Thesis
Reference
Description and modeling of phytoplankton with an emphasis of cyanobacteria in deep peri-Alpine lakes under warmer climatic
conditions
GALLINA, Nicole
Abstract
The aim was to investigate the impacts of climate change on the behavior of phytoplankton, with a special emphasis on harmful cyanobacteria in the peri-Alpine region. It was hypothesized that in this highly sensitive region more important episodes of harmful cyanobacteria outbreaks under warmer climatic conditions could lead to negative impacts on water quality. The objectives were i) to analyze if air temperature is able to influence the cyanobacteria community, ii) to define the main drivers for the phytoplankton/cyanobacteria community, iii) to predict the cyanobacteria biomass under warmer climatic. Multivariate analysis and statistical modeling were applied on data from seven peri-Alpine lakes. Air temperature significantly influences the phytoplankton community, which is mainly driven by nutrients and temperature. Oscillatoriales, may be favored with increased water temperature and a longer stratification period. Planktothrix rubescens biomass was projected to increase in abundance and frequency thus potentially could induce a community function change.
GALLINA, Nicole. Description and modeling of phytoplankton with an emphasis of cyanobacteria in deep peri-Alpine lakes under warmer climatic conditions . Thèse de doctorat : Univ. Genève, 2012, no. Sc. 4467
URN : urn:nbn:ch:unige-258603
DOI : 10.13097/archive-ouverte/unige:25860
Available at:
http://archive-ouverte.unige.ch/unige:25860
Disclaimer: layout of this document may differ from the published version.
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UNIVERSITÉ DE GENÈVE FACULTÉ DES SCIENCES
Institut des sciences de l’environnement Professeur Martin Beniston
Description and modeling of phytoplankton with an emphasis of cyanobacteria in deep peri-Alpine lakes under
warmer climatic conditions
THÈSE
Présentée à la Faculté des Sciences de l’Université de Genève pour obtenir le grade de Docteur ès Sciences, mention interdisciplinaire
par Nicole Gallina
de Littau (LU)
Thèse N° XXXX
GENÈVE
Ateliers d’impression ReproMail
2012
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On fait la science avec des faits, comme on fait une maison avec des pierres: mais une accumulation de faits n'est pas plus une science qu'un tas de pierres n'est une maison.
Henri Poincaré
Pour mes enfants Elias et Liv, qui illuminent ma vie à chaque instant.
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Remerciements
Une thèse est un travail scientifique, une œuvre artistique, qui se crée et s’accomplit avec l’aide et les réflexions de plusieurs personnes.
Je tiens à remercier en tout premier lieu mes deux directeurs de thèse, Martin Beniston et Nico Salmaso.
Merci à vous deux pour votre confiance sans faille pendant ces quelques années, votre aide et votre appui, ainsi que vos conseils que ce soit pour former ma réflexion ou pour me donner les moyens de travailler et accomplir cette recherche. Vous m’avez également et précieusement formé en tant que chercheuse. J’ai eu la chance d’avoir pu être à vos cotées, ce qui m’a permis d’enrichir considérablement mes connaissances scientifiques, tout comme vous étiez d’une grande valeur sur le plan humain et personnel. Merci Martin, tu étais présent à tout moment quand il le fallait. Merci Nico, tu m’as appris énormément sur le phytoplancton et je ne vais jamais oublier les moments de grandes réfections philosophiques que l’on a eu sur le sujet de ma thèse.
Mes éternels remerciements vont également à Orlane Anneville, avec qui j’ai pu avoir des échanges forts intéressants sur le comportement du phytoplancton. J’ai apprécié ces moments de
Brainstorming qui ontnon seulement nourri ma compréhension mais aussi mon aptitude à faire de la recherche. J’ai beaucoup estimé ton humour et ta modestie.
Ensuite, j’aimerais remercier l’équipe du Service de l’Ecologie d’Eau, de l’Etat de Genève, notamment Jean Perfetta, Arielle Cordonier, Sophie Lavigne et Vincent Ebener. C’est vous qui m’avait planté la graine pour que je puisse m’épanouir dans la limnologie et découvrir le monde merveilleux du phytoplancton. Je vous remercie aussi pour votre amitié qui m’est devenue chère.
Le groupe climat, incluant mes collègues proches mais aussi les collaborateurs et les secrétaires, est, et on ne peut pas mieux le dire, une équipe fine de choc. J’ai pu travailler dans une magnifique ambiance, avec des personnes de grandes valeurs. Je tiens à spécialement remercier Kri in da Yourte, Thierry, Charly, Margot, Marjorie, Anna, Maura, Bastienne, Stéphane, Denis, Enrique, Francis et Anthony. C’était toujours un plaisir de venir travailler, car je savais que vous étiez présents.
Une grande pensée et un éternel et profond remerciement à mes amis. Vous étiez là, c’est sûr, et comment.
Sans vous, cette thèse n’aura jamais abouti de la même façon. Merci pour vos soutiens et les moments de grande joie que l’on a pu partager ensemble. Merci Pascale, Anne la Conj, Chanti, Ceska, Albertine, Lidia, Neyda, Nat, Sonja, Aline, Anne E., Anneso, Claudia, Lionel, Gregiboy, Pascal, Stroumpf, Alex, Pablo et Genti. Merci aussi à Mealy, à Oussa et à Kétia, ça fait du bien de vous savoir si purs et proches.
Merci à ma famille qui me donne un amour inconditionnel, à ma mère qui m’a toujours fait confiance, à
mon père qui m’a appris la persévérance, à mon frère qui veille toujours sur moi, à ma sœur qui est avec
moi. Et surtout merci Elias et Liv, vous êtes mes rayons de soleil, vous me donnez le sourire chaque jour,
je suis fière de vous.
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Résumé
Les écosystèmes lacustres s’avèrent être particulièrement vulnérables aux changements climatiques, de façon telle que même de petits changements dans certains paramètres climatiques ont la capacité d’avoir des effets disproportionnés, non seulement sur leur composition chimique mais également sur leurs compartiments biologiques. Les lacs possèdent une valeur écologique, économique mais aussi sociale inestimable. Par conséquent, les effets du changement climatique sur les écosystèmes aquatiques, plus précisément sur la qualité et la disponibilité de l'eau, ont été déclarés menace majeure par l’Union européenne, soulignant l'urgence et la nécessité de pouvoir évaluer les impacts possibles liés à ces changements.
En Europe centrale, les modèles climatiques régionaux prévoient une augmentation de la température de l'air jusqu’à 6 °C pour la période à venir de 2071 à 2100. Une attention particulière doit toutefois être portée sur la région alpine, car elle symbolise non seulement une région écologique importante, souvent qualifiée de ''château d'eau'' de l'Europe, mais elle représente également une région caractéristique face aux changements climatiques. Pour cette région distincte, les modèles climatiques ont prévu des températures plus élevées que la moyenne mondiale avec des réponses particulièrement sensibles à des modifications des conditions météorologiques à court terme.
