibilités en molybdène (Mo) et en phosphore (P), mais également être contrebalancée par l’azote minéral apporté via les dépositions atmosphériques. La complexité des systèmes naturels, notamment les caractéristiques de la symbiose cyanobactérie-végétaux supérieurs rends difficile l’étude de l’effet de l’augmentation du CO2 et des dépositions atmosphérique sur la fixation d’azote, dépendamment des disponibilités en Mo et P. Nous avons donc tenté de déterminer les effets de ces facteurs environnementaux (CO2 et apport d’azote minéral), en utilisant des cultures pures de cyanobactéries (Anabaena variabilis) dans différentes conditions de disponibilités en Mo et P. Cette approche expérimentale réductionniste nous a permis de mettre en évidence un effet significatif du P sur la concentration de la chlorophylle, indépendant des conditions environnementales (CO2, N) ainsi qu’une interaction entre le Mo et le P pour la variable chlorophylle, dans des conditions de CO2 augmenté sans apport d’azote. Cette interaction entre le Mo et le P pourrait être expliquée par un compromis entre l’acquisition du carbone, facilitée dans des conditions de CO2 augmentée, et la fixation biologique d’azote. Notre étude a également permis de mettre en évidence un effet significatif du P sur la croissance d’A. variabilis, dans des conditions de CO2 ambient et en présence d’azote. De façon plus spécifique, à haut P (2.5 10-5 M) le taux de croissance était plus faible (P = 0.02), alors que la quantité de chlorophylle et l’activité de la nitrogénase étaient toutes deux plus élevée (P = 0.002 et P = 0.015 respectivement). Cet effet du P pourrait être expliqué par une différence de stratégie de croissance de la cyanobactérie. A faible P, la cyanobactérie pourrait investir dans de courts filaments composés de cellules volumineuses, contrairement à des conditions de concentration en P élevées, où la cyanobactérie pourrait investir plus dans la formation d’hétérocystes et de filaments plus longs, composés de cellules plus petites. Cette étude apporte des pistes de réflexion quant aux stratégies permettant aux cyanobactéries filamenteuses de s’adapter à l’augmentation du CO2 atmosphérique et des dépositions azotées, en fonction des disponibilités en Mo et P. Un compromis entre l’acquisition de C et la fixation d’azote ou un changement de la stratégie de croissance des filaments pourrait permettre aux cyanobactéries filamenteuses d’être moins sensibles aux stress environnementaux (augmentation du CO2 atmosphérique et disponibilité des nutriments tels que l’azote).

Cyanobacteria are ancient prokaryotic organisms capable of performing oxygenic photosynthesis. An increase in the temporal and spatial distribution of cyanobacteria blooms worldwide has drawn considerable research attention in recent decades because of the health risks cyanobacteria pose to humans and wildlife through the production of cyanotoxins, interference with recreation, and ecosystem changes. A variety of hypotheses have sought to explain the increasing frequency and severity of cyanobacteria blooms around the world, with the relationship between cyanobacteria abundance and eutrophication receiving considerable attention. While the impacts of phosphorus concentration on cyanobacteria success are relatively well-studied, less is known about how nutrient stoichiometry and nitrogen uptake kinetics of different species contribute to cyanobacteria dominance. The underlying mechanism for the impacts of nitrogen to phosphorus (N:P) ratio and nitrogen form on cyanobacteria involves internal cycling of nitrogen within lakes and aspects of cyanobacteria cell physiology. The primary objective of this study was to assess the impacts of N:P ratios and nitrogen form on the growth of Microcystis aeruginosa and Anabaena flos-aquae in both axenic cultures and natural phytoplankton assemblages from Missisquoi Bay, Lake Champlain. A second objective was to determine whether treatment condition affected the production of the cyanotoxin microcystin. A final objective was to document the presence of benthic ammonium in Missisquoi Bay and the vertical migration of cyanobacteria throughout the water column in the bay, to provide evidence in support of the underlying mechanisms that might provide advantages to cyanobacteria in the bay. In laboratory culture experiments with M. aeruginosa and A. flos-aquae alone and in a mixed community, N:P ratios were varied between 5, 15, 30 and 45:1, and nitrogen was supplied as both nitrate and ammonium at each ratio. Triplicate samples were preserved after one, three and six days for cell enumeration using the standard Ütermohl method. Differences in density between initial and later times were used as an estimate of growth. Microcystin concentration was measured with the ELISA method. Weekly field sampling was conducted in the summer of 2006 in Missisquoi Bay to measure benthic nitrogen concentrations. Nocturnal sampling at varied depths in the bay was used to explore the vertical migration of cyanobacteria throughout the water column. There were weak associations between ammonium-nitrogen and M. aeruginosa growth and nitrate-nitrogen and A. flos-aquae growth, while the effects of N:P ratio on growth was highly variable across time and treatment condition. Ammonium-nitrogen was documented in the benthic water of Missisquoi Bay throughout the growing season, and M. aeruginosa dominated the vertical migration of cyanobacteria throughout the water column. The lack of clear trends visible within the data from laboratory experiments can be in part attributed to high variability of cell density within treatment conditions and the limitations of the methodology used for cell enumeration. Taken together these data suggest that the distribution of nitrogen within an aquatic system and the ability of M. aeruginosa to vertically migrate may contribute to the M. aeruginosa dominance of the summer phytoplankton community.

