Coastal and shallow lakes are often subjected to eutrophication due to nutrients from catchment farming activities. Lake Ellesmere (Te Waihora) is a hyper-eutrophic lake which has gained recent attention because of concerns over its ecological health and fishery status. This study investigated the benthic ecology of the lake by extensive spatial and temporal sampling. Eight littoral sites were sampled on a single occasion, and 20 benthic sites were sampled once per season for one year. Water chemistry conditions, substrate and invertebrate communities varied significantly around the lake. Salinity, pH, DO and seston were primarily affected by freshwater inputs from inflow streams and salt water intrusion due to the lake opening to the sea. On these occasions, salinity reached 32 ‰ at the lake outlet. The lake invertebrate community was depauperate, comprising of only two species of invertebrate predators restricted to the littoral zone and eight benthic invertebrate taxa, dominated by oligochaetes, amphipods and chironomids. Benthic invertebrate abundances also reflect the dominant local substrate, where oligochaetes and chironomids preferred areas of silt substrate, whereas Potamopyrgus preferred harder substrate. Stable isotope and gut analysis determined that the primary food sources within the lake were phytoplankton and algae. Macrophytes provided a minimal contribution to the food web, possibly relating to the change in status from a clear water, macrophyte dominated lake to a turbid, phytoplankton dominated condition since the Wahine Storm in 1968. Isotope analysis also showed that the lake food web was markedly different in its carbon values from food webs of its inflow streams and nearby marine source. However the lake food web did show a marine-derived carbon signature. A mesocosm experiment testing the effect of common lentic predators on the abundance of the lake chironomid Chironomus zealandicus, showed that if invertebrate predators were present in the lake they could markedly reduce the abundance of the pest prey species. This study highlights that the frequent re-suspension of bottom sediments, lake level fluctuation resulting in wetting and drying of littoral zones, and the management of the lake opening to the sea all have an effect on the benthic ecology of Te Waihora.

Shallow lakes in Muskoka-Haliburton have been largely ignored in previous limnological and paleolimnological studies, as many are considered to be less desirable for cottage development and other cultural activities. Nonetheless, shallow lakes offer important habitat for many animals. 30 oligo- and mesotrophic, shallow lakes were chosen for a paleolimnological study to assess the impacts of multiple stressors on cladoceran invertebrates. Fossil cladoceran remains preserved in the sediment samples of the study lakes were used to evaluate the nature and magnitude of any changes in assemblages from both modern and pre-industrial times. Relationships between present-day assemblages and key environmental variables were investigated using redundancy analysis, which identified that lake area (p<0.01) and Secchi depth (p<0.05) were significant predictors of assemblage composition in the shallow lakes. Secchi depth was not correlated to water clarity measures, as it usually is, but rather to macrophyte abundance. The modern-day assemblages were compared to the pre-industrial assemblages using the snapshot “top-bottom” paleolimnological approach. The top and bottom assemblages were compared using an ANOSIM which was not significant (p=0.2), confirming that modern assemblages were similar to pre-industrial assemblages in shallow lakes. Full core analysis from three shallow lakes also showed only subtle changes in littoral assemblage composition through time, further supporting results from the top/bottom study. The changes recorded were