Le phytoplancton est constitué par les organismes photosynthétiques capables de vivre en suspension dans la colonne d'eau et il occupe un rôle central dans l’écosystème lacustre. Il est largement utilisé comme indicateur de la qualité de l'eau en raison de sa réponse rapide aux changements environnementaux. Parmi le phytoplancton, les cyanobactéries sont le seul groupe phytoplanctonique d’eau douce capable de produire des substances toxique qui ont le potentiel de contaminer non seulement l’eau potable, mais également de nuire à l'écosystème lacustre entière et de ce fait représentent une menace particulière. En outre, les cyanobactéries ont long histoire de vie sur terre étaient les premiers organismes répertoriés ayant la capacité de produire de l’oxygène et sont par conséquence responsables de l’évolution de la vie sur terre. Leur longue histoire leur a permis d'acquérir des traits éco-physiologiques adaptés qui leur attribuent la capacité d’être potentiellement compétitifs par rapport aux autres groupes phytoplanctoniques, en condition de changement et de stress environnementaux. Par conséquent, face aux changements climatiques, des efflorescences cyanobactériennes ont été présumées et déclarées se produire avec une biomasse et une fréquence plus élevées, ce qui provoque aujourd’hui pour les autorités de gestion de la qualité des eaux un problème considérable. En outre, l’étude de l'effet des changements climatiques sur la composante biologique lacustre présente des interactions très complexes et difficiles à isoler d'autres influences.
Le but de cette thèse était d'étudier les impacts du changement climatique sur le comportement du phytoplancton, avec un accent particulier sur les cyanobactéries potentiellement nuisibles dans la région péri-Alpine. L’hypothèse formulée était que, dans cette région, les épisodes d’efflorescence de cyanobactéries sous conditions climatiques plus chaudes pourraient entraîner des impacts négatifs sur la qualité de l'eau et la santé publique. Les objectifs à atteindre étaient d'analyser, premièrement, si la température de l'air est capable d'influencer la communauté cyanobactérienne des lacs péri-Alpins, deuxièmement, de définir les principaux facteurs capables d’influencer significativement le phytoplancton et les cyanobactéries, et, troisièmement, de prédire la biomasse des cyanobactéries sous les conditions climatiques plus chaudes prévues pour les décennies à venir.
Pour atteindre ces objectifs, une matrice a été construite, dérivée des sept lacs péri-Alpins et profonds
suivants: le Lac de Constance, le Lac de Zürich (Haut et Bas), le Lac de Walen, le Lac de Lucerne, le
Léman (Petit et Grand), le Lac Majeur et le Lac de Garde. Les variables environnementales et les données
du phytoplancton à différentes périodes ont été fournies en vue d'obtenir une matrice couvrant le gradient
trophique comprenant des lacs allant de l'état oligotrophe à l’état eutrophe. En outre, un gradient de
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température de l'eau associé à un gradient altitudinal était inclus par rapports à la position des lacs qui sont situés dans la même région géographique et climatologique. Ces conditions ont permis une évaluation de la communauté phytoplanctonique d'une manière synoptique. Le phytoplancton et les cyanobactéries ont été classés en fonction de leurs groupes pigmentaires classiques, mais étaient aussi évalués en se basant sur les groupes morpho-fonctionnels (MFG), ce qui a permis d'inclure leur fonction écologique.
Afin de prendre en compte l’absence de séries temporelles, des données de la température de l'air lors d'événements extrêmes du climat actuel étaient utilisés comme un proxy pour un réchauffement climatique futur. Ceci a permis d’évaluer ce qui pourrait être le potentiel d’impact des températures d’air sur le phytoplancton. En outre, pour définir quels sont les principaux facteurs responsables et comment ils influencent la composition de la communauté phytoplanctonique, une analyse multivariée a été employée (Non-Metric Multidimensional Scaling (NMDS). Le modèle statistique MARS (Multi Adaptive Regression Spline) a été utilisé pour prédire la biomasse de
Planktothrix rubescense, une cyanobactériepotentiellement toxique, dans le Léman.
Les résultats démontrent la capacité de la température de l'air d’influencer significativement la communauté phytoplanctonique et suggèrent qu’un climat futur plus chaud favorisera l'augmentation de la biomasse phytoplanctonique, notamment celle des cyanobactéries, ce qui entrainera une perte de diversité parmi les groupes taxonomiques. Des cyanobactéries productrices et non-productrices de toxines répondent de la même manière à la température de l’air plus chaude. La température (eau et air) et les nutriments (P, N) sont les facteurs majeurs contrôlant la communauté phytoplanctonique, selon une force équivalente mais indépendamment les uns des autres. La durée de la période de stratification et le broutage par les cladocères influencent aussi significativement le phytoplancton mais de façon moins forte. Les différents MFG des cyanobactéries réagissent inégalement aux facteurs environnementaux et une attention particulière devrait être accordée aux groupements filamenteux des Oscillatoriales qui semblent être spécialement favorisés avec l’augmentation de la température et une plus longue période de stratification.
La concentration des éléments nutritifs au printemps a été démontrée comme étant décisive et capable de contrôler la succession saisonnière du phytoplancton au cours de l'année. L'étude synoptique a clairement montré que chaque lac étudié présentait une communauté phytoplanctonique significativement différente, ce qui a été désigné par « l'effet lac ». Il était présumé que cet « effet lac » est principalement dû à la position altidudinal, donc vincgraphique, de chaque lac et ainsi est responsable de générer une communauté phytoplanktonique bien distincte. Les résultats concernant la modélisation de
P.rubescensdans le « Grand Lac Leman » ont démontré que les températures de l'air, saisonnières et extrêmement chaudes favorisaient une biomasse cyanobactérienne plus importante. Les prédicteurs sélectionnés du modèle de
P. rubescens sont directement liés aux facteurs climatiques ce qui confirme la vulnérabilité de P. rubescens aux perturbations lorsqu'ils sont sous l'influence du changement climatique, indiquant ainsila tendance de
P. rubescens d'être affecté lors de ces scénarios. Le modèle a prévu non seulement uneaugmentation de la biomasse de P. rubescens, mais également de sa fréquence. De plus, une tendance de
P. rubescens à induire un changement de la composition de la communauté a été démontré, ainsi qu’unepériode de croissance avec un développement estival plus tôt dans l’année. Il a été proposé que les printemps plus chauds aient pu jouer un rôle décisif favorisant une période de stratification plus longue et plus forte, entraînant ainsi un épuisement plus précoce des nutriments en été. Ce phénomène pourrait représenter les conditions idéales pour que P. rubescens surpasse les espèces concurrentes et se développe avec une biomase plus important en été. De la même manière, cette situation pourrait favoriser d’autres espèces adaptées à ces conditions metalimniques.
