Écologie et implications trophiques de la cyanobactérie Lyngbya wollei dans le fleuve Saint-Laurent

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.

Biomasses et compositions relatives des communautés de macroinvertébrés associées à différents types d'habitats au lac Saint-Pierre (Québec, Canada)

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

Integrating science through Bayesian Belief Networks : case study of Lyngbya in Moreton Bay

Bayesian Belief Networks (BBNs) are emerging as valuable tools for investigating complex ecological problems. In a BBN, the important variables in a problem are identified and causal relationships are represented graphically. Underpinning this is the probabilistic framework in which variables can take on a finite range of mutually exclusive states. Associated with each variable is a conditional probability table (CPT), showing the probability of a variable attaining each of its possible states conditioned on all possible combinations of it parents. Whilst the variables (nodes) are connected, the CPT attached to each node can be quantified independently. This allows each variable to be populated with the best data available, including expert opinion, simulation results or observed data. It also allows the information to be easily updated as better data become available ----- ----- This paper reports on the process of developing a BBN to better understand the initial rapid growth phase (initiation) of a marine cyanobacterium, Lyngbya majuscula, in Moreton Bay, Queensland. Anecdotal evidence suggests that Lyngbya blooms in this region have increased in severity and extent over the past decade. Lyngbya has been associated with acute dermatitis and a range of other health problems in humans. Blooms have been linked to ecosystem degradation and have also damaged commercial and recreational fisheries. However, the causes of blooms are as yet poorly understood.

From science to management : using Bayesian networks to learn about Lyngbya

Toxic blooms of Lyngbya majuscula occur in coastal areas worldwide and have major ecological, health and economic consequences. The exact causes and combinations of factors which lead to these blooms are not clearly understood. Lyngbya experts and stakeholders are a particularly diverse group, including ecologists, scientists, state and local government representatives, community organisations, catchment industry groups and local fishermen. An integrated Bayesian Network approach was developed to better understand and model this complex environmental problem, identify knowledge gaps, prioritise future research and evaluate management options.

A Bayesian network approach to modelling temporal behaviour of Lyngbya majuscula bloom initiation

Lyngbya majuscula is a cyanobacterium (blue-green algae) occurring naturally in tropical and subtropical coastal areas worldwide. Deception Bay, in Northern Moreton Bay, Queensland, has a history of Lyngbya blooms, and forms a case study for this investigation. The South East Queensland (SEQ) Healthy Waterways Partnership, collaboration between government, industry, research and the community, was formed to address issues affecting the health of the river catchments and waterways of South East Queensland. The Partnership coordinated the Lyngbya Research and Management Program (2005-2007) which culminated in a Coastal Algal Blooms (CAB) Action Plan for harmful and nuisance algal blooms, such as Lyngbya majuscula. This first phase of the project was predominantly of a scientific nature and also facilitated the collection of additional data to better understand Lyngbya blooms. The second phase of this project, SEQ Healthy Waterways Strategy 2007-2012, is now underway to implement the CAB Action Plan and as such is more management focussed. As part of the first phase of the project, a Science model for the initiation of a Lyngbya bloom was built using Bayesian Networks (BN). The structure of the Science Bayesian Network was built by the Lyngbya Science Working Group (LSWG) which was drawn from diverse disciplines. The BN was then quantified with annual data and expert knowledge. Scenario testing confirmed the expected temporal nature of bloom initiation and it was recommended that the next version of the BN be extended to take this into account. Elicitation for this BN thus occurred at three levels: design, quantification and verification. The first level involved construction of the conceptual model itself, definition of the nodes within the model and identification of sources of information to quantify the nodes. The second level included elicitation of expert opinion and representation of this information in a form suitable for inclusion in the BN. The third and final level concerned the specification of scenarios used to verify the model. The second phase of the project provides the opportunity to update the network with the newly collected detailed data obtained during the previous phase of the project. Specifically the temporal nature of Lyngbya blooms is of interest. Management efforts need to be directed to the most vulnerable periods to bloom initiation in the Bay. To model the temporal aspects of Lyngbya we are using Object Oriented Bayesian networks (OOBN) to create ‘time slices’ for each of the periods of interest during the summer. OOBNs provide a framework to simplify knowledge representation and facilitate reuse of nodes and network fragments. An OOBN is more hierarchical than a traditional BN with any sub-network able to contain other sub-networks. Connectivity between OOBNs is an important feature and allows information flow between the time slices. This study demonstrates more sophisticated use of expert information within Bayesian networks, which combine expert knowledge with data (categorized using expert-defined thresholds) within an expert-defined model structure. Based on the results from the verification process the experts are able to target areas requiring greater precision and those exhibiting temporal behaviour. The time slices incorporate the data for that time period for each of the temporal nodes (instead of using the annual data from the previous static Science BN) and include lag effects to allow the effect from one time slice to flow to the next time slice. We demonstrate a concurrent steady increase in the probability of initiation of a Lyngbya bloom and conclude that the inclusion of temporal aspects in the BN model is consistent with the perceptions of Lyngbya behaviour held by the stakeholders. This extended model provides a more accurate representation of the increased risk of algal blooms in the summer months and show that the opinions elicited to inform a static BN can be readily extended to a dynamic OOBN, providing more comprehensive information for decision makers.

Effects of temperature and light on the growth and geosmin production of Lyngbya kuetzingii (Cyanophyta)

The effects of temperature and light on the growth and geosmin production of Lyngbya kuetzingii were determined. Of the three temperatures tested, 10, 25 and 35A degrees C, the maximal geosmin concentration and geosmin productivity were yielded at 10A degrees C, while the highest chl a production was observed at 25A degrees C. In the studies on light intensity, the maximal geosmin concentration and geosmin productivity were observed at 10 mu mol m(-2) s(-1), while the highest chl a production was at 20 mu mol m(-2) s(-1). It was suggested that more geosmin was synthesized with lower chl a demand. Meanwhile, the relative amounts of extra- and intracellular geosmin were investigated. Under optimum growth conditions (20 mu mol m(-2) s(-1), 25A degrees C; BG-11 medium), the amounts of extracellular geosmin increased as the growth progressed and reached the maximum in the stationary phase, while the intracellular geosmin reached its maximum value in the late exponential phase, and then began to decline. However, under the low temperature (10A degrees C) or light (10 mu mol m(-2) s(-1)) conditions, more intracellular geosmin was synthesized and mainly accumulated in the cells. The proportions of extracellular geosmin were high, to 33.33 and 32.27%, respectively, during the stationary phase at 35A degrees C and 20 mu mol m(-2) s(-1). It was indicated that low temperature or light could stimulate geosmin production and favor the accumulation of geosmin in cells, while more intracellular geosmin may be released into the medium at higher temperatures or optimum light intensity.

Effects of phosphorus on the growth and nitrogen fixation rates of Lyngbya majuscula: implications for management in Moreton Bay, Queensland

Significant acetylene reduction and therefore N-2 fixation was observed for Lyngbya majuscula only during dark periods, which suggests that oxygenic photosynthesis and N-2 fixation are incompatible processes for this species. Results from a series of batch and continuous-flow-culture reactor studies showed that the specific growth rate and N-2 fixation rate of L, majuscula increased with phosphate (P-PO4) concentration up to a maximum value and thereafter remained constant. The P-PO4 concentrations corresponding to the maximum N-2 fixation and maximum growth rates were -0.27 and -0.18 muM respectively and these values are denoted as the saturation values for N-2 fixation and growth respectively. Regular monitoring studies in Moreton Bay, Queensland, show that concentrations Of P-PO4 generally exceed these saturation values over a large portion of the Bay and therefore, the growth of the bloom-forming L, majuscula is potentially maximised throughout much of the Bay by the elevated P-PO4 concentrations. Results from other studies suggest that the elevated P-PO4 concentrations in the Bay can be largely attributed to discharges from waste-water treatment plants (WWTPs), and thus it is proposed that the control of the growth of L. majuscula in Moreton Bay will require a significant reduction in the P load from the WWTP discharges. If the current strategy of N load reduction for these discharges is maintained in the absence of substantial P load reduction, it is hypothesised that the growth of L, majuscula and other diazotrophs in Moreton Bay will increase in the future.

Ecophysiology of the marine cyanobacterium, Lyngbya majuscula (Oscillatoriaceae) in Moreton Bay, Australia

Large blooms of the marine cyanobacterium Lyngbya majuscula in Moreton Bay, Australia (27 degrees 05'S, 153 degrees 08'E) have been re-occurring for several years. A bloom was studied in Deception Bay (Northern Moreton Bay) in detail over the period January-March 2000. In situ data loggers and field sampling characterised various environmental parameters before and during the L. majuscula bloom. Various ecophysiological experiments were conducted on L. majuscula collected in the field and transported to the laboratory, including short-term (2h) C-14 incorporation rates and long-term (7 days) pulse amplitude modulated (PAM) fluorometry assessments of photosynthetic capacity. The effects of L. majuscula on various seagrasses in the bloom region were also assessed with repeated biomass sampling. The bloom commenced in January 2000 following usual December rainfall events, water temperatures in excess of 24 degrees C and high light conditions. This bloom expanded rapidly from 0 to a maximum extent of 8 km(2) over 55 days with an average biomass of 210 g(dw)(-1) m(-2) in late February, followed by a rapid decline in early April. Seagrass biomass, especially Syringodium isoetifolium, was found to decline in areas of dense L. majuscula accumulation. Dissolved and total nutrient concentrations did not differ significantly (P > 0.05) preceding or during the bloom. However, water samples from creeks discharging into the study region indicated elevated concentrations of total iron (2.7-80.6 mu M) and dissolved organic carbon (2.5-24.7 mg L-1), associated with low pH values (3.8-6.7). C-14 incorporation rates by L. majuscula were significantly (P < 0.05) elevated by additions of iron (5 mu M Fe), an organic chelator, ethylenediaminetetra-acetic acid (5 mu M EDTA) and phosphorus (5 mu M PO4-3). Photosynthetic capacity measured with PAM fluorometry was also stimulated by various nutrient additions, but not significantly (P > 0.05). These results suggest that the L. majuscula bloom may have been stimulated by bioavailable iron, perhaps complexed by dissolved organic carbon. The rapid bloom expansion observed may then have been sustained by additional inputs of nutrients (N and P) and iron through sediment efflux, stimulated by redox changes due to decomposing L. majuscula mats. (c) 2004 Elsevier B.V. All rights reserved.

Blooms of the cyanobacterium Lyngbya majuscula in coastal Queensland, Australia: disparate sites, common factors

During the last decade there has been a significant rise in observations of blooms of the toxic cyanobacterium, Lyngbya majuscula along the east coast of Queensland, Australia. Whether the increase in cyanobacterial abundance is a biological indicator of widespread water quality degradation or also a function of other environmental change is unknown. A bioassay approach was used to assesses the potential for runoff from various land uses to stimulate productivity of L. majuscula. In Moreton Bay, L. majuscula productivity was significantly (p < 0.05) stimulated by soil extracts, which were high in phosphorus, iron and organic carbon. Productivity of L. majuscula from the Great Barrier Reef was also significantly (p < 0.05) elevated by iron and phosphorus rich extracts, in this case seabird guano adjacent to the bloom site. Hence, it is possible that other L. majuscula blooms are a result of similar stimulating factors (iron, phosphorus and organic carbon), delivered through different mechanisms. (c) 2004 Elsevier Ltd. All rights reserved.

The fate of Lyngbya majuscula toxins in three potential consumers

Blooms of Lyngbya majuscula have been reported with increasing frequency and severity in the last decade in Moreton Bay, Australia. A number of grazers have been observed feeding upon this toxic cyanobacterium. Differences in sequestration of toxic compounds from L. majuscula were investigated in two anaspideans, Stylocheilus striatus, Bursatella leachii, and the cephalaspidean Diniatys dentifer. Species fed a monospecific diet of L. majuscula had different toxin distribution in their tissues and excretions. A high concentration of lyngbyatoxin-a was observed in the body of S. striatus (3.94 mg/kg(-1)) compared to bodily secretions (ink 0.12 mg/kg- 1; fecal matter 0.56 mg/kg(-1); eggs 0.05 mg/kg(-1)). In contrast, B. leachii secreted greater concentrations of lyngbyatoxin-a (ink 5.41 mg/kg(-1); fecal matter 6.71 mg/kg(-1)) than that stored in the body (2.24 mg/kg(-1)). The major internal repository of lyngbyatoxin-a and debromoaplysiatoxin was the digestive gland for both S. striatus (6.31 +/- 0.31 mg/kg(-1)) and B. leachii (156.39 +/- 46.92 mg/kg(-1)). D. dentifer showed high variability in the distribution of sequestered compounds. Lyngbyatoxin-a was detected in the digestive gland (3.56 +/- 3.56 mg/kg(-1)) but not in the head and foot, while debromoaplysiatoxin was detected in the head and foot (133.73 +/- 129.82 mg/kg(-1)) but not in the digestive gland. The concentrations of sequestered secondary metabolites in these animals did not correspond to the concentrations found in L. majuscula used as food for these experiments, suggesting it may have been from previous dietary exposure. Trophic transfer of debromoaplysiatoxin from L. majuscula into S. striatus is well established; however, a lack of knowledge exists for other grazers. The high levels of secondary metabolites observed in both the anaspidean and the cephalapsidean species suggest that these toxins may bioaccumulate through marine food chains.