The Orange River is an important source of freshwater and like many other wetlands in semi-arid regions, supports various social (Spurgeon, 1998), economic (Spurgeon, 1998; Bornman et al., 2005) and ecological functions. The saltmarsh at the Orange River Mouth has become degraded over time following numerous anthropogenic impacts. As a result the Transboundary RAMSAR site was placed on the Montreux record emphasising the importance for rehabilitation. The potential of the marsh for natural rehabilitation was assessed through three physical factors which were considered to have the most influence on the saltmarsh i.e. 1) sediment 2) water 3) wind. Three sampling areas were chosen to investigate the sediment characteristics of the ORM saltmarsh and the suitability for seed germination and adult survival. Site A was representative of the general marsh area, Site B was thought to have favourable sediment conditions for saltmarsh growth because of the large numbers of seedlings and Site C was prone to inundation by wind blown sediment. The sites were sampled in 2005 (dry conditions) and in 2006 after high rainfall and river flooding. Electrical conductivity (EC) of the sediment throughout the marsh was hypersaline in many instances above the tolerance range for S. pillansii (> 80 mS.cm-1) The freshwater event in 2006 lowered salinity significantly in two of the three sites. Differences in sediment characteristics were also compared for three habitats i.e. driftlines, open sites and under vegetation. Driftlines (C. coronopifolia = 872 seedlings m-2; S. pillansii = 1296 seedlings m-2) and the microhabitat associated with adult plants (C. coronopifolia = 803 seedlings m-2; S. pillansii = 721 seedlings m-2) created favourable conditions for seedling growth, however open unvegetated (C. coronopifolia = 56 seedlings m-2; S. pillansii = 49 seedlings m-2) areas had significantly lower seedling density. Due to the marsh currently being in a desertified state this study aimed to establish whether the remaining vegetation could produce enough seed to revegetate the marsh. Laboratory studies indicated that seeds of both species germinated best in freshwater (0 psu). The germination of S. pillansii seeds was 40 percent at 0 psu compared to 5 percent at 35 psu. After storage under hypersaline conditions (35 psu) C. coronopifolia showed 100 percent seed germination when returned to freshwater whereas storage at 70 psu decreased the viability of S. pillansii seeds. The plants are producing adequate seed that will allow for regrowth and rehabilitation if sediment and groundwater characteristics are suitable for seed germination, seedling growth and adult survival. However the increase in bare areas at the Orange River mouth as a result of salt marsh dieback has increased the available sediment source. The wind blown sediment has covered large areas of the remaining adult salt marsh vegetation, particularly in the northern corner at Site C, causing further die-back.

Os sistemas lóticos são considerados sistemas abertos e de fluxo unidirecional de energia. Nestes sistemas a intensa relação com o meio terrestre adjacente faz com que as condições ecológicas dos rios sejam um reflexo das transformações ocorridas em sua bacia de drenagem. O objetivo do presente estudo foi traçar um perfil dos aspectos ecológicos do rio Uberaba-MG, em função dos usos preponderantes da bacia hidrográfica. Foram avaliadas as características limnológicas, a comunidade de macroinvertebrados bentônicos e o estado de conservação da área de entorno do rio. Procurou-se ainda avaliar a evolução espacial e a variação temporal das variáveis mensuradas à luz de teorias ecológicas de sistemas lóticos. Os resultados mostraram que no rio Uberaba três zonas com impactos preponderantes, diferentes, devem ser avaliadas com maior cautela: a) antes do município de Uberaba existe a predominância de atividades agrícolas, que promovem impactos pela entrada contínua de sedimentos e oferecem riscos pela entrada de agrotóxicos e fertilizantes, b) abaixo do município de Uberaba, a entrada de efluentes gera acentuada degradação da qualidade da água e o conseqüente perecimento das comunidades biológicas, até a região próxima ao município de Veríssimo, e c) a região compreendida entre os municípios de Veríssimo, Conceição das Alagoas e Planura, pelo crescente risco de degradação da qualidade da água decorrente da entrada de efluentes e do aumento do desmatamento nas margens do rio Uberaba e afluentes. Discute-se ainda a necessidade de serem aprimoradas teorias ecológicas para sistemas lóticos tropicais, com o intuito de aumentar a capacidade de predição sobre este sistema e remeter estratégias de conservação mais eficientes aos gestores ambientais. / Lotic systems are considered opened and one-way energy flux systems. In these cases, the intense relationship with adjacent terrestrial environment renders rivers ecological conditions a reflex of transformations occurred at its watershed. The present study aimed to profile an Uberaba river ecological features as a function of the watershed main uses. Limnological characteristic, benthic macroinvertebrates community and conditions at river nearby area were evaluated. Based on the lotic systems ecological theories, it was attempted to evaluate the measured variable space evolution and temporal variation. Results showed that in Uberaba river, three zones with different impacts should be carefully evaluated: a) before Uberaba city, agricultural activities predominates, promoting impacts due to the continuous sediment input, and offering risks of pesticide and fertilizers contribuition; b) below Uberaba city, wastewater effluent generates high water quality degradation, resulting in biological communities decay until the area closed to Veríssimo City; and c) at the region embracing Veríssimo, Conceição das Alagoas and Planura cities, in function of the growing water quality degradation risk due to effluents entrance and deforesting increase at the margins of Uberaba river and tributaries. The needs of improved ecological theories for tropical lotic systems are also discussed, aiming to increase the prediction capacity on this lotic systems and to transmit more efficient conservation strategies to environmental managers.