Les autorités de gestion des eaux devraient être conscientes des périodes printanières plus chaudes que la
normale, au cours desquelles une surveillance accrue devrait être entreprise afin de détecter plus
rapidement les efflorescences de cyanobactéries potentiellement nuisibles, en particulier celles des
Oscillatoriales. Cela pourrait aider les gestionnaires à mettre en œuvre des mesures pour prévenir les
impacts négatifs possibles sur la qualité de l'eau et l'approvisionnement en eau potable.
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Abstract
Lake ecosystems are stated to be particularly vulnerable to changes in climatic parameters, as even small changes have been demonstrated to have disproportionate effects on their chemical and biological components. As natural ecosystems, lakes have immeasurable ecological, economical but also social values. Therefore climate change impacts on lakes, more precisely on their water quality and availability, have been stated as a major threat, suggesting an urgency and necessity to understand the possible effects.
In central Europe an increase in air temperature of as much as 6° C by 2071-2100 is forecasted, in which the alpine region has to be taken into special consideration, as it typifies an ecologically important region often stated as the “water–tower” of Europe. Furthermore, also it represents a region for which climate models project an even higher temperature increase than the global average, responding particularly sensitively to short- term changes in weather.
Phytoplankton, photosynthetic organisms adapted to live in suspension in the water column, embody a central role in aquatic ecosystems and are widely used as indicators of water quality due to their rapid response to environmental changes. Among the different groups of phytoplankton, cyanobacteria represent a special threat as this group is the only freshwater phytoplankton capable of forming blooms and producing toxins, which have the potential to harm the entire lake ecosystem. Moreover, cyanobacteria have long life histories which allow them to acquire specific improved eco-physiological traits, thus enabling them to be potentially competitive over other phytoplankton groups under conditions of environmental changes and stress. Therefore cyanobacteria outbreaks have been stated as becoming increasingly important in biomass and frequency under climate change conditions, and represent a significant threat to lake water management authorities. However, the effect of changes in climatic conditions on the biological component, are complex and difficult to disentangle from other influences.
The aim of this thesis was to investigate the impacts of climate change on the behavior of phytoplankton, with a special emphasis on harmful cyanobacteria in the peri-Alpine region. It was hypothesized that in this highly sensitive region more important episodes of harmful cyanobacteria outbreaks under warmer climatic conditions could lead to negative impacts on water quality and public health. The objectives were to first analyze if air temperature is able to influence the cyanobacteria community, second to define the main drivers for the phytoplankton/cyanobacteria community in peri-Alpine lakes, and third to predict the cyanobacteria biomass under warmer climatic conditions projected for the coming decades.
To achieve these objectives, a matrix derived from seven deep peri-Alpine lakes was constructed: Lake Constance, Lake Zürich (Upper and Lower), Lake Walen, Lake Lucerne, Lake Geneva (Small and Big), Lake Maggiore and Lake Garda. Environmental variables and phytoplankton data from different time periods were provided in order to obtain a matrix that covered the entire trophic gradient from lakes in the oligotrophic state to those in a eutrophic state. Additionally, the lakes included an altitudinal gradient associated with a water temperature gradient, and moreover are situated in the same climatological and geographical region. These conditions allowed an assessment of the phytoplankton community in a general synoptical manner. The phytoplankton/cyanobacteria community was classified according their conventional pigmentary groups, and assessed using Morpho-Functional Groups (MFG), which enabled to include their ecological function.
In order to overcome the challenge of missing long-term datasets, an appropriate solution was
hypothesized as using data on air temperature during extreme events under current climate as a proxy for
future mean climate change. Thus the potential of and the way how air temperature impacts the
phytoplankton community could be investigated. Furthermore, in order to define the main drivers and how
they influence the phytoplankton community composition, a multivariate analysis of Non-Metric
Multidimensional scaling (NMDS) was employed. The statistical Multi Adaptive Regression Spline
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(MARS) Model was used to predict the biomass of the potentially toxic species Planktothrix rubescens in Big Lake Geneva.
The outcomes clearly show the capacity of air temperature to significantly influence the phytoplankton community and suggest that future warmer climate will enhance the increase of phytoplankton biomass, especially cyanobacteria in the peri-Alpine region, leading to a loss in diversity among the phytoplankton groups. Toxin and non-toxin producing cyanobacteria generally respond in the same manner to warmer air temperature. Throughout the year, the phytoplankton community is manly driven by temperature and nutrients with equal strength, but, independently to each other. The duration of the stratification period and grazing by the Cladocerans as well significantly impacted the phytoplankton community. The cyanobacteria MFG’s respond differently to the drivers and special attention should be paid to the filamentous Oscillatoriales, which may be favored with increased water temperature and a longer stratification period. The nutrient concentration in spring was revealed to be a decisive factor able to control the seasonal succession of phytoplankton during the remaining year. The synoptical study clearly showed that each lake studied had a statistically-significant different phytoplankton community. It was hypothesized that this differential behavior is mainly due to the geographic/altitudinal position of the lakes able to generate a different phytoplankton community composition. In Big Lake Geneva, seasonal extreme warm air temperatures led to higher cyanobacteria biomass. The predictors selected to model P. rubescens were climatologically related predictors, therefore vulnerable to disruption when under the influence of climate change, thus indicating the potential of
P. rubescensto be affected under these scenarios.
P.rubescens biomass was projected to increase not only in abundance but also in frequency thus potentially
able to induce a community function change. Furthermore, the growth period is predicted to start earlier. It was suggested that a warmer spring might be the decisive factor, leading to a longer, stronger stratification period, thus resulting in earlier nutrient depletion during summer. This could represent not only an ideal circumstance for P. rubescense to outcompete other species and boost their growth earlier and with higher biomass, but also favor other species adapted to these metalimnic conditons.
Water management authorities should be aware of warmer than normal spring periods, during which
increased monitoring should be undertaken to detect potential harmful cyanobacteria outbreaks earlier,
especially Oscillatoriales. This could help water managers to implement measures to prevent possible
negative impacts on water quality and drinking water supply .
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Preface
This thesis is based on the following papers,
I Gallina N., O. Anneville, M. Beniston. Impacts of extreme air temperatures on cyanobacteria in five deep peri-Alpine lakes. Journal of Limnology, 70(2) : 186-196, 2011.
II Gallina N., N. Salmaso, G. Morabito, M. Beniston. Phytoplankton configuration in six deep lakes in the peri-Alpine region: Are the key drivers related to eutrophication and climate. (Submitted to Aquatic Ecology).
III Gallina N, M. Beniston, S. Jacquet. Will Lake Geneva turn red? (Submitted to Limnology
& Oceanography).