mainly in the pelagic taxa, with the littoral taxa relatively very stable through time. These small, shallow lakes were then compared and combined with a deep lake set from the same region. The results show some striking differences between shallow and deep lakes. For example, pelagic taxa appear to be driving changes within assemblages across a depth gradient, with most of the changes in the deeper lakes. Water chemistry has an increasingly important role in structuring cladoceran assemblage as lakes become deeper, although lake morphometry does play an important role in defining cladoceran assemblages in all the study lakes. Cladoceran assemblages in shallow lakes appear to be more stable than deeper lakes, despite being exposed to the same regional stressors, such as acidification, calcium decline, and climate change. / Thesis (Master, Biology) -- Queen's University, 2012-12-31 13:41:52.662

Coastal and shallow lakes are often subjected to eutrophication due to nutrients from catchment farming activities. Lake Ellesmere (Te Waihora) is a hyper-eutrophic lake which has gained recent attention because of concerns over its ecological health and fishery status. This study investigated the benthic ecology of the lake by extensive spatial and temporal sampling. Eight littoral sites were sampled on a single occasion, and 20 benthic sites were sampled once per season for one year. Water chemistry conditions, substrate and invertebrate communities varied significantly around the lake. Salinity, pH, DO and seston were primarily affected by freshwater inputs from inflow streams and salt water intrusion due to the lake opening to the sea. On these occasions, salinity reached 32 ‰ at the lake outlet. The lake invertebrate community was depauperate, comprising of only two species of invertebrate predators restricted to the littoral zone and eight benthic invertebrate taxa, dominated by oligochaetes, amphipods and chironomids. Benthic invertebrate abundances also reflect the dominant local substrate, where oligochaetes and chironomids preferred areas of silt substrate, whereas Potamopyrgus preferred harder substrate. Stable isotope and gut analysis determined that the primary food sources within the lake were phytoplankton and algae. Macrophytes provided a minimal contribution to the food web, possibly relating to the change in status from a clear water, macrophyte dominated lake to a turbid, phytoplankton dominated condition since the Wahine Storm in 1968. Isotope analysis also showed that the lake food web was markedly different in its carbon values from food webs of its inflow streams and nearby marine source. However the lake food web did show a marine-derived carbon signature. A mesocosm experiment testing the effect of common lentic predators on the abundance of the lake chironomid Chironomus zealandicus, showed that if invertebrate predators were present in the lake they could markedly reduce the abundance of the pest prey species. This study highlights that the frequent re-suspension of bottom sediments, lake level fluctuation resulting in wetting and drying of littoral zones, and the management of the lake opening to the sea all have an effect on the benthic ecology of Te Waihora.

Η λίμνη Παμβώτιδα των Ιωαννίνων είναι μια τυπική εύτροφη ρηχή λίμνη που παρουσιάζει μεγάλο οικολογικό ενδιαφέρον (ανήκει στο δίκτυο «ΦΥΣΗ 2000» , αποτελεί ενδιαίτημα πολλών ελληνικών ενδημικών ειδών ) . Ο ευτροφισμός και η εισαγωγή πολλών αλλόχθονων κυπρινοειδών ειδών τα τελευταία χρόνια έχουν προκαλέσει τα τελευταία χρόνια δραματική μείωση της αφθονίας της υδρόβιας βλάστησης. Στα πλαίσια αυτής της μεταπτυχιακής διατριβής μελετήθηκε η δομή της υπάρχουσας υδρόβιας βλάστησης και αξιολογήθηκε η επίδραση της παρόχθιας ζώνης στα φυσικοχημικά χαρακτηριστικά του νερού κατά τη δειγματοληπτική περίοδο 2004-2005. Εφαρμόστηκαν οι μέθοδοι TWINSPAN