Understanding energy fluxes within and between floodplain water bodies, and between rivers and their floodplains is essential for comprehending the dynamics of modern, altered river systems. Floodplain aquatic invertebrates may move between habitats deliberately (through emergence and dispersal) or through passive transport during flooding. This movement may represent a significant flux of energy and an essential food web subsidy. I assessed the usefulness of the stable isotopes of hydrogen and oxygen (D and 18O respectively) for identifying the origins and movements of macroinvertebrates in Mississippi River floodplain water bodies. I sampled water and invertebrates from the Mississippi River, intermittent and permanent floodplain wetlands, and tributaries during 2007 and 2008. Results showed consistent relationships between δD and δ18O signatures in invertebrate tissues and their home water bodies. I also investigated whether δD and δ18O could be used as a multivariate "fingerprint" to trace a captured invertebrate back to its environment of origin. Results showed that δD can be a useful tracer of the movement of floodplain invertebrates in some cases, although δ18O is likely not suitable for that purpose.

Responses of phytoplankton productivity in the Red River to unbleached pulp and paper mill wastewaters were monitored using in situ ^14C incubation. Preoperational studies, conducted prior to the discharge of mill wastewaters varied seasonally, but revealed similar productivity trends when compared with postoperational studies, conducted after mill discharges began entering the Red River. Carbon assimilation rates measured downstream of mill discharge were generally greater than upstream levels in both preoperational and postoperational studies. Selected physical, chemical, and biological parameters varied seasonally, but showed similar upstream-downstream values and preoperational-postoperational values. Total Organic Carbon (TOC), Dissolved Organic Carbon (DOC), and Biochemical Oxygen Demand (BOD_5) were positively correlated with postoperational productivity rates. Apparent color was negatively correlated with productivity rates.