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Table of contents
1. INTRODUCTION ... 9
1.1 Climate change and Lakes: An Overview ... 9
1.1.1 Climate change in peri-Alpine Lakes: the importance of regional- geographical location of lakes ... 10
1.2 Phytoplankton in lake ecosystems... 11
1.2.1 Definition and importance ... 11
1.2.2 Classifications ... 11
1.2.3 The seasonal succession ... 13
1.3 Cyanobacteria ... 13
1.3.1 Origin, morphological characteristics, distribution and eco-physiological traits ... 14
1.3.2 Factors influencing Cyanobacteria ... 15
1.3.3 Environmental changes, causes for cyanobacteria outbreaks ... 18
1.3.4 Consequences of Cyanobacteria blooms for Lake Ecosystems ... 21
1.4 Aims and objectives ... 21
1.5 Data and Methods used ... 22
1.5.1 Data characteristics ... 22
1.5.2 Methods ... 27
2. IMPACTS OF EXTREME AIR TEMPERATURE ON CYANOBACTERIA IN FIVE DEEP PERI-ALPINE LAKES ... 31
3. PHYTOPLANKTON CONFIGURATION IN SIX DEEP LAKES IN THE PERI-ALPINE REGION: ARE THE KEY DRIVERS RELATED TO EUTROPHICATION AND CLIMATE? ... 49
4. WILL LAKE GENEVA TURN “RED” IN THE FUTURE? A POSSIBLE SCENARIO FOR THE DEVELOPMENT OF THE CYANOBACTERIUM PLANKTOTHRIX RUBESCENS... 68
5. CONCLUSIONS AND PERSPECTIVES ... 88
6. ANNEXES ... 93
7. BIBLIOGRAPHY ... 96
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Chapter 1
1. Introduction
1.1 Climate change and Lakes: An Overview
Aquatic ecosystems are considered to be an integral part of human existence on earth that in addition provides an essential dimension to our lives (Williamson and al. 2009a). Lakes are freshwater bodies highly valued for recreational activities, providing drinking water supply and sustaining natural ecosystems that are home to many of species. Their economic importance is highlighted by the use of water for agricultural purposes (irrigation), their role as an important source of food (fishing), and their use for waste water and sewage treatment and in terms of hydro-power production. In Europe, the substantial concern to maintain or restore lake water quality was the keystone for the Water Frame Work Directive (WFD, Directive 2000/60/EC), which came into effect in 2000. The WFD represented a fundamental change in water management in Europe expressing the need by which aquatic ecosystems need to be assessed in a holistic way in order to achieve, by the end of 2015, a good ecological status for all ground and surface waters. However, at the time the WFD initiative was created, the climate change issues were not taken in account as they should have been. In 2008, a technical report of the Intergovernmental Panel on Climate Change (Bates and al., 2008) clearly stated with their following outcome the urgency and necessity to understand the effect of climate change on lakes:
"Water and its availability and quality will be the main pressures on, and issues for, societies and the environment under climate change”.
Climate change is thus considered to be a genuine threat to many natural systems, and has
become the body of research interest among limnologists around the world. Research in
limnology was initiated to answer the question of how lake ecosystems may respond to a
changing world, in which even small changes were demonstrated to have a disproportionate
effect on their chemistry and biology (Wehenmeyer and al., 1999). Ten years later, lakes were
stated to be effective sentinels, integrators and regulators of climate change (Adrian and al. 2009,
Williamson and al., 2009a, 2009b), due to their rapid responses to environmental changes, and
their capacity to integrate information about changes in the catchment zones (Adrian and al.,
2009).
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Changes in climate forcing have been found to affect the physical environment of lake ecosystems and thereby induce an alteration in their chemical and biological properties, which ultimately have the ability to jeopardize the ecosystem services that lakes provide (Vincent 2009). Of particular interest to limnologists are the interactions between variables, the feedback effects that accelerate or dampen environmental change.
1.1.1 Climate change in peri-Alpine Lakes: the importance of regional- geographical location of lakes
Of a particular interest when investigating lakes related to climate changes is the geographic location of the lakes. In Europe, different weather systems are used to classify weather types.
George and al. (2010a) demonstrated by using three closely related weather systems, that each type is able to influence the observed variation in the surface temperature of lakes situated in Northern Europe, Western Europe and Central Europe.
In Central Europe, climate change scenarios suggest an increase by as much as 6°C by 2071-2100 (IPCC, 2007. Special attention should be paid to the Alpine region, where the temperature increase is projected by climate models may be even higher than the global average rise (Beniston, 1997). This region is defined as being particularly sensitive to short-term changes in the weather (Psenner, 2003; Thompson and al., 2005). Thus, these changes have a strong impact on the water bodies of the Alpine region, and because the Alps are often considered to be the
‘water tower’ of Europe, these lakes are therefore of major ecological importance.
Another important aspect in studying lakes on a same geographical region is the regional coherence in-between lakes, which was first demonstrated by the work of Magnuson and al.
(1990). The features of lakes within the same region respond coherently to drivers such as
climate forcing and catchment processes. Recent work from Livingstone and al. (2010) showed
that spatial coherence for physical variables is the highest whereas for biological variables the
weakest thereby emphasizing the difficulty to assess these variables in different lakes. In other
words, the effect of changes in climatic conditions on the biological component, especially the
phytoplankton community, are complex, difficult to disentangle from other influences and not
easy to generalize (Livingstone and al., 2007). This complexity of interactions should be
constantly taken into consideration if the growth, structure and dynamics of phytoplankton
community are assessed.
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1.2 Phytoplankton in lake ecosystems
1.2.1 Definition and importance
The definition of phytoplankton is based on the term “phyto” meaning plants and designates organisms able to do the photosynthesis and “plankton”, organisms adapted to live in suspension in the water column. However, due to the widespread variety between species, the following re- definition from Reynolds (2006) was selected as follows: Phytoplankton is a collective of photosynthetic microorganisms adapted to live partly or continuously in open waters. As such it is the photoautotrophic part of the plankton and a major primary producer of organic carbon in the pelagic of the seas and of inland waters.
Phytoplankton plays an important role in lake ecology. In the process of photosynthesis, phytoplankton produces half of the world's oxygen (all kind of water bodies included). Moreover, by primary production, death and sinking they effectively transport carbon from surface layer to sediments, a process by which phytoplankton exert a global-scale influence on climate (carbon dioxide and the greenhouse effect). Furthermore, phytoplankton constitutes the bottom level of aquatic food webs (Arrigo, 2005). In addition, due to their rapid response to environmental changes, phytoplankton constitutes an important indicator of water quality. (Stoermer, 1978;
Directive 2000/60/EC), and its use as an indicator of environmental changes became more important under the pressures of a changing climate.
1.2.2 Classifications
The classification of phytoplankton is a complicated and complex task for taxonomists, demanding a certain experience and knowledge in order to identify correctly these microorganisms, which have vast morphological and physiological characteristics. Their taxonomy has been continuously revised, improved with the discovering of new species or new traits on already discovered species and still remains subject to discussion (Komarék, 2003).
