In the Arctic, shallow sea-floor communities have been documented to shift abruptly from an invertebrate-dominated state to a state with high macroalgal abundance. Climate warming, resulting in increased water temperatures and decreased sea ice cover, could trigger such regime shifts in benthic ecosystems, but the underlying mechanisms are not clear. To gain a better understanding, a mechanistic model of algal growth and interspecific competition was applied in two marine ecosystems in northwestern Svalbard. Unravelling the effects of light and temperature on the model parameters, the current study showed that light is a key factor determining the algal dominance in the two ecosystems. Changes in sea-ice cover, which alters the underwater light regime, is therefore considered the most likely trigger for invertebratealgae regime shifts. A continued prolonging of the ice-free season in the Arctic is likely to occur in the upcoming years, considering the current climatic development. Thus, it is expected that macroalgal cover in shallow rocky-bottom communities will continue to increase, altering species composition and function in seasonally ice-covered arctic ecosystems. / I ekosystem på grunda havsbottnar i Arktis har snabba, kraftiga förändringar i artsammansättning och ekosystemstruktur dokumenterats. Organismsamhällen som tidigare främst utgjorts av ryggradslösa organismer och hårda kalkalger har plötsligt blivit dominerade av olika tångarter. Den globala uppvärmningen medför högre ytvattentemperatur och minskat havsistäcke i Arktis, vilket i sin tur innebär förbättrade ljusförhållanden. Dessa förändringar tros kunna utlösa regimskiften där de ekosystemen övergår från ett stadium dominerat av ryggradslösa djur, till ett stadium dominerat av tång. Mekanismerna bakom sådana regimskiften är emellertid inte kartlagda. Denna studie syftade därför till en bättre förståelse för regimskiften i Arktiska havsbottenekosystem. En mekanistisk modell för algtillväxt och konkurrens tillämpades på två marina organismsamhällen i nordvästra Svalbard, som genomgått sådana regimskiften. Temperatur- och ljusberoendet hos modellens parametrar uppskattades först utifrån kunskap från ett antal tidigare studier. Med hjälp av modellen testades sedan hur fördelningen av alger påverkades av temperatur- och ljusförändringar. Modelleringen indikerade att ljusförhållanden var avgörande för expansionen av tång i de två studerade organismsamhällena på Svalbard, och att ljusförändringar således kan vara en huvudsaklig orsak till regimskiften i Arktis. Med tanke på den nuvarande klimatutvecklingen är en fortsatt förlängning av den isfria säsongen mycket sannolik. Detta kommer troligen innebära en vidare expansion av tång samt förändrad artsammansättning och ekosystemfunktion i grunda, arktiska havsbottenekosystem.

Some of the major problems we face today are human induced changes to the nitrogen (N), phosphorus (P) and carbon (C) cycles. Predicted increases in rainfall and temperature due to climate change, may also increase dissolved organic matter (DOM) inflows to freshwater ecosystems in the boreal zone. N, P, C and light, are essential resources that most often limit phytoplankton (PPr) and bacterial production (BP) in the pelagic zone of lakes. PPr and BP not only constitute the total basal C resource for the pelagic aquatic food web, but also influence ecosystem function and biogeochemical cycles. In this thesis I studied how N, P, C and light affect the relative and absolute rates of PPr and BP, along a wide latitudinal and trophic gradient using published data, and in two in situ mesocosm experiments in a clear water oligotrophic lake. In the experiments I manipulated bottom-up drivers of production and top-down predation to examine how these factors interact to affect pelagic food web structure and function. The most important predictors of PPr globally (Paper I) were latitude, TN, and lake shape. Latitude alone explained the most variation in areal (50%) and volumetric (40%) PPr. In terms of nutrients PPr was primarily N-limited and BP was P-limited. Therefore bacteria and phytoplankton were not directly competing for nutrients. BP:PPr was mostly driven by PPr, therefore light, N, temperature and other factors affecting PPr controlled this ratio. PPr was positively correlated with temperature, but not BP, consequently, higher temperatures may reduce BP:PPr and hence the amount of energy mobilised through the microbial food web on a global scale. In papers II and III interaction effects were found between C-additions and top-down predation by young-of-the-year (YOY) perch. Selective predation by fish on copepods influenced the fate of labile