A bacia do rio Betari, localizada em uma das poucas áreas ainda preservadas do estado de São Paulo, apresenta características particulares quanto a sua geologia e geomorfologia (carste) e a Mata Atlântica, ainda preservada. Também é importante o fluxo de turistas que tem aumentado, provocando uma ocupação sem planejamento. Parte da bacia é ocupada pelo Parque Equatorial Estadual Turístico do Alto Ribeira - PETAR. Com o objetivo de caracterizar o rio Betari e sua bacia hidrográfica, relacionar as características limnológicas do rio com o uso e ocupação do solo e determinar os impactos do turismo no bairro da Serra, para obter um diagnóstico ambiental da bacia e fornecer subsídios para seu desenvolvimento sustentável, foram feitas coletas em quatro estações do ano, em 14 pontos ao longo do rio. Nesses pontos, foram analisados o sedimento e as características hidráulicas, físicas e químicas e biológicas da água. Também foram utilizados sedimento remoto e geoprocessamento para integração das informações sobre a bacia hidrográfica e o corpo d\'água. Foram, ainda realizadas análises estatísticas para a determinação de grupos, entre os pontos de coleta, com similaridades ou descontinuidades quanto aos dados obtidos. Um estudo dos impactos econômicos e sócio-culturais foi realizado no bairro Serra, principal agrupamento humano na bacia. Os resultados mostram a existência de uma homogeneidade espacial dos parâmetros físicos e químicos analisados no rio. As características geológicas da bacia (carste) parecem exercer grande influência nos parâmetros hidrológicos analisados. A qualidade da água é boa, demonstrando que a ação antrópica ainda não comprometeu o sistema lótico, apesar da existência de metais pesados no sedimento reflexo dos rejeitos das atividades minerárias presentes na área. A bacia hidrográfica, por sua vez, não apresentou grandes alterações na sua cobertura vegetal nos últimos anos. A comunidade bentônica apresentou variação temporal e espacial e, aparentemente, os fatores hidráulicos e as características do canal foram os principais determinantes destas variações. Os impactos econômicos e sócio-culturais do turismo, positivos e negativos, foram detectados sobre a população residente. Aparentemente, a comunidade Serrana passa por um período de euforia, graças aos benefícios econômicos propiciados pela atividade turística, porém a falta de planejamento pode comprometer a atividade e os ecossistemas regionais, impedindo assim, o desenvolvimento sustentável. / The Betari River basin, localized in one of the few areas still preserved in the São Paulo Sate, presents particular characteristics regarding its geology and geomorphology and the Atlantic Forest, still preserved. The increasing flow of tourists is also important, provoking a plan less occupation of the area. Part of the basin is occupied by the Parque Estadual Turístico do Alto Ribeira - PETAR. With the objective of characterizing the Betari River and its hydrographic basin, relate the river\'s limnologic characteristics with the use and occupation of the land and determine the impact of tourism in the Serra borough, to obtain an environmental diagnostic of the basin and supply subsidiaries for its sustainable development, samples were collected in 4 seasons of the year, in 14 points throughout the river. In these points, sedimentation and hydraulic, physical, chemical and biological characteristics of the water were studied. Remote sensoring and geo-processing were also used to integrate information about the hydrographic basin and the body of water. Statistical analyses were then carried out to determine groups. A study of the economic and social-cultural impact was carried out in the Serra borough, the main human settlement in the basin. The results show an existence of spatial homogeneity in the physical and chemical parameters analyzed in the river. The geological characteristics of the basin seem to have a great influence in the hydrological parameters analyzed. The quality of the water is good, showing that the antropic action has not yet compromised the lotic system, despite the existence of heavy metals in sediment, a reflex of the rejects of mineral activities present in the area. The hydrographic basin did not present great changes in its vegetation in the last few years. The bentonic community presented temporal and special variation and, apparently, the hydraulic factors and the canal\'s characteristics were the main determiners of these variations. The economical and social-cultural impacts of tourism, positive or negative, were detected on the residing population. Apparently, the Serrana community passes through a euphoric period, thanks to the economical benefits brought by the tourist activity; however, the lack of planning can compromise the activity and these regional ecosystems, impeding, this way the sustainable development.