Moreover, the advance of science added the knowledge of phylogenetic classification based on genomic resemblance in between species, which required updating the existing classification. The major taxonomic groups are based upon their primary characteristic, the photosynthetic pigments:
chlorophylls, carotenoids, xanthophylls and the phycobiliproteins. In deep peri-Alpine lakes, the
resulting seven algal groups were identified and considered following the more recent
monographs of the series ‘’Süsswasserflora von Mitteleuropa” established by A. Pasher (Gustave
Fisher Verlag) as well as The Süsswasserflora of the British Isles (John and al., 2002). The
definition of the major groups of eukaryotic algae is coherent with the recent status in the
systematic described by Krienitz (2009). However, the contribution of Chrysophyceae, and
Bacillariophyceae, which were included, by Krienitz (2009) in the division Heterokontophyta,
were considered separately during the thesis.
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CYANOBACTERIA (blue green algae). They are structurally and physiologically like bacteria, but they photosynthesize functionally like plants in aquatic systems. An enhanced description will be given in a separate paragraph.
BACILLARIOPHYCEAE (Diatoms). A most important group of the phytoplankton and highly distinctive, through their characteristics including a unique cell wall of silica (termed as
‘frustule’). The beautiful structures of their frustule are complex and used as taxonomic characteristic. Diatoms are widely used as indicators of water quality in rivers (Gallina and al., 2009) since distinct nutritional requirements favor growth of one group over another (Wetzel 2001, Reynolds 2006).
CHLOROPHYCEAE (Green algae). This is an extremely large and morphologically diverse group of algae that is almost totally distributed in freshwater. Many members are flagellate, at least in the gametes stages (John 2002, Reynolds 2006).
CONJUGATOPHYCEAE (Charophyta – Green algae). Well defined group among the green algae’s and characterized by their sexual reproduction by the conjugation of the gametes. The genera Mougeotia bloomed in Lake Geneva and Lake Garda notably.
DINOPHYCEAE. Unicellular flagellated providing weak locomotion, which is relatively resistant to most grazing organisms, due to their size and are limited by weak nutrients concentrations. They are conspicuously represented in freshwaters (Reynolds, 2006)
CHRYSOPHYCEAE. Most are unicellular and only few are colonial; the presence of flagella is variable. Many species lack cell wall. They are major components in temperate oligotrophic lakes (Hutchinson 1967).
CRYPTOPHYCEAE. Most are naked, unicellular, and motile, occurring in most lakes, regardless their trophic state having a characteristic intermittent numerical dominance and qualified as ecologically significant internal stabilizing component of plankton communities (Wetzel, 2001).
1.2.2.1 An ecological way of classification to assess the phytoplankton community
Phytoplankton ecologists soon stated that the taxonomical classification of phytoplankton based
upon their pigmentation characteristics, is not always convenient to use for the description of the
ecological function and quality of the aquatic environment. With the idea to not only name the
species, but also have a close idea of their affinities, in order to group species together with the
same ecological traits and shared adaptive features, Reynolds and al. (2002) published a review
in which they promote a new classification, a functional classification to study the phytoplankton
community. The resulting function groups are based not only on individual functional traits, but
also on the range of environmental conditions for which the species are found to occur. Salmaso
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and Padisàk (2007) joined this idea and developed the morpho-functional groups (MFG), which include the morphological as well as the functional features of phytoplankton species. Recently, Kruk and al. (2011) tested all these classification methods, and stated that the community composition can be best predicted in using the new classification methods, namely the morphological groups.
1.2.3 The seasonal succession
The growth and seasonal successions of phytoplankton are regulated by variety of external as well as internal factors (Reynolds 2006). The main controlling factors are related to light, nutrients, water temperature, turbulence, and trophic interactions (grazing and competition) (Harris 1986). The Plankton Ecology Group of the international Society of Limnology proposed the conceptual Model (PEG- model) (Sommer and al. 1986), in which “24 Sequential Statements of Seasonal Succession of Plankton in Freshwater” (Annexe 1) were exposed for temperate lakes. The PEG-model defines the most important mechanisms governing the phytoplankton community during the year and clearly indicates the importance of physical forcing (light, temperature, and mixing) during autumn and winter, chemical conditions (depletion of nutrients) and biological interactions (grazing and competition) during spring and summer. Further work confirmed the role of physical constraints in shaping both the biomass and the composition of phytoplankton, especially in winter (Padisàk, 2010).
Therefore, as climate directly affects light, mixing and water temperature, its capacity to influence the phytoplankton community and its seasonal evolution becomes more obvious.
1.3 Cyanobacteria
The following chapter introduces the importance cyanobacteria present in lake ecosystems. It
focuses on their origin, their morphological and eco-physiological traits which give cyanobacteria
their competitive advantage over the rest of the phytoplankton community. Moreover, the
environmental factors controlling their growth are delineated, including climate change and
eutrophication. Finally, their potential harmful effects to lake ecosystems and the reasons why
lake management authorities consider cyanobacteria as a major concern is discussed.
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1.3.1 Origin, morphological characteristics, distribution and eco-physiological traits
Cyanobacteria are known to be the world’s oldest known oxygen-producing and nitrogen-fixing organism (Schopf 2000, Chorus and Bartram, 1999). They are considered as major actors responsible for the accumulation of oxygen in the Earth’s early atmosphere and are thus considered as being at the origin of the evolution of life on earth. Cyanobacteria have a long life history and their first appearance was confirmed to date back to about 2.15 billion years ago (Hoffmann 1975, Knopf 2006, Ramussen 2008).
Cyanobacteria are photosynthetic prokaryotes as they lack nuclei and other organelles, and have a peptidoglycan cell wall that is typical of gram-negative Eubacteria (Hoiczyk and Hansel, 2000).
However, they are classified as blue-green algae because of their algal-like appearance, their possession of chlorophyll and their photosynthetic production of oxygen by a two photosystem process (I+II) (Castenholz, 2002). All cyanobacteria contain chlorophyll a and most comprise blue phycobiliproteins phycocyanon and phycocyanin, giving the cells their characteristic blue–
green color (Grossman and al., 1995). Although cyanobacteria lack membrane-bound organelles, they have a variety of cellular structures and inclusions that have specialized functions that contribute to their ecological success, as these inclusions allow cells to accumulate energy and nutrients far in excess of their present requirement when they are under favorable conditions and to subsequently use these reserves for growth maintenance when they encounter source-poor conditions (Vincent, 2009). Some planktonic species contain in their cell up to several thousand gas vacuoles, providing buoyancy to the cells and colonies and thus float among the water column to require ideal light and nutrient conditions (Walsby and Hayes, 1988; Walsby, 1994;
Walsby and al., 2004). Moreover cyanobacteria produce different cell types, like the heterocyte (e.g. genera Nostocales) which are the location of the enzyme nitrogenase for nitrogen fixation.
Another specialized cell type is the akinete, formed under unfavorable conditions and that allows cyanobacteria to overwinter in the sediments (Mur and al., 1999).
Cyanobacteria are distributed worldwide and can be found in environments ranging from the tropics to the arctic, and from alkaline to acidic environments, and can proliferate in freshwater, estuarian and marine ecosystems (Chorus and Bartram, 1999, Mur and al. 1999).