C-addition, as rotifer biomass increased with C-addition, but only when fish were absent. Interaction effects between these top-down and bottom-up drivers were evident in middle of the food web, which is seldom examined in this type of study. Although the energy pathway from bacteria to higher consumers is generally longer than from phytoplankton to higher trophic levels, increased BP still stimulated the biomass of rotifers, calanoid copepods and YOY fish. However, this appeared to be mediated by intermediate bacterial grazers such as flagellates and ciliates. Light was an important driver of crustacean zooplankton biomass (paper IV), but the light:nutrient hypothesis was inadequate to predict the mechanisms behind the decrease in zooplankton biomass at low light. Instead, it appeared that reduced edibility of the phytoplankton community under low light conditions and reduced BP most strongly affected zooplankton biomass. Thus, the LNH may not apply in oligotrophic lakes where PPr is primarily N-limited, Daphnia is rare or absent and mixotrophic phytoplankton are abundant. N, P, C and light manipulations have very different effects on different parts of the pelagic food web. They influence the relative rates of PPr and BP, affect phytoplankton community composition, alter the biomass of higher trophic levels and change pathways of energy transfer through the pelagic food web. This thesis adds valuable information as to how major changes in these resources will affect food web structure and function under different environmental conditions and future climate scenarios. / Lake ecosystem response to environmental change

The minimum ecologically viable population size (MEVP) of a species describes the minimum size at which the species itself or another species in the same ecosystem goes extinct as a result of the loss of inter-specific interactions. The MEVP shows a good potential for use as a tool for exploring the mechanisms behind species extinctions, but presently only a small amount of research has been done that takes advantage of this. In this study the MEVP is used to investigate what properties of species can be used as good indicators of ecological importance. 100 large computer generated food webs were created with an assembly model and the reduction in density that was necessary to induce an extinction event in the web was subsequently determined for each species within the webs. This change in density was then correlated with 28 different properties, measured for each species. The results show that properties that measure how well connected a species is, as well as measures of the species role as a prey item in the web are the ones with the greatest potential to find species with high MEVP. Further, the results put emphasis on the importance of regarding the web as a whole when working with species extinctions, while also highlighting the usefulness of the MEVP concept.

Female reproductive decisions have been suggested to be highly influenced by mate quality. I have studied whether offspring quality may be adjusted by females to match the attractiveness of males and how strong control females have over their reproductive investment focusing on egg size. This was done in the Banggai cardinalfish (Pterapogon kauderni), a sex-role reversed obligate paternal mouthbrooder where males invest heavily into reproduction. As this species is suitable for both laboratory experiments and field studies it is an ideal candidate for the study of reproductive investment. Mating was size-assortative and both males and females benefited from pairing with large partners. However, male size determined the reproductive output of a pair. Females courted large males more intensively and produced larger, but not fewer eggs when mated to large males as compared to small males. Further, this matching of egg size to mate attractiveness may be fast. Female courtship behaviours contained honest information regarding both clutch weight and egg maturity, traits that may be highly important for male mate choice. Surprisingly, males played an important part in territory defence suggesting relatively equal sex-roles in this species. Also, this species showed stable group structures which may be important for the evolution of female plasticity in reproductive investment due to high variance in quality of available mates. This thesis suggest that females have a remarkable control over their reproductive investments and that male quality may be highly influential on reproductive decisions regarding offspring quality. Furthermore, it suggest that sexual selection may have strong effects on the evolution of egg size and parental care on a whole.