A bacia do rio Betari, localizada em uma das poucas áreas ainda preservadas do estado de São Paulo, apresenta características particulares quanto a sua geologia e geomorfologia (carste) e a Mata Atlântica, ainda preservada. Também é importante o fluxo de turistas que tem aumentado, provocando uma ocupação sem planejamento. Parte da bacia é ocupada pelo Parque Equatorial Estadual Turístico do Alto Ribeira - PETAR. Com o objetivo de caracterizar o rio Betari e sua bacia hidrográfica, relacionar as características limnológicas do rio com o uso e ocupação do solo e determinar os impactos do turismo no bairro da Serra, para obter um diagnóstico ambiental da bacia e fornecer subsídios para seu desenvolvimento sustentável, foram feitas coletas em quatro estações do ano, em 14 pontos ao longo do rio. Nesses pontos, foram analisados o sedimento e as características hidráulicas, físicas e químicas e biológicas da água. Também foram utilizados sedimento remoto e geoprocessamento para integração das informações sobre a bacia hidrográfica e o corpo d\'água. Foram, ainda realizadas análises estatísticas para a determinação de grupos, entre os pontos de coleta, com similaridades ou descontinuidades quanto aos dados obtidos. Um estudo dos impactos econômicos e sócio-culturais foi realizado no bairro Serra, principal agrupamento humano na bacia. Os resultados mostram a existência de uma homogeneidade espacial dos parâmetros físicos e químicos analisados no rio. As características geológicas da bacia (carste) parecem exercer grande influência nos parâmetros hidrológicos analisados. A qualidade da água é boa, demonstrando que a ação antrópica ainda não comprometeu o sistema lótico, apesar da existência de metais pesados no sedimento reflexo dos rejeitos das atividades minerárias presentes na área. A bacia hidrográfica, por sua vez, não apresentou grandes alterações na sua cobertura vegetal nos últimos anos. A comunidade bentônica apresentou variação temporal e espacial e, aparentemente, os fatores hidráulicos e as características do canal foram os principais determinantes destas variações. Os impactos econômicos e sócio-culturais do turismo, positivos e negativos, foram detectados sobre a população residente. Aparentemente, a comunidade Serrana passa por um período de euforia, graças aos benefícios econômicos propiciados pela atividade turística, porém a falta de planejamento pode comprometer a atividade e os ecossistemas regionais, impedindo assim, o desenvolvimento sustentável. / The Betari River basin, localized in one of the few areas still preserved in the São Paulo Sate, presents particular characteristics regarding its geology and geomorphology and the Atlantic Forest, still preserved. The increasing flow of tourists is also important, provoking a plan less occupation of the area. Part of the basin is occupied by the Parque Estadual Turístico do Alto Ribeira - PETAR. With the objective of characterizing the Betari River and its hydrographic basin, relate the river\'s limnologic characteristics with the use and occupation of the land and determine the impact of tourism in the Serra borough, to obtain an environmental diagnostic of the basin and supply subsidiaries for its sustainable development, samples were collected in 4 seasons of the year, in 14 points throughout the river. In these points, sedimentation and hydraulic, physical, chemical and biological characteristics of the water were studied. Remote sensoring and geo-processing were also used to integrate information about the hydrographic basin and the body of water. Statistical analyses were then carried out to determine groups. A study of the economic and social-cultural impact was carried out in the Serra borough, the main human settlement in the basin. The results show an existence of spatial homogeneity in the physical and chemical parameters analyzed in the river. The geological characteristics of the basin seem to have a great influence in the hydrological parameters analyzed. The quality of the water is good, showing that the antropic action has not yet compromised the lotic system, despite the existence of heavy metals in sediment, a reflex of the rejects of mineral activities present in the area. The hydrographic basin did not present great changes in its vegetation in the last few years. The bentonic community presented temporal and special variation and, apparently, the hydraulic factors and the canal\'s characteristics were the main determiners of these variations. The economical and social-cultural impacts of tourism, positive or negative, were detected on the residing population. Apparently, the Serrana community passes through a euphoric period, thanks to the economical benefits brought by the tourist activity; however, the lack of planning can compromise the activity and these regional ecosystems, impeding, this way the sustainable development.