Their long life history through which cyanobacteria had to evolve has resulted in an enhanced
ability of adaptation, which is evidenced by their improved eco-physiological traits (Litchman
and al., 2010; Cayelan 2011). These traits are hypothesized to confer cyanobacteria the ability to
compete with other phytoplankton groups in situations of environmental stress. The following
lists evoke these traits. As already mentioned, the first three traits result from the production of
their differentiated cell structures.
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a) Heterocysts, which enable some genera of cyanobacteria to fix the atmospheric nitrogen via the nitrogenase enzyme, when nitrogen is scarce in the environment (Ganf and Oliver, 2000).
b) Akinetes, which are produced under stress situations (low light conditions and deficiency of nutrients) (Kaplan and Levy, 2010) and allow the cyanobacteria to survive under unfavorable environmental conditions. Akinetes are resistant to cold and dry weather (Adams et Duggan, 1999).
c) Buoyancy enables cyanobacteria to migrate in stratified lakes between the surface where light is abundant to produce photosynthesis and to the nutrient rich metalimnion, where it absorbs the accumulated carbohydrates from the surface layer by respiration (Ganf and Oliver, 2000; Walsby 1994)
d) UV radiation tolerance in developing sunscreen pigments that envelope the cell and function even when cells are at rest. Moreover cyanobacteria are able to develop efficient systems for repair of damaged DNA and for replacement of UVR-damaged compounds, and to implement directed gliding motility for escaping the diurnally high intensities of solar irradiance (Castenholz and Garcia-Pichel, 2002).
e) Warmer temperature optima for growth are features of most cyanobacteria, and bloom- forming species prefer temperatures exceeding 15°C. The temperature at which maximum replication rates occurred for cyanobacteria varied from just over 20 °C for Aphanizomenon flos-aquae and Planktothrix agardhii, to 28 °C for Microcystis aeruginosa, and even 41 °C for Synechococcus sp. (Reynolds, 1989, 2006; Robarts and Zohary, 1987).
f) Large light-harvesting antenna. Cyanobacteria, are reputed to be strong competitors for light due to their large light-harvesting antenna pigments (Reynolds 1997, Jacquet 2005).
g) Toxin productions. Cyanobacteria are the only freshwater phytoplankton group to produce a diverse range of toxins (Codd and al., 1989, 1999, Carmichael 1997.), which have been hypothesized to confer allelopathic advantages to grazing and competition (e.g.
Schatz, 2007, Roy, 2009).
1.3.2 Factors influencing Cyanobacteria
It is well established that the success of cyanobacteria is a result of complex and synergistic
environmental factors, rather than a single dominant variable (Hyenstrand and al., 1998; Dokulil
and Teubner, 2007). We introduce here the main factors which are supposed to govern
significantly Cyanobacteria.
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1.3.2.1 Physical forcing
a) Thermodynamics and stratification
The thermodynamics of the water masses have an influence of the spatial distribution of phytoplankton communities (Hedger and al., 2004).
The horizontal distribution is a consequence of the wind, which is known to play an important role in creating advection in the water, thus accumulating mainly species capable of floating in matter to form scums, mats or blooms (Hedger and al., 2004).
The vertical distribution is a consequence of temperature and light. The atmosphere imposes a temperature signal on the lake surface inducing a density difference in water bodies, facilitating an evolution of chemical differences with many consequences for living organisms in lakes (Boehrer and Schulze, 2008). Peri-Alpine, temperate lakes are classified as warm monomictic lakes (Hutchinson, 1957), which are lakes that never freeze, and are thermally stratified throughout much of the year. The density difference between the warm surface waters (the epilimnion) and the colder bottom waters (the hypolimnion) prevents these lakes from mixing in summer. During winter the surface waters cool to a temperature equal to the bottom waters.
Lacking significant thermal stratification, these lakes mix thoroughly in winter (holomictic lakes) and in deep lakes during very cold winters or to a certain extent during warmer winters (oligomictic lakes) (Lewis, 1983).
Vertical movements favor species that are able to develop under low light conditions during mixing periods (Huismann 1999; Hedger, 2004) and for the opposite case, during stratification periods, species able to move vertically in the water column in order to seek ideal light and nutrient conditions (Huisman, 1999).
b) Temperature
Temperature is known to be the most important environmental driving factor affecting directly
the metabolism, growth, reproduction and survival of living organisms, as well as the interaction
among species (Ibelings and al., 2011) and therefore impacts the ecosystem functioning (De
Stasio, 2009). Furthermore temperature is also indirectly involved in the strength and duration of
the stratification period and enhanced availability of nutrients in the epilimnion and hypolimnion
(Søndergraad and al. 2003). As already mentioned, most of the cyanobacteria are blooming under
their optimal growth temperature of 20°-25°C (Reynolds, 1987; Robarts and Zohary, 1987),
nevertheless their optimal growth varies among the cyanobacteria considered.
17 c) Light
The accessibility of light is an important factor for organisms performing photosynthesis. In the water column, the fraction of light energy available for photosynthesis, (PAR, Photosynthetically-Available Radiation) is situated around wavelengths between 400 and 700nm, and is highly influenced by the depth, turbidity, particles in suspension, chlorophyll concentrations, and also by the existing, self-shading phytoplankton community (Tilzer and al., 1995). The depth of water transparency is measured by the help of the Secchi disc (z
s), from which the euphotic depth (z
e) can be derived as follows: z
e= 2.7*z
s.The euphotic depth is considerate as the limit of the photosynthetic activity, corresponding to 1% of the light intensity present at the lake surface (Lemmin, 1995). Light thus divides the lake water column into the euphotic zone, where the proliferation of phytoplankton takes place, and the aphotic zone.
Another measure of the light availability used is the underwater light climate index, which is expressed by the ratio of the depth of the mixing layer and the euphotic depth (z
m/z
eu). The capacity to access and to use the available light is a determinant factor for cyanobacteria growth, which moreover have the capacity to modify, by expressing genes the quality of phycobilisomes, the light harvesting antennae of photosystem II (Kehoe et Gutu, 2006)
1.3.2.2 Nutrients
Tilman & Khilman (1976) and further Reynolds (2006) demonstrated that the differentiated abilities of phytoplankton species to gather resources necessary to support cell growth and replication might influence the dynamics and ecology of phytoplankton populations.
In addition to light, growth of cyanobacteria consumes carbon and iron (photosynthesis),
‘nutrients’ and equally, may often be constrained by their availability and fluxes. Since the earliest days of phytoplankton ecology, phosphorus (P) and nitrogen (N) have been emphasized to be the most important variables controlling the phytoplankton community structure and biomass (Hutchinson 1967), and their role as limiting nutrients was demonstrated (Tilman 1982).
The term of limiting nutrients is due to the fact that it is not the quantity of these required elements that constrains the growth, but the ease with which they are obtained (Reynolds 2006).