και NMS προκειμένου η υδρόβια βλάστηση να ταξινομηθεί σε ευδιάκριτες ομάδες βλάστησης και η CCA ώστε να διαπιστωθούν συσχετίσεις μεταξύ των υδρόβιων μακροφύτων και των περιβαλλοντικών παραμέτρων. Πραγματοποιήθηκε ανάλυση διασποράς (one-way ANOVA) προκειμένου να διαπιστωθούν στατιστικά σημαντικές διαφορές μεταξύ των φυσικοχημικών χαρακτηριστικών του νερού της παρόχθιας ζώνης ,του νερού υδρόβιας βλάστησης εκτός παρόχθιας ζώνης και των ανοικτών νερών. Επιπλέον διερευνήθηκαν πιθανά εποχιακά πρότυπα στη διακύμανση των φυσικοχημικών παραμέτρων ενώ διερευνήθηκαν και πιθανές συσχετίσεις μεταξύ των παραμέτρων. Χρησιμοποιήθηκαν οι τροφικοί δείκτες Carlson για την αξιολόγηση της τροφικής κατάστασης της λίμνης και την εξαγωγή χρήσιμων συμπερασμάτων. Τα αποτελέσματα των αναλύσεων έδειξαν πως η υφυδατική βλάστηση έχει μειωθεί σε δραματικά επίπεδα ,ενώ έχουν απομείνει ελάχιστα είδη που παρουσιάζουν μέγιστη τιμή πληθοκάλυψης την άνοιξη. Διακρίθηκαν 4 ομάδες υδρόβιας βλάστησης εκ των οποίων οι 3 αντιστοιχούν σε ομάδες εφυδατικής βλάστησης. Η παρόχθια ζώνη παρουσίασε χαμηλότερα επίπεδα SRP ,pH ,διαλυμένου οξυγόνου ,ανθρακικών και διαφάνειας και υψηλότερα επίπεδα όξινων ανθρακικών ,αγωγιμότητας και λόγου διαφάνειας προς βάθος σε σχέση με τους άλλους δύο οικότυπους. Τα φωσφορικά και τα αμμωνιακά κυμάνθηκαν σε υψηλά επίπεδα το καλοκαίρι ενώ τα νιτρικά εμφάνισαν την υψηλότερη συγκέντρωσή τους το φθινόπωρο. Ο λόγος DIN /SRP κυμάνθηκε σε χαμηλές τιμές ( <15) υποδεικνύοντας περιοριστικές συνθήκες αζώτου ενώ η τροφική κατάσταση της λίμνης κατατάσσεται ως εύτροφη σύμφωνα με τον τροφικό δείκτη Carlson. Σύμφωνα με τα παραπάνω υπάρχουν ενδείξεις πως η παρόχθια ζώνη, είτε λόγω των υδρόβιων μακροφύτων είτε λόγω άλλων μηχανισμών ,επηρεάζει με διάφορους τρόπους ορισμένα φυσικοχημικά χαρακτηριστικά του νερού. Τα αυξημένα επίπεδα φωσφόρου το καλοκαίρι υποδεικνύουν την ύπαρξη μηχανισμών απελευθέρωσης φωσφόρου από το ίζημα ,ενώ οι χαμηλές τιμές του λόγου DIN/SRP αποτελούν ένδειξη για την επικράτηση αζωτοδεσμευτικών κυανοβακτηρίων με δυσμενείς επιπτώσεις για την υδρόβια ζωή. / Lake Pamvotis is a shallow eutrophic lake situated in northwestern Greece (Region of Epirus). The lake has been stocked with grass carp and common carp during the last years and since then a serious decline of submerged vegetation has been observed. The aquatic vegetation of the lake was studied during the sampling period 2004 –2005 and was classified in groups using TWINSPAN and NMS. CCA was performed in order to examine relationships between the aquatic macrophyte species abundance and the environmental variables. Moreover ,it was investigated the effect of the littoral zone on physical and chemical characteristics of water quality and the seasonal patterns of the environmental variables. One –way ANOVA was used to determine statistical significant differences of environmental variables between the littoral zone and the non –littoral plots with aquatic vegetation and without aquatic vegetation. The trophic state index of Carlson was calculated to evaluate the trophic state of the lake. The results showed a great decline of submerged aquatic vegetation and only a few submersed species were recorded (Ranunculus trichophyllus ,Potamogeton crispus ,Callitriche stagnalis). One-way ANOVA showed that SRP ,pH ,surface D.O ,carbonates and transparency were lower in the littoral zone while bicarbonates ,conductivity and transparency /depth ratio were higher. Higher SRP was recorded during the summer due to increased internal loading from the sediment ,while higher nitrate concentrations were occured during the autumn. The ratio DIN/SRP ranged in low values (<15) from spring to autumn indicating N limiting conditions where cyanobacteria blooms are likely to be dominant. The trophic state of the lake is classified as eutrophic according to the Kratzer and Brezonik classification system.