This thesis deals with the question of whether planktonic protits of the genus Dinophysis have permanent plastids (=chloroplasts) or practice kleptoplasty, i.e. acquire plastids via predation on other microorganisms. Sequencing the plastid 16S rDNA of Dinophysis spp. collected from 4 different geographical regions unveiled two different plastid genotypes within this genera: one that was found at all locations investigated, identical to that of the free-living cryptophyte Teleaulax amphioxeia, and another found only in the Greenland Sea, closely related to that of the cryptophyte Geminigera cryophila. Both types were found within the species D. acuminata. These findings imply that the plastids in Dinophysis spp. were not inherited from a common ancestor, but acquired from feeding. By using flow cytometry in combination with an acidotrophic probe, it was shown that 71 % of the cells in a D. norvegica population in the aphotic zone of the Baltic Sea had food-vacuoles. Dinophysis used to be regarded as a primarily phototrophic organism, and this was a higher proportion of cells with food-vacuoles than reported earlier. To further study if Dinophysis needs constant refill of new plastids from the environment, a new method combining flow-cytometry and quantitative real-time PCR was developed to compare the levels of nuclear and plastid DNA in different phases of the cell-cycle. Results showed that plastid acquisition in Dinophysis was uncoupled with the cell-cycle, which is different than the pattern seen in microalgal species with permanent plastids. Furthermore, when quantitative real-time PCR combined with flow-cytometry was used to follow D. caudata cultures during a 65 days starvation/feeding experiment, the cells first went through a steady decrease in plastid DNA during starvation. In contrast, after feeding on the ciliate Myrionecta rubra, plastid DNA in starved cells increased 7-fold, thereby directly revealing the kleptoplastic behavior. The main conclusion from this thesis is that Dinophysis cells are actively taking up kleptoplastids from the ciliates on which they feed, and that kleptoplasty is an important key to understand Dinophysis ecology. Part of this thesis work has also been dedicated to the application and optimization of new methods, and it shows how quantitative real-time PCR, flow cytometry and molecular methods in different combinations can be used as powerful tools for the study of plankton ecology.

La Zone de Fracture d'Owen (ZFO) est une faille de 800 km de long qui accommode actuellement le mouvement dextre entre l'Inde et l'Arabie. Elle s'inscrit le long d'un petit cercle qui implique un mouvement purement dextre sur la majeure partie de la faille. La couverture bathymétrique complète de la ZFO recueillie au cours de la campagne Owen-1 révèle plusieurs segments de failles séparés par des relais, dont un bassin pull-apart de 90 km de long sur 30 km de large à la latitude 20°N. Un bassin pull-apart rhomboédrique (Beautemps-Beaupré) marque la terminaison sud de l'OFZ, tandis qu'un bassin plus complexe, la fosse de Dalrymple, constitue la terminaison Nord. La ZFO recoupe la Ride d'Owen-Murray, qui forme une série d'imposants reliefs océaniques. Les décalages morphologiques de la Ride d'Owen confirment un décalage dextre d'environ 10-12 km le long de l'OFZ. En considérant constant un mouvement de 3±1 mm.a-1 estimé à partir de données géologiques et GPS, l'âge de la ZFO est compris entre 3 et 6 Ma. De même, l'âge du bassin 20°N et de la fosse de Dalrymple sont estimés à 2-3 Ma à partir de calibration avec les forages ODP-DSDP. L'âge de ces bassins est contemporain d'une discordance majeure observée dans la plaine abyssale de l'Oman (discordance "M") associée au prisme du Makran. D'autre part, la Ride d'Owen est érodée par des systèmes complexes de glissements de terrains sous-marins, qui représentent une source potentielle d'aléa tsunami pour les côtes de l'Oman. Les données stratigraphiques indiquent que la récurrence de tels glissements (et des tsunamis associés) est faible, de l'ordre de 105-106 années. Les données de sismique réflexion collectées lors de la campagne Owen-2 ont permis d'identifier les traces Miocène et Paléogène de la limite de plaque, entraînant une révision complète de son histoire géologique. Tout d'abord, l'identification de lambeaux de l'ophiolite de Masirah sur le socle du Bassin d'Owen aux abords