It is the demand relative to the supply that is ultimately critical, bearing in mind that the presence
of nutrient should be in their assimilable form requested by the cells. Cyanobacteria have the
ability to avoid periods of nutrient deficiency to allow achieving their need. The enzyme
nitrogenase allows fixing the atmospheric nitrogen, as already evoked. In periods of phosphorus
insufficiency, some cyanobacteria have the capacity to cleave the organic compounds by the
activation of extracellular enzymes (Dyhrman and Ruttenberg, 2006). It is also shown that the
stoichiometry of the primary nutrients present in the aquatic environment seems to influence the
composition of the phytoplankton community (Smith and Bennet, 1999; Huisman and Hulot,
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2005). Especially, a weak ratio TN:TP (total nitrogen (TN) and total phosphorus (TN)) is known to have the tendency to favor the cyanobacteria (Huisman and Hulot, 2005, Smith 2005).
1.3.2.3 Biological interactions
a) Competitons
With the famous “paradox of plankton”, published 50 years ago, Hutchinson (1961) raised the question of “how it is possible for a number of species to coexist in a relatively isotropic or unconstructed environment, all competing for the same sorts of materials”. With this statement, he highlighted the observed contradiction between the numbers of species competing for the same resource and the principle of competitive exclusion (Hardin 1960), in which the final equilibrium is attained if the population were to reduce to one species which outcompeted all the others. A number of investigations followed to describe the abiotic and biotic factors which prevent to attain the equilibrium (e.g. Scheffer and al., 2003, Benincà and al., 2008). The eco- physiological adaption mentioned above, clearly indicate the advantage of cyanobacteria in comparison to their competitors, therefore putting their hypothetical success in situations of environmental stress into perspective.
b) Grazing by zooplankton
For cyanobacteria, grazing seems not to be an important threat (Psenner, 1995). The reason of resistance to grazing can be summarized by first the important size of their colonial forms or filaments therefore difficult to ingest, second by the bad food quality they represent for grazers due to their weak content in polyunsaturated acid (DeMott and Müller-Navarra, 1997) and third, as already evoked, by their production of toxic allopathic compounds. On the contrary, even a negative impact of cyanobacteria on zooplankton needs to be discussed (Pearl, 2001), as well as the positive influence of zooplankton favoring cyanobacteria by reducing their competitors (Fulton and Pearl, 1987).
1.3.3 Environmental changes, causes for cyanobacteria outbreaks
During the last decades, an increasing number of publications have stated the rise in
cyanobacteria outbreaks in freshwater ecosystems. This increase is partly due to more frequent
surveys (Sellner and al., 2003), but several authors also observed cyanobacteria increases in
biomass, duration and distribution (Anderson, 2002). The reasons for these increases are due to
two anthropogenic related phenomena which are identified to be the most important threats in
lake ecology: Eutrophication and Climate Change.
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1.3.3.1 Eutrophication
Over several centuries human nutrient over-enrichement (particularly nitrogen and phosphorus), associated with urban, agricultural and industrial development, has promoted accelerated rates of primary production, or eutrophication (Pearl and Paul, 2011). Moreover eutrophication is projected to be favored in relation to climate change (Moss, 2011). A strong correlation in between eutrophication and blooms of cyanobacteria (Reynolds and Petersen, 2000; Downing and al., 2001) has been recorded. Some results also clearly point out that eutrophication favors the proliferation and dominance of harmful blooms of cyanobacteria (Fogg, 1969; Huismann and Hulot, 2005; Pearl and Fulton 2006, Moss 2011). Nevertheless, even after restoration of the largest natural French lake (Lake Bourget in Savoie), the proliferation of a cyanobacteria species was observed (Jacquet and al., 2005). Likewise Anneville and al. (2002) indicated in Lake Geneva higher phytoplankton biomass even though the nutrient loads decreased after management efforts. This non-linear relationship between nutrient and phytoplankton/cyanobacteria was termed ‘’Hysteresis”, translated by the resilience of the ecosystem to maintain its integrity once challenged to environmental modifications (Carpenter and Cottingham, 1997). Therefore the recovery of lakes after reductions in nutrient loading may be confounded by concomitant environmental changes such as global warming. However, effects of global change are likely to run counter to reductions in nutrient loading rather than reinforcing re-oligotrophication (Jeppesen and al., 2005).
1.3.3.2 Climate change
Cyanobacteria have been hypothesized to benefit from environmental changes associated with global warming (Paul, 2008; Paerl and Huisman 2008; Paerl and Huisman, 2009; Mooij and al., 2005; Pearl and Paul, 2011). Their increase may be mainly due to their high capacity to be well adapted when subjected to various stresses related to climate change (Cayelan and al., 2011).
Moreover several authors observed the potential of cyanobacteria to become dominant (Elliot 2006, Jöhnk and al. 2008).), but also able to change in their phenology of bloom events (Elliot 201o, Zhang, 2011). Indirecty, deeper stratification (Anneville, 2002), favor cyanobacteria adapted to low light conditions. Moreover, the migration from tropical species to northern temperate situated lakes also has been observed (Briand and al., 2004; Wiedner and al., 2007).
However many of these factors could be the direct consequences of temperature due to their optimal growth at higher temperatures compared to other phytoplankton groups (Reynolds, 1984;
Robarts and Zohary, 1987), but also to indirect positive effects of temperature, which are
multiple. First, buoyant cyanobacteria will be favored under thermal conditions that are likely to
become more stable, prolonged and intense (Paerl 1988; Dokulil and Teubner 2000; Winder and
Schindler 2004; Wagner and Adrian, 2009). Another important trait is that climate change may
reinforce the symptoms of eutrophication that favor cyanobacteria (Moos, 2011; Adrian 2009).
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Figure 2 summarizes the factors involved in the growth of cyanobacteria under a climate change scenario.
Figure 2 highlights the different factors and mechanisms involved in mediating the climatic response of cyanobacteria (adapted from Nõges and al., 2010).
1.3.3.3 Climate change and Nutrients: A synergistic effect?
When it comes to assess impacts on phytoplankton, identification of climate signals can be further complicated by the influence of other environmental changes, such as eutrophication (Adrian and al., 2009). The questions about the synergy and the relative importance of influence of these two elements of global changes arise, when affecting the phytoplankton community.
Moss and al. (2003) found that warming had a considerably smaller effect on the phytoplankton community than did fish and nutrients through a mesocosm study. Research on sediment cores of Lake Biwa indicated a stronger effect of nutrient during the eutrophication period, whereas meteorological forcing was the driving factor during oligotrophicaton (Tsugeki and al, 2009).
The results of Stich and al. (2009) indicate that oligotrophication outweights the effect of global warming in Lake Constance. The phytoplankton community model PROTECH (Phytoplankton Responses To Environmental Change; Reynolds and al. 2001) was used to investigate the impacts of changing water temperature and nutrient loading upon the phytoplankton in Loch Leven (Elliott and May 2008). Change in water temperature had relatively little effect on phytoplankton biomass and species diversity in comparison with changes in nutrient loading.