nÃo hà / Identificar se as comunidades estÃo estruturadas de forma aleatÃria ou com base em interaÃÃes biolÃgicas trata-se de um dos aspectos mais importantes em ecologia . Neste sentido a identificaÃÃo dos eventos envolvidos em sucessÃo de espÃcies pode contribuir de forma significativa. As lagoas temporÃrias do semi-Ãrido brasileiro sÃo ambientes Ãmidos temporÃrios, com caracterÃsticas peculiares que permitem uma rÃpida sucessÃo de espÃcies, uma vez que, apresentam variaÃÃes ambientais, fase Ãmida e seca, responsÃveis pela seleÃÃo de espÃcies de acordo com suas formas de vida. Estes ambientes apresentam comunidades, dentre as quais se destacam as macrÃfitas aquÃticas, adaptadas aos perÃodos sazonais de inundaÃÃo. Assim, teve-se como objetivo descrever as variaÃÃes na estrutura de uma comunidade de macrÃfitas aquÃticas, em uma lagoa temporÃria, durante seu perÃodo de inundaÃÃo, relacionando as alteraÃÃes na dominÃncia e composiÃÃo de espÃcies com variÃveis ambientais. Como tambÃm, levantar consideraÃÃes sobre a co-ocorrÃncia de espÃcies atravÃs do c-score. As amostragens de dados foram realizadas quinzenalmente atravÃs do mÃtodo dos pontos a fim de se obter dados quanto a estratificaÃÃo, composiÃÃo e dominÃncia de espÃcies na comunidade , totalizando13 amostragens. AlÃm dos dados da vegetaÃÃo foram tambÃm obtidos dados de pH, condutividade, turbidez, alcalinidade, nitrogÃnio total, nitrato, fosfato total, ortofosfato, oxigÃnio dissolvido e demanda bioquÃmica de oxigÃnio da Ãgua. Estes foram relacionados com os dados de dominÃncia de espÃcies mostrando que as variÃveis ambientais afetam de formas diferentes as espÃcies estudadas. O c-score foi calculado tanto para cada coleta individualmente como para todo o perÃodo de inundaÃÃo. Nos estÃgios do perÃodo de inundaÃÃo predominaram as interaÃÃes biolÃgicas na comunidade e considerando todo o perÃodo o c-score mostra uma ocupaÃÃo das espÃcies de acordo com um padrÃo de sucessÃo / Identify whether communities are structured randomly or based on bio logical interactions is one of the most important aspect in ecology. In this sense the identification of events involved in the succession of species can significantly contribute. The temporary ponds in the semi-arid environments are temporary wet, with distinctive features that allow a rapid species succession, as they present environmental variations, wet and dry phase, responsible for the selection of species according to their ways of life. These environments present communities, which emphasize the aquatic macrophytes, adapted to seasonal flooding periods. So, it came up to describe the variations in the structure of a aquatic macrophytes community in a temporary pond, during the flooding period, relating the changes in dominance and species composition to environmental variables. As well, raising considerations of co-occurrence of species through the c-score. Data samples were made fortnightly by the method of points in order to obtain data on the stratification, composition of dominant species in the community, totaling 13 samples. In addition to the vegetation data were also obtained data for pH, conductivity, turbidity, alkalinity, total nitrogen, nitrate, total phosphate, orthophosphate, dissolved oxygen and biochemical oxygen demand of water. These were related to the data of dominance of species showing that environmental variables affect different forms of the species studied. The c-score was both calculated for each sample individually and for the duration of flooding. For the each sample separate ly prevailed biological interactions in the community and considering the whole period the c-score shows an occupation of the species according to a pattern of succession