de la marge est-omanaise indique que le socle est à cet endroit d'âge Crétacé (au minimum). Cela contraste avec l'âge Paléocène des roches forées au niveau du socle de la Ride d'Owen. Ces observations indiquent que le socle du Bassin d'Owen est d'âge mixte, et que des portions de lithosphère d'âge différent ont été mises en contact par une faille transformante majeure. Le système de zones de fracture structurant le socle du Bassin d'Owen était associé à la dorsale de Carlsberg au cours du Paléocène-Eocène. L'arrêt de son activité, associé à un épisode de transpression à la fin de l'Eocène, est marqué par le soulèvement de rides marginales et une discordance angulaire régionale. La limite de plaque Inde-Arabie est ensuite retrouvée au niveau de l'actuelle Ride d'Owen dès la fin de l'Oligocène. Cette relocalisation de la plaque Arabe a impliqué le transfert d'un vaste segment de la plaque Indienne à la plaque Arabe. Le soulèvement de la Ride d'Owen a eu lieu autour de 8.2-8.8 Ma, en réponse à un changement cinématique de la plaque Indienne, probablement en réponse à la croissance des reliefs himalayens et au début de l'étalement gravitaire de la chaîne. Cet épisode de déformation est responsable de l'inactivation de la limite de plaque Miocène, et de sa relocalisation au niveau de l'OFZ actuelle. Un changement environnemental majeur a été précédemment identifié sur le pourtour de la Mer d'Arabie dans la séquence de Siwalik au Pakistan. L'augmentation drastique des abondances de G. Bulloides dans la couverture sédimentaire de la Ride d'Owen, précédemment interprétée comme le résultat d'une abrupte intensification de la mousson autour de 8.5 Ma, marque en réalité un changement des conditions de préservation des foraminifères suite au soulèvement de la ride. Nous proposons que l'épisode de déformation identifié dans le Bassin d'Owen ait pu également favoriser le soulèvement de reliefs au niveau de la marge du Dhofar et du prisme d'accrétion du Makran, conduisant à une réorganisation de la circulation atmosphérique à l'origine du changement environnemental de Siwalik.

Coral reefs belong to the most diverse and the most threatened ecosystems on earth. Anthropogenic stressors and climate change have led to mortalities at levels unprecedented in modern times. The aims of this thesis are to investigate aspects of the corals’ ability to reproduce, disperse, adapt and survive. Papers I-III study reproduction in a common soft coral species, Sarcophyton elegans, with previously unknown reproductive modes. Paper IV investigates genetic distribution of coral-symbiont associations in Galaxea fascicularis focusing on adaptation to the environment along the coastline of Vietnam. Sarcophyton  elegans is a gonochoric broadcast spawner with a 1:1 sex ratio. Reproduction is strictly size dependent. Oogenesis takes 19-24 months, with a new cycle commencing every year. Spermatogenesis takes 10-12 months. The majority of gametes were released during the annual austral mass spawning event after full moon in November, but spawning also occur between August and February. The polyps at the outer edge of the colonies released their gametes first, followed by polyps situated closer to the center during subsequent months. Colonies upstream in the prevailing current spawn earlier than those downstream. The colonies were arranged in clusters of alternating males and females, which spawned simultaneously and were of the same genotype. Fission and buddying is a common mode to expand locally. Additionally, females undergoing fission divided into the most fecund size classes. The G. fascicularis and their associated symbionts were not genetically coupled to each other but to environmental factors. The host displayed an inshore-offshore zonation, with higher diversity offshore. The D1a symbiont exhibited an inshore- offshore zonation. In contrast; the 5 different C symbiont types showed a latitudinal distribution gradient, which shifted in dominance north to south. The study highlights the importance of protecting resilient coral and algal genotypes in stressed areas and the need to understand reproductive modes for coral conservation. / Các rạn san hô là một trong những hệ sinh thái có tính đa dạng và bị đe dọa cao nhất trên trái đất. Các áp lực từ con người và nhiệt độ nước biển tăng (SSTs) đã gây ra hiện tượng “tẩy trắng” gây chết san hô ở mức độ cao chưa từng thấy trong thời