However, phytoplankton varied according to the way in which nutrient loading changed. In
another study, phytoplankton community models predicted a greater impact of nutrient loading
over temperature, but with a large dominance of cyanobacteria when high water temperatures
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faunawere combined with high nutrient loads (Elliott and al. 2006). Although these questions are important to answer when it comes to find the most adapted and efficient solution for water management authorities, the relative importance of nutrients versus temperature as the dominant impact on cyanobacteria still remains unclear.
1.3.4 Consequences of Cyanobacteria blooms for Lake Ecosystems
The consequences of cyanobacteria blooms and the way they may damage the lake ecosystem represent a major concern for lake management authorities.
The increased biomass during a bloom episode increases the turbidity of lakes which in turn affects the transparency and therefore restricts the light availability necessary for the aquatic vegetation, which in turn leads to habitat disappearance for fish and benthic flora and fauna (Scheffer and al., 1997). The phenomena of shading due to their outbreaks entrain the suppression of the phytoplankton via competition for light (Jöhnk and al., 2008). A drastic reduction of the phytoplankton diversity may be observed (Crosetti and al., 2008). Moreover, during nighttime, dense blooms of cyanobacteria are able to cause oxygen depletion through respiration and bacterial decomposition, which can results in massive fish mortality and loss of fauna and flora (Pearl and Fulton 2006), and to a complete imbalance in the entire trophic chain (Vanni and al., 1997).
The major concern of cyanobacteria is related to health risk and their ability to produce toxic compounds able to harm both animal and human health, as a result of drinking the water or swimming (Chorus and Bartram 1999, Dokulil and Teubner 2000, Briand and al. 2003). These toxins can be grouped into three families, depending on their toxic effects: Hepatotoxins, neurotoxins and dermatotoxins. Hence numerous hypotheses were formulated about the reasons underlying the production of these toxins (Vasconcelos, 2001; Wiegand and Pflugamcher, 2005;
Schatz and al., 2007). The results suggest that the target of their allopatic effect may be to harm potential grazers or competitors (Paerl and Millie, 1996; Tillmans and al., 2008). The impact of these cyanotoxins on aquatic ecosystems still remains, however, largely misunderstood.
1.4 Aims and objectives
The aim of this thesis is to investigate the impacts of climate change on the behavior of
phytoplankton, with a special emphasis on harmful cyanobacteria in the peri-Alpine region, a
region known for its vulnerability to global warming and its ecological importance as the “water-
tower” of Europe. It is hypothesized that in this region more important episodes of harmful
cyanobacteria outbreaks under warmer climatic conditions could lead to negative impacts on
water quality and public health. The outcomes should lead to a better understanding of what
water management authorities have to expect in order to avert the more negative risks arising
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from a warming climate. The enhanced knowledge should therefore allow the responsible authorities to successfully maintain water quality and drinking water supply in a world in which climate is changing and population is growing.
To achieve this goal the following objectives were defined:
I. To analyse of whether air temperature (a driver directly related to warming) is able to influence cyanobacteria in peri-Alpine lakes (cf. Chapter two).
II. To define which are the main drivers for the phytoplankton/cyanobacteria community in peri-Alpine lakes (cf. Chapter three).
III. To predict cyanobacteria biomass under warmer climatic conditions projected for the coming decades of the 21st century (cf. Chapter four).
The first two objectives (I and II) are meant to lead to a better understanding concerning the behavior of the phytoplankton, and especially the cyanobacteria, in the particular area of the peri- Alpine lakes. Even though some of the six lakes were investigated together (Anneville, 2004;
Salmaso, 2006), this study represents the first synoptic investigation, involving a consistent dataset with an interesting number of lakes, focusing on cyanobacteria. This step was achieved by using descriptive statistics, with the aim to identify specific characteristic of the phytoplankton / cyanobacteria of peri-Alpine lakes. Further, the results obtained provide the necessary information needed to attempt to predict the evolution of cyanobacteria biomass, in a way that it represents realistic adapted features for the peri-Alpine region. This last step (III) was achieved with the help of statistical modeling methods.
1.5 Data and applied Methods
1.5.1 Data characteristics
The idea was to compile a unique and extensive matrix for the peri-Alpine region. When investigating which lake to include, three principle aims were pursued:
1) Having similar types of lakes in the same climatological and geographical regions, allowing for comparability and integration into one common matrix. Such a matrix moreover, thus enables to perform for a potential synoptic study in a defined region.
2) Having lakes covering the whole trophic gradient, from oligotrophic to eutrophic. As nutrients were supposed to be important drivers for the phytoplankton community, the assessment of phytoplankton at each trophic state could be integrated.
3) Having lakes at different altitudes, which potentially could facilitate the integration of a heterogenic phytoplankton community resulting from the temperature gradient.
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In the end, an original matrix derived from seven lakes was able to be included into the final peri- Alpine Lake dataset, namely Lake Constance, Lake Zürich, Lake Walen, Lake Lucerne, Lake Geneva, Lake Maggiore, and Lake Garda. Data from Lake Geneva and Lake Zürich were collected at two points, referred to as “Small Lake Geneva” and “Big Lake Geneva”, and “Upper Lake Zürich” and “Lower Lake Zürich”, respectively. Consequently, nine datasets were derived from the seven lakes. The data for each lake were sampled at and during different time periods.
The data were kindly provided by the state water authorities responsible for lake monitoring as well as from limnological research institutes, namely the LUBW for Lake Constance, the Wasserversorung Zürich and EAWAG for Upper and Lower Lake Zürich, Lake Walen and as well as for Lake Lucerne, SECOE for Small Lake Geneva, the CIPEL for Big Lake Geneva, the FEM-IASMA for Lake Garda and the CNR-ISE for Lake Maggiore. Figure 1 shows the geographical position of the considered lakes related to the alpine arc.
Figure 1: Geographical representation of the peri-Alpine region and the lakes which were taken into account during the thesis, with the length of the timeseries available for each lake. ZH is the abbreviation for Lake Zürich, while GE is the abbreviation for Lake Geneva.
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The seven lakes are all deep, warm monomictic lakes (Hutchinson, 1957), belonging to the same geographical (peri-Alpine) and climatological (continental) region. Moreover, based on total phosphorus concentrations, and according to OECD (1982), the assessed lakes cover the entire trophic gradient, reaching from oligotrophic to eutrophic lakes. An altitudinal gradient could be derived from the highest lake situated at 434 m. a. s. l. (Lake Lucerne) to the lowest situated lake at 65 m. a. s. l. (Lake Garda). Figure 2 highlights the lake features with respect to the trophic state, the altitudinal position and the length of the time series. The hydro-morpho-metrical characteristics, and the time period during which the data were collected, figure in in chapter 3, table 1. Lake Lucerne is not included for reasons which will be explained below.
Figure 2: Lake data sets represented upon their trophic level, their altitudinal position, and the length of the
timeseries, which is indicated propotionally by the length of the black bar. (O = oligotroph; O-M= oligo-mesotroph;
M = mesotroph, M-E= meso-eutroph; E= eutroph).