The Lower Karori Reservoir (LKR) is a small, monomictic lake of 2.34 ha situated in the Karori Wildlife Sanctuary (KWS), Wellington. Over the past decade cyanobacterial blooms have become a common occurrence in this water body. In 2005 Anabaena planktonica was detected for the first time in the LKR and this species now forms dense blooms during summer. These blooms are problematic as they reduce aesthetic appeal and have resulted in odour problems for visitors to this high profile wildlife sanctuary. The objectives of this study were to identify key physical, chemical and biological variables influencing phytoplankton dynamics in the LKR and to use ecological models to investigate plausible management options. The key parameters investigated, that may cause bloom formation were; summer stratification, high nutrient levels, and the food web effects of a large population of European perch (Perca fluviatilis). High resolution sampling was carried out every six hours over a 72 hour period during pre-bloom, bloom and post-bloom periods in 2006/7 to elucidate short term temporal and spatial variations in biological and physico-chemical parameters. Quantitative polymerase chain reaction (QPCR) was used to enumerate A. planktonica populations, allowing a large number of samples to be simultaneously evaluated. Algal densities were also estimated using conventional phytoplankton enumeration and chlorophyll a analysis. Water samples were collected for nutrient analysis at discrete depths and profiles were taken for temperature, dissolved oxygen and photosynthetic active radiation. Secchi depth and pH were also measured. Weekly or fortnightly phytoplankton and zooplankton samples and physical variables have been collected at LKR since September 2005 as part of an independent sampling program carried out by the KWS, Waikato University and Cawthron Institute. In this project the 2-year data set was used to assist with analysis of lake processes and for validation of the hydrodynamic-ecological model DYRESM-CAEDYM. Between 12 and 15 February, 2007, electric fishing was undertaken within the LKR. A total of 3,946 P. fluviatilis were removed and the effects on phytoplankton and zooplankton concentrations were investigated. To increase knowledge of the physiology of A. planktonica, laboratory experiments were undertaken using cultures subjected to a range of different light intensities and temperature regimes The phytoplankton assemblage of the LKR shows very distinct temporal variations. Summer stratification occurred in the LKR for ~4 months each summer. During these periods A. planktonica comprised up to 99.9% of the surface phytoplankton population. During isothermy chlorophytes, bacillariophytes and small flagellated dinophytes are co-dominant in the phytoplankton assemblage. The results of the QPCR showed distinct diurnal vertical movement of A. planktonica, with the highest cell concentrations occurring at 1900 hours at the surface. Ammonium (NH4-N) is the dominant species of inorganic nitrogen during periods of stratification, while nitrate (NO3-N) is generally dominant during times of isothermy. Phosphate concentrations at surface and depth remained at low levels throughout the sampling period. The large surface populations of A. planktonica, are probably responsible for the elevated total nitrogen concentrations in surface waters during stratified periods. There appeared to be some short term effects of the P. fluviatilis removal with an increase in large crustaceans (e.g., Daphnia sp.) and a reduction in A. planktonica densities observed in the months following the P. fluviatilis removal. Only a small proportion of the total P. fluviatilis population was removed and it is unlikely that the effects will be long-lasting without subsequent removal steps. However, it seems likely that P. fluviatilis is one of the factors contributing to cyanobacterial blooms and management of this fish species should be considered in future lake restoration plans. Growth experiments indicated A. planktonica grow over a wide range of light intensities and temperatures, although highest growth rates were generally associated with higher temperatures (25 C) and light intensities (60 - 140 μmol m-2 s-1). Ecological and hydrodynamic trends within the LKR over a two year period were simulated with adequate success using the model DYRESM-CAEDYM. Management scenarios simulated using DYRESM-CAEDYM suggest implementation of an artificial destratification system in the LKR may be the most practical and effective means of controlling A. planktonica blooms. The addition of an artificial aeration system emitting air at a rate of approximately 50 l-1 s-1 should result in an isothermal system. Without summer stratification some of the physiological features of A. planktonica (e.g., buoyancy regulation and nitrogen-fixation) that give it a competitive advantage over other phytoplankton species will be reduced.