Made available in DSpace on 2015-03-26T13:30:53Z (GMT). No. of bitstreams: 1 texto completo.pdf: 986817 bytes, checksum: e4ef3e7967a1c4ddb759b319e3255b70 (MD5) Previous issue date: 2006-07-06 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The longitudinal distribution of adult Odonata was examined in streams of different channel width in the Pindaíba River Basin, in the municipalities of Barra do Garças and Nova Xavantina. The general purpose was to evaluate the existence of environmental gradients that affect the longitudinal distribution of Odonata, and to establish testable predictions to this predator group regarding the River Continuum Concept. Two hypotheses were tested: (1) increasing channel width longitudinally along the basin causes an increase of light input, and, assuming restraints and distinct abilities, there would be a decrease of Zygoptera and increase of Anisoptera species richness. (2) Dragonflies are affected by the gradient generated through river continuum mechanisms, increasing species richness in medium-sized streams. I sampled 19 sites in rivers and streams from 1st to 6th orders, and in each site I sample once in the dry and once in the rainy seasons. Quantitative survey was conducted through scan method in fixed areas, counting visually on Odonata adults along 100 meters of the waterbody, divided into 20 stretches of 5 meters each. Channel width and depth measures were taken at the beginning, middle, and end of each 20-meter region. A total of 549 individuals were collected, distributed in one family, 13 genera and 17 species of Anisoptera, and six families, 15 genera and 30 species of Zygoptera. The abundance and proportion of Zygoptera species decrease while Anisoptera increase with channel width and mean depth of rivers and streams. The channel width was considered the best predictor of Odonata species distribution. The distribution of Odonata, species richness, did not corroborate the hypotheses of higher species richness in the middle courses of streams. The results obtained in this study confirm the thermoregulation hypothesis as a determining factor in the distribution of Odonata species in the system. / A distribuição longitudinal de Odonata adultos foi avaliada em riachos de diferentes larguras na Bacia do Rio Pindaíba nos municípios de Barra do Garças e Nova Xavantina-MT. O objetivo geral foi avaliar a existência de gradientes ambientais influenciando a distribuição longitudinal de Odonata, estabelecendo predições testáveis para o grupo predador sobre o Conceito de Continuum fluvial. Foram testadas duas hipóteses: (1) o aumento da largura do canal longitudinalmente na bacia causa um aumento da entrada de luz e, assumindo restrições e habilidades distintas, haveria uma diminuição da riqueza de espécies de Zygoptera e aumento da riqueza de Anisoptera. (2) as libélulas respondem ao gradiente gerado pelo mecanismo do continuum fluvial, aumentando de riqueza nos riachos de tamanho intermediário. Foram amostrados 19 trechos em oito rios e riachos de 1a a 6a ordem, sendo que em cada um dos trechos foram efetuadas duas coletas, uma na estação seca e outra na estação chuvosa. A amostragem quantitativa foi feita pelo método de varredura com áreas fixas que consistiu na contagem visual do número de adultos em 100 metros do corpo d água, divididos em 20 segmentos de cinco metros. As medidas de largura e profundidade do canal foram tomadas no início, meio e final de cada segmento de 20 metros. Um total de 549 indivíduos foi coletado, pertencentes a uma família, 13 gêneros e 17 espécies de Anisoptera, e seis famílias, 15 gêneros e 30 espécies de Zygoptera. A abundância e a proporção de espécies de Zygoptera diminuem enquanto as de Anisoptera aumentam em relação à largura e a profundidade média dos rios e riachos. A largura foi considerada o melhor preditor da distribuição de espécies de Odonata. A distribuição de Odonata, considerando agregadamente Zygoptera e Anisoptera, não corrobora a hipótese de maior riqueza nos trechos médios dos riachos. Os resultados desse estudo suportaram a hipótese de termorregulação como fator determinante da distribuição das espécies de Odonata no sistema.

Thesis (MTech (Nature Conservation))--Cape Peninsula University of Technology, 2010. / Many urban rivers are heavily engineered and no longer function naturally. The City of Cape Town has designed and implemented many stormwater and river management projects. Very little monitoring has occurred as to whether these engineering projects and remediation measures, have had a positive effect on our urban river ecosystem function. The study investigated the influence of specific engineering interventions such as the placement or rocks in stream to create weirs, gabion lining of stream channels, removal of canal walls, establishment of artificial wetlands, and approaches to urban river management, on river ecosystem function of the Keysers River, Little Lotus River, Langevlei Canal, Silvermine River, Moddergat River and Big Lotus River, as measured by specific indices including water chemistry and aquatic community structure (macroinvertebrates and diatoms).