điểm hiện tại. Mục tiêu của nghiên cứu này là tìm hiểu khả năng của san hô trong thích nghi, phân tán và sống sót nhằm duy trì quần thể. Bài báo số II-III là những nghiên cứu đầu tiên về đặc điểm sinh sản của loài san hô mềm phổ biến, Sarcophyton elegan tại Australia. Bài báo số IV nghiên cứu về phân bố nguồn gen của tảo cộng sinh trong loài san hô Galaxea fascicularis, tập trung vào sự thích nghi với môi trường dọc theo vùng biển Việt Nam, khu vực bị ô nhiễm từ lục địa. Sarcophyton elegans được biết với đặc điểm sinh sản cả vô tính và hữu tính. Loài này là loài sinh sản bằng cách phân tán trứng, với tỷ lệ giới tính là 1:1 và sự sinh sản hữu tính bị khống chế nghiêm ngặt bởi kích cỡ của tập đoàn (Bài báo II, phần phương pháp của Bài báo I). Quá trình tạo trứng kéo dài từ 19 đến 24 tháng với chu kỳ sinh sản lặp lại hàng năm, và sự sinh tinh kéo dài từ 10 đến 12 tháng. Phần lớn giao tử được giải phóng trong một thời gian ngắn sau ngày trăng tròn của tháng 11, nhưng giao tử vẫn được giải phóng trong ngày trăng tròn của các tháng từ tháng 8 đến tháng 1 năm sau. Các polyp autozooid nằm phía ngoài của tập đoàn giải phóng giao tử trước, sau đó là các polyp nằm gần lõi trong các tháng tiếp theo. Các tập đoàn ngược lên trong dòng chảy thịnh hành đẻ trứng sớm hơn các tập đoàn xuôi dòng khoảng một tháng (Bài báo II). Các tập đoàn được sắp xếp thành từng đám từ 7 đến hàng trăm tập đoàn trong mỗi nhóm, bao gồm cả đực và cái. Các tập đoàn trong cùng một nhóm sinh sản cùng một thời điểm. (Bài báo II) và mỗi nhóm có cùng một kiểu di truyền (Bài báo III) có đầy đủ 13 (có thể là 22) kiểu di truyền  khác nhau. Sự phân đôi và kết đôi phụ thuộc hoàn toàn vào kích thước và có lẽ là phương thức mở rộng phổ biến nhất. Sự phân đôi phải mất 2 năm hoặc hơn mới hoàn thành. Thêm vào đó, con cái trải qua quá trình phân đôi thành kích cỡ có khả năng sinh sản cao nhất (Bài báo III). Có 6 nhóm haplotypes (mtDNA) của loài G. fascicularis và tảo cộng sinh Symbiodinium (ITS2 rDNA) không đóng cặp với nhau nhưng lại gắn với các yếu tố môi trường, có thể như kết quả của phương thức sinh sản của vật chủ (Bài báo IV). Vật chủ có sự phân vùng rõ rệt giữa gần bờ và xa bờ, với sự đa dạng cao hơn hẳn của các rạn xa bờ so với các rạn gần bờ, khu vực thường xuyên bị độ đục, ô nhiễm và lắng đọng trầm tích tác động. Tảo cộng sinh Symbiodinium D1a ITS2 điểm hình của sự phân vùng gần bờ và xa bờ. Ngược lại, 5 loại C khác lại có sự phân vùng theo vĩ tuyến, với sự tăng lên rõ rệt theo chiều Bắc-Nam, cùng với sự ổn định SST và sự tăng lên của các SST. Nghiên cứu này đã chỉ rõ tầm quan trọng trong bảo vệ các loài san hô và tảo biển bản địa tại các khu vực bị đe dọa (Bài báo IV) và sự cần thiết phải hiểu các phương thức sinh sản (Bài báo II-III) và các thông số môi trường trong việc xác định mức độ đa dạn sinh học và sự hấp thụ của sinh vật cộng sinh trong san hô cứng và san hô mềm. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

The colour choice of spiders and the effects of competition on this phenomenon were examined in the laboratory using the sit-and-wait predator goldenrod crab spider (Misumena vatia, (Thomisidae)). This species can  be both white and yellow (could also change colour)  and it was here examined whether individuals preferred to sit on flowers which has the same colouration as their bodies. I found that M.vatia showed preference for flowers that matched their own body colour, but when competition was introduced this preference disappear. It seems, however, that position is a factor that can influences the substrate decisions of M. vatia, where the edge of the flower is preferred.

This research aims to investigate the effects that dredging may have on benthic fauna. The dredging took place in the stream Ireån on northern Gotland during the early summer 2010. My area of research was taking place in a scenic area downstream in Ireån close to the Baltic Sea. I chose this area due to the dredging that started just upstream of this area and continued further upstream. Thus it is not the direct impact of the dredging that is the focus for this paper but the side-effect the dredging may have on the area downstream. In this research I used the M42-method and BQI- index in order to evaluate the results. The effects on the dredging were weaker than expected, with only minor changes occurring within the benthic fauna (BQI-value before dredging: 3.35, BQI-value after dredging: 3.51).