Grunda, näringsrika sjöar förekommer i två stadier: antingen ett med grumligt vatten och hög biomassa av växtplankton, eller ett med klart vatten och riklig undervattensvegetation. Det klara stadiet är eftersträvansvärt eftersom det skapar goda förutsättningar för en rad ekosystemtjänster, inklusive vattenkvalitet, rekreation och förutsättningar för biodiversitet. Vid eutrofiering, d v s ökat tillskott av näringsämnen, kvarstår det klara stadiet tills näringsbelastningen blir så hög att sjön övergår till ett grumligt stadium. Sjön Tåkern, i Östergötland, har under de senaste åren blivit alltmer eutrofierad och kalkning av vattnet har diskuterats som en möjlig åtgärd för att bevara det klara stadiet som sjön numera befinner sig i. Undersökningar om kalkning av eutrofierade sjöar har gjorts tidigare, men det har varit få studier av effekterna av kalkning på undervattensvegetation, substratlevande djur och påväxtalger. För att undersöka detta, gjordes ett fält- och ett laboratorieexperiment inriktade på effekter på överlevnad av vattengråsugga, Asellus aquaticus, fysiologisk status (halten klorofyll a) i kransalgen Chara globularis och tillväxt av nyetablerade påväxtalger i sjön Tåkern. Resultaten visar att A. aquaticus hade lägre överlevnad efter kalkning till pH 10 i laboratorieexperiment, men i fältförsöket hade kalkning till detta pH inte någon inverkan. C. globularis hade en högre halt klorofyll a efter kalkning till pH 10 i laboratorieexperimentet, men i fältexperimentet hade kalkning ingen effekt på halten klorofyll a. Påväxtalger hade en lägre tillväxthastighet efter kalkning i fältexperimentet. Slutsatsen är att kalkning kan vara en metod för att motverka eutrofiering av sjön Tåkern. Några inkonsekvenser mellan mesokosm- och laborationsresultaten tyder på att fler studier behövs göras för att undersöka hur kalkning till pH 10 påverkar bentiska organismer. / Shallow, nutrient-rich lakes can occur in two alternative states: either a turbid water state with high biomass of phytoplankton, or a clear water state with abundant submerged vegetation. The clear water state is worth striving for because it creates good conditions for a range of ecosystem services and biodiversity. Lake Tåkern, Östergötland, has in recent years become increasingly eutrophic and liming has been discussed as a method to reduce the phosphorus level and maintain, or trigger a shift to, the clear water state. Investigations regarding the effects of liming of eutrophic lakes on phosphorus level and plankton have been performed in the past, but few studies have examined the effects on submerged vegetation, benthic invertebrates and periphyton. To study this, a field and a laboratory experiment were performed to examine how liming to pH 10 affects survival of the benthic invertebrate Asellus aquaticus, the chlorophyll a content in the stonewort Chara globularis and growth of periphyton. Survival of A. aquaticus was not affected in a field mesocosm experiment in Lake Tåkern, but decreased after liming in a laboratory experiment. The chlorophyll a content of C. globularis was not affected in the field experiment but higher than in the controls in the laboratory experiment. Growth of periphyton was reduced in the field experiment. Based on the field experiment, effects on A. aquaticus and C. globularis were negligible, but the laboratory experiment indicate that liming to pH 10 may under certain conditions affect benthic organisms. The conclusion is, however, that liming can be a suitable method to counteract the eutrophication of Lake Tåkern. Some inconsistencies between the field and laboratory results suggest that more studies needs to be made to see how liming to pH 10 affect benthic organisms.