Les proliférations nuisibles de la cyanobactérie filamenteuse benthique Lyngbya wollei qui forme des tapis déposés sur les sédiments ont augmenté en fréquence au cours des 30 dernières années dans les rivières, lacs et sources de l'Amérique du Nord. Lyngbya wollei produit des neurotoxines et des composés organiques volatils (géosmin, 2-méthylisobornéol) qui ont des répercussions sur la santé publique de même que des impacts d'ordre socioéconomiques. Cette cyanobactérie est considérée comme un habitat et une source de nourriture de piètre qualité pour les invertébrés en raison de sa gaine robuste et de sa production de toxines. Les proliférations de L. wollei ont été observées pour la première fois en 2005 dans le fleuve Saint-Laurent (SLR; Québec, Canada). Nous avons jugé important de déterminer sa distribution sur un tronçon de 250 km afin d'élaborer des modèles prédictifs de sa présence et biomasse en se basant sur les caractéristiques chimiques et physiques de l'eau. Lyngbya wollei était généralement observé en aval de la confluence de petits tributaires qui irriguent des terres agricoles. L’écoulement d’eaux enrichies à travers la végétation submergée se traduisait par une diminution de la concentration d’azote inorganique dissous (DIN), alors que les concentrations de carbone organique dissous (DOC) et de phosphore total dissous (TDP) demeuraient élevées, produisant un faible rapport DIN :TDP. Selon nos modèles, DOC (effet positif), TP (effet négatif) et DIN :TDP (effet négatif) sont les variables les plus importantes pour expliquer la répartition de cette cyanobactérie. La probabilité que L. wollei soit présent dans le SLR a été prédite avec exactitude dans 72 % à 92 % des cas pour un ensemble de données indépendantes. Nous avons ensuite examiné si les conditions hydrodynamiques, c'est-à-dire le courant généré par les vagues et l'écoulement du fleuve, contrôlent les variations spatiales et temporelles de biomasse de L. wollei dans un grand système fluvial. Nous avons mesuré la biomasse de L. wollei ainsi que les variables chimiques, physiques et météorologiques durant trois ans à 10 sites le long d'un gradient d'exposition au courant et au vent dans un grand (148 km2) lac fluvial du SLR. L'exposition aux vagues et la vitesse du courant contrôlaient les variations de biomasses spatiales et temporelles. La biomasse augmentait de mai à novembre et persistait durant l'hiver. Les variations interannuelles étaient contrôlées par l'écoulement de la rivière (niveau d'eau) avec la crue printanière qui délogeait les tapis de l'année précédente. Les baisses du niveau d'eau et l'augmentation de l'intensité des tempêtes anticipées par les scénarios de changements climatiques pourraient accroître la superficie colonisée par L. wollei de même que son accumulation sur les berges. Par la suite, nous avons évalué l'importance relative de L. wollei par rapport aux macrophytes et aux épiphytes. Nous avons examiné l'influence structurante de l'échelle spatiale sur les variables environnementales et la biomasse de ces producteurs primaires (PP) benthiques. Nous avons testé si leur biomasse reflétait la nature des agrégats d'habitat basées sur l'écogéomorphologie ou plutôt le continuum fluvial. Pour répondre à ces deux questions, nous avons utilisé un design à 3 échelles spatiales dans le SLR: 1) le long d'un tronçon de 250 km, 2) entre les lacs fluviaux localisés dans ce tronçon, 3) à l'intérieur de chaque lac fluvial. Les facteurs environnementaux (conductivité et TP) et la structure spatiale expliquent 59% de la variation de biomasse des trois PP benthiques. Spécifiquement, les variations de biomasses étaient le mieux expliquées par la conductivité (+) pour les macrophytes, par le ratio DIN:TDP (+) et le coefficient d'extinction lumineuse (+) pour les épiphytes et par le DOC (+) et le NH4+ (-) pour L. wollei. La structure spatiale à l'intérieur des lacs fluviaux était la plus importante composante spatiale pour tous les PP benthiques, suggérant que les effets locaux tels que l'enrichissement par les tributaire plutôt que les gradients amont-aval déterminent la biomasse de PP benthiques. Donc, la dynamique des agrégats d'habitat représente un cadre général adéquat pour expliquer les variations spatiales et la grande variété de conditions environnementales supportant des organismes aquatiques dans les grands fleuves. Enfin, nous avons étudié le rôle écologique des tapis de L. wollei dans les écosystèmes aquatiques, en particulier comme source de nourriture et refuge pour l'amphipode Gammarus fasciatus. Nous avons offert aux amphipodes un choix entre des tapis de L. wollei et soit des chlorophytes filamenteuses ou un tapis artificiel de laine acrylique lors d'expériences en laboratoire. Nous avons aussi reconstitué la diète in situ des amphipodes à l'aide du mixing model (d13C et δ15N). Gammarus fasciatus choisissait le substrat offrant le meilleur refuge face à la lumière (Acrylique>Lyngbya=Rhizoclonium>Spirogyra). La présence de saxitoxines, la composition élémentaire des tissus et l'abondance des épiphytes n'ont eu aucun effet sur le choix de substrat. Lyngbya wollei et ses épiphytes constituaient 36 et 24 % de l'alimentation in situ de G. fasciatus alors que les chlorophytes, les macrophytes et les épiphytes associées représentaient une fraction moins importante de son alimentation. Les tapis de cyanobactéries benthiques devraient être considérés comme un bon refuge et une source de nourriture pour les petits invertébrés omnivores tels que les amphipodes. / Harmful proliferations of the filamentous cyanobacterium L. wollei forming conspicuous benthic mats on the bottom sediment have been reported with increasing frequency in the last 30 years in rivers, lakes, and springs in North America. It is a known producer of neurotoxins and volatile organic compounds (geosmin, 2-methylisoborneol) thus exerting socioeconomic and public health impacts. Lyngbya wollei is also considered a poor nutritional source for invertebrates because of its robust sheath and toxin production. Proliferation of L. wollei in St. Lawrence River (SLR; Quebec, Canada) was first noticed in 2005. We deemed important to determine its distribution over a 250 km stretch of the SLR to elaborate predictive models of its presence and biomass based on chemical and physical characteristics. Lyngbya wollei was generally found downstream of the inflow tributaries draining farmlands. As enriched waters flowed slowly through submerged vegetation, dissolved inorganic nitrogen (DIN) concentration dropped but dissolved organic carbon (DOC) and total dissolved phosphorus (TDP) remained high, leading to a low DIN:TDP ratio. Models identified DOC (positive effect), TP (negative effect), and DIN:TDP (negative effect) as the most important variables explaining L. wollei distribution. The risk of L. wollei occurrence in the SLR was correctly forecasted in 72%-92% of all cases with an independent data set. We then examined if hydrodynamic conditions, namely currents generated by waves and river flow, control spatial and temporal variations of L. wollei biomass in a large river system. We measured L. wollei biomass together with meteorological, physical, and chemical variables over three years at 10 sites along a gradient of exposure to current and wind in a large (148 km2) fluvial lake of SLR. Wave exposure and current velocity controlled spatial and temporal biomass variations. Biomass increased from May to November and persisted during winter. Interannual variations were primarily controlled by river flow (water level) with spring discharge dislodging mats from the previous year. As anticipated under climate change scenarios, drops in water level and rising storm intensity may lead to an increase in the areas colonized by L. wollei, together with more frequent episodes of mat disruption and beach fouling. Additionally, we evaluated the relative importance of L. wollei with respect to macrophytes and epiphytes. We assessed the influence of the spatial scale in structuring environmental variables and biomass of these benthic primary producers (PP). We also test to which extent their biomass reflected the nature of patches based on ecogeomorphology or the river continuum. To address these two questions, we used a nested design at 3 spatial scales within the SLR: 1) along a 250-km-long upstream-downstream river stretch, 2) among three fluvial lakes located within that river stretch and 3) within each fluvial lake. Environmental factors (conductivity and TP) and spatial structure together explained 59% of the variability in biomass of all three benthic PP. Spatial variability of biomass was best explained by conductivity (+) for macrophytes, DIN:TDP ratio (+) and water extinction coefficient (+) for epiphytes and DOC (+) and NH4+ (-) for L. wollei mats. Within-lake structure was the most important spatial component for all benthic PP, suggesting that local effects, such as enrichment by the inflow of tributaries, rather than upstream-downstream gradients, determined the biomass and composition of benthic PP. Therefore patch dynamics represents a general framework which adequately covers the spatial variability and wide variety of environmental conditions experienced by aquatic organisms found in large rivers. Finally, we investigated the ecological role of L. wollei mats in aquatic ecosystems, especially as a food source and shelter for the amphipod Gammarus fasciatus. We offered amphipods a choice between mats of L. wollei and either chlorophytes or an artificial mat made of acrylic wool in laboratory experiment. Moreover, we reconstructed in situ amphipod diet using mixing model (δ13C and δ15N). Gammarus fasciatus selected the substratum offering the best light refuge (Acrylic > Lyngbya = Rhizoclonium > Spirogyra). Presence of saxitoxins, tissue elemental composition and epiphyte abundance had no significant effect on substratum choice. Lyngbya wollei and its epiphytes constituted 36 and 24% of the in situ diet of G. fasciatus whereas chlorophytes, macrophytes and associated epiphytes represented a less important fraction of its diet. Benthic cyanobacterial mats should be considered a good shelter and food source for small omnivorous invertebrates such as amphipods.