Depuis la fin de la dernière période glaciaire, chaque région du globe a connu une évolution du niveau marin relatif - ou niveau marin « ressenti » - qui lui est propre, principalement conditionnée par la conjonction de la remontée globale « eustatique » du niveau marin et des dynamiques isostatiques, responsables des mouvements verticaux de la surface terrestre. Ces derniers, encore en cours aujourd’hui, jouent un rôle majeur dans l’augmentation actuelle du niveau marin. Malgré d’importants travaux menés au Royaume-Uni au cours des dernières décennies, la connaissance de l’évolution du niveau marin relatif holocène et des dynamiques isostatiques sur la façade Atlantique ouest-européenne demeure largement imparfaite, notamment en ce qui concerne les côtes françaises. De par sa position géographique, le Finistère se pose comme un jalon idéal afin de renforcer les connaissances dans ce domaine. A ce titre, cette thèse a eu pour principaux objectifs d’obtenir une courbe fiable de remontée du niveau marin relatif holocène pour le Finistère et d’évaluer l’apport de nouvelles connaissances à ce sujet pour la compréhension des dynamiques isostatiques régionales. Dans un premier temps, un important travail de terrain a été réalisé sur différents sites répartis tout autour de la péninsule Finistérienne. Une nouvelle méthode multi-proxies, développée à partir de l’utilisation novatrice d’indicateurs géochimiques et microstratigraphiques, a permis de reconstituer l’évolution du niveau marin au cours des 8000 dernières années à partir des séquences sédimentaires prélevées. Nos résultats démontrent que le Finistère a connu une remontée continue du niveau marin relatif à des rythmes décroissants durant l’Holocène. Cette évolution a provoqué de profonds bouleversements dans l’organisation et le fonctionnement des systèmes sédimentaires côtiers dont les implications sont discutées. En particulier, de nouvelles hypothèses sont proposées quant à l’origine de la baisse pluri-métrique du niveau marin jusqu’alors invoquée autour de 3000 B.P. Dans un second temps, nos résultats ont été confrontés aux reconstitutions du niveau marin obtenues dans le sud-ouest de l’Angleterre ou dans le sud du golfe de Gascogne, ainsi qu’aux résultats produits par des modèles géophysiques de réajustement isostatique. Ces comparaisons ouvrent de nouvelles perspectives quant à la compréhension de l’amplitude et de la chronologie des dynamiques isostatiques holocène à l’échelle du nord-ouest de l’Europe. / Since the Last Glacial Maximum, the different regions of the world underwent specific relative sea-level evolutions, principally under the joint influences of the worldwide “eustatic” sea-level rise and regional isostatic dynamics. The latter, still going on today, appear to play a major role in the contemporaneous sea-level rise. Despite of the several studies made on the subject in the UK within the last decades, only little knowledge appears available on the relative sea-level evolution and on the isostatic dynamics at the scale of the Western Europe Atlantic façade, and particularly on the French Atlantic coasts. Due to its geographical position, the Finistère region stands out as a potentially ideal region for providing new material on the subject.This PhD work aimed to produce a new and reliable Holocene relative sea-level curve for the Finistère region. At first, sedimentary sequences were retrieved from several sites located all around the Finistère peninsula. In order to reconstruct RSL from these sequences, an innovative multi-proxy method was developed, conjointly using geochemical and microstratigraphic indicators. Our results bear out that, in the Finistère region, the Holocene RSL followed a continuous rise at progressively decreasing rates during the last 8000 yrs B.P. This evolution led to massive disruptions in the organization and functioning of the coastal sedimentary systems which are discussed. In particular, new hypotheses are proposed to explain the pluri-metric drop in RSL that was formerly invoked ca. 3000 B.P. The results we obtained were then compared to the RSL data available in the south-western UK or the south of the Bay of Biscay, along with glacio-isostatic adjustment geophysical models. These comparisons provide new enlightments on both the amplitude and the chronology of isostatic dynamics at the scale of the north-western Europe during the Holocene.