Les écosystèmes côtiers, interface entre terre et mer, sont soumis au changement climatique ainsi qu’à de fortes pressions anthropiques. Par conséquent, les eaux côtières sont sujettes à eutrophisation. Le phytoplancton demande une attention particulière dû à son rôle de producteur primaire des écosystèmes marins. Récemment, l’efflorescence des algues nuisibles est devenue mondialement, une inquiétude croissante. Le but de la thèse a été de décrire et de mesurer les réponses temporelles et les causalités de la structure de communauté phytoplanctonique sous impact des changements globaux, en présence d’une algue nuisible. Pour ce faire, le concept de niche écologique et une méthode statistique, ont été adaptés. Les "Within Outlying Mean Indexes" ont été proposés pour affiner l’analyse "Outlying Mean Index" en combinant ses propriétés avec la décomposition de la marginalité de l’analyse "K-select". Les dynamiques des sous-niches de la communauté d’espèce ont été étudiées dans des conditions environnementales d’ abondances basses (L) ou fortes (H) de Phaeocystis spp. Le sous-ensemble H était caractérisé par une large niche de Phaeocystis spp. ainsi qu’une forte diversité de diatomées. Dans le sous-ensemble L, Phaeocystis spp. a subit une forte contrainte biologique probablement induite par la compétition des diatomées pour les ressources. La relation diversité-productivité du phytoplancton s’est avérée plus forte à l’échelle saisonnière que sur le long-terme. Le déséquilibre des ressources n’a pas eu de lien direct avec la productivité à long terme. Le succès à long terme de l’espèce invasive et de son impact sur la productivité, est favorisé par une suite d’années froides avec des ressources élevées et déséquilibrées, augmentant le nombre de petites espèces de diatomées et donc son efflorescence. Enfin, je discute des améliorations méthodologiques, du potentiel d’utilisation de l’approche par traits, et d’éventuelles montages expérimentaux pour supporter les résultats de la thèse. / Coastal ecosystems, the interfaces between land and sea, are subject to climate change and high anthropogenic pressure. Consequently, most coastal waters are prone to eutrophication. The phytoplankton require a special attention because of its role of primary producer in marine ecosystems. Recently, Harmful Algae Bloom outbreaks has raised concern worldwide. The thesis aim was to describe and to measure the temporal responses and causalities of the phytoplankton community structure, with the occurrence of a harmful algae, under global changes. To do so, the ecological niche concept and a statistical method were adapted. The Within Outlying Mean Indexes was proposed to refine the Outlying Mean Index analysis by combining its properties with the K-select analysis species marginality decomposition. The subniche dynamics of the species composing the community were studied under environmental conditions hosting low (L) and high (H) Phaeocystis spp. abundance. Subset H was characterized by a large Phaeocystis spp. niche and a high diatom diversity. In subset L, Phaeocystis spp. was subject to great biological constrain suspected to be caused by diatom competition for resources. The phytoplankton diversity productivity was stronger at a seasonal scale