The majority of lakes on Earth are shallow, unproductive and located at high latitudes. These lakes are experiencing big changes due to climate change, where two environmental drivers operate simultaneously, browning and warming. How they affect lake ecosystems is not well understood. Here, I addressed this issue by using a theoretical and an experimental approach. In particular, I generated model predictions and compared them with the results of a realistic large-scale experiment, where browning and warming were manipulated in a factorial design. In addition, model outcomes were compared with data from 12 unproductive lakes sampled along a gradient of browning. Another novelty of my thesis is that it integrates benthic and pelagic food web components in the model and experimental approaches. I found that browning affected the resources availability for benthic and pelagic producers in the model and in the experiment. With browning, benthic primary producers became increasingly light limited and declined, while pelagic producers became less nutrient limited and increased. Pelagic nutrient limitation was alleviated by two non-exclusive mechanisms. Browning directly enriched the water with nutrients, and browning indirectly increased the nutrient flowing from the sediment to the pelagic habitat via suppression of benthic producers. To tease apart these two mechanisms I applied structural equation modeling (SEM). The indirect evidence by SEM suggests that both mechanisms contributed equally to the pelagic nutrient concentration in the experiment. Interestingly, a model food web with only primary producers shows similar qualitative behavior as a food web with grazers and carnivores included. This happens because carnivorous fish exert strong top-down control in the more productive habitat, which relaxes grazing pressure on primary producers and increases resource limitation in the adjacent habitat. Biomass of benthic and pelagic consumers followed the same pattern as their resources. The lake data were largely congruent with model expectations and supported the findings of the experiment. Furthermore, the model also predicted a negative relationship between total phosphorus and both primary and fish production, which was observed across the 12 lakes. Warming effects were more complex. The model predicts that warming effects should depend on browning and are expected to be strongest in the more productive of the two (benthic and pelagic) habitats. For example, at low levels of browning the biomasses of benthic algae and fish are expected to decline with warming, which was observed in the experiment. In contrast, observed warming effects at high levels of browning deviated from model expectations. The mechanisms by which browning and warming interactively affect lake food webs are still poorly understood. This thesis offers a conceptual foundation for their further study through the integration of within- and between-habitat interactions.

Climate changes alter species composition in both aquatic and terrestrial ecosystems. For establishment of species in a fish community, dispersal barriers, habitat structure and interactions between species act as a filter. The aim of this study was to examine changes in the fish community in Lake Tåkern, Sweden, by repeating a test-fishing in a lake 40 years after a previous test-fishing. Special interest was directed towards the possible establishment of bream (Abramis brama) and (Cyprinus carpio) which recently had been found in the lake. In addition, a decline of the low oxygen tolerant species crucian carp (Carassius carassius) and tench (Tinca tinca) was expected. Test-fishing was performed in June and August 2019 and combined with analyses of Environmental-DNA from samples in September 2019. A comparison with the average catch of 1978-1979 together showed that all cyprinid species had decreased in 2019. The hypoxic tolerant crucian carp decreased 16-fold in numbers, which was expected due to fewer events of cold winters and hypoxia. One bream was caught from the test-fishing and was also detected through eDNA-analysis. The ecological status of the fish community was classified as moderate but showed a high proportion of piscivore percids contra cyprinids, compared to six other shallow lakes in Sweden with similar characteristics. The high proportion of piscivore percids is probably contributing to the stability of the present clear-water state of the lake.

Le vent joue un rôle clé dans la structure et le fonctionnement des lacs. Le brassage de la colonne d’eau est notamment impliqué dans la compétition entre les différentes espèces de phytoplancton dont les stratégies de positionnement vertical varient largement entre groupes. Par exemple, les cyanobactéries sont globalement favorisées par des conditions de brassage faible grâce à des mécanismes physiologiques de flottaison leur permettant de se maintenir à une profondeur désirée, alors que les diatomées et les algues vertes qui n’ont pas de système de flottaison dépendent d’un brassage important pour être suspendues dans la zone euphotique. Dans les lacs peu profonds, le brassage généré par le vent est généralement suffisant pour entrainer la resuspension des sédiments. Ceci a pour conséquence directe une augmentation de la turbidité qui peut directement impacter la croissance et la productivité algale. La seconde conséquence est la libération de phosphore précédemment stocké dans les sédiments. La pollution et l’usage d’engrais ont provoqué un important import de phosphore dans les lacs qui sont pour la plupart dans un état d’eutrophisation. Dans un premier temps, cette thèse a eu pour objectif de modéliser la compétition entre une cyanobactérie et une algue verte dans une colonne d’eau de lac peu profond prenant en compte la possibilité de resuspension des sédiments et de phosphore. Différents niveaux de brassage ont été testés : de quasi-stagnant à tempête. Des centaines de simulation ont été réalisées dans différentes conditions de brassage, d’eutrophisation (de oligo- à hypereutrophe) et de réchauffement climatique (actuel et +2°C) permettant d’établir des domaines de conditions de dominance d’une espèce ou de l’autre. Dans une seconde partie, la thèse a eu pour objectif d’explorer les impacts potentiels du brassage sur l’ensemble du réseau trophique pélagique. En effet, la grande majorité des études portant sur le brassage sont des études in situ focalisées sur le phytoplancton. Dans le cas présent, nous avons utilisé des mésocosmes de 15m3 équipés de batteurs à vagues dont les amplitudes, fréquences et longueurs d’ondes sont modifiables. Une première expérience en 2012 a eu pour objet de suivre durant 9 semaines la dynamique des variables physico-chimiques standards et des communautés de phytoplancton, zooplancton, bactéries et virus dans deux modalités de brassage : brassage superficiel et brassage de l’ensemble de la colonne d’eau avec resuspension des sédiments. L’année suivante, la même expérience a été conduite mais cette fois-ci couplée à un traitement de réchauffement appliqué grâce à des bâches de polyéthylène transparentes posées sur les mésocosmes. Ces deux expériences montrent un effet important du brassage sur le phytoplancton avec une augmentation du contenu en chlorophyll a et une augmentation de la productivité. Au niveau du zooplancton, la réponse dépend du sous-groupe. Dans nos expériences, uniquement les copépodes ont répondu négativement au régime de brassage avec un potentiel effet cascade sur la population de rotifers. L’expérience de 2012 suggère également un renforcement de l’activité de lyse bactérienne par les virus dans des conditions de brassage important. En 2013, aucun effet du réchauffement de l’eau de 1°C n’a été observé sur les variables mesurées. Des analyses supplémentaires en cours devraient permettre de compléter les informations obtenues et de vérifier l’absence d’effet du réchauffement. / The wind plays a key role in the structure and functioning of lakes. Water column mixing is involved in the competition between different species of phytoplankton with different vertical positioning strategies. For example, buoyant cyanobacteria are generally favored by low mixing conditions, while sinking diatoms and green algae depend on mixing to be suspended in the photic zone. In addition, in shallow lakes, wind-induced mixing is usually sufficient to cause sediment resuspension. This leads to an increase in turbidity which can directly impact the algal growth and productivity. Resuspension can also induce the release of phosphorus previously stored in sediments. This internal loading via resuspension can boost algal production and growth. The first goal of this thesis was to model the competition between the buoyant cyanobacterium Microcystis aeruginosa and a sinking green algae under different mixing conditions: from quasi-stagnant to storm-like events. Hundreds of simulations were carried out in different conditions of mixing, phosphorus concentration and water temperatures in order to establish the conditions necessary for the dominance of one species on the other. In the second part of this thesis, we explored the potential impacts of mixing on the pelagic food web. The vast majority of previous studies on mixing in shallow lakes are in situ studies focused on phytoplankton. In the summer 2012, we used 15m3 mesocosms equipped with wave-makers and followed during nine weeks the dynamics of physical and chemical variables and the dynamic of phytoplankton, zooplankton, bacteria and viruses under two mixing conditions: whole column mixing with sediment resuspension and superficial mixing without resuspension. The following year, the same experiment was conducted but this time crossed with a warming treatment. Both experiments gave similar results in regards to mixing. We observed a significant effect of mixing on phytoplankton with an increase in chlorophyll content and an increase in productivity. The response of zooplankton to mixing depended on the subgroup. In our experiments, only copepods responded negatively to mixing with a potential cascade effect on the rotifer population while bosminas remained unaffected throughout the experiments. The 2012 experience also suggests an increase in bacterial lysis activity by virus in mixed enclosures. In 2013, no effect of water warming (+1°C) was observed on the measured variables, however, additional analysis are still underway to confirm or infirm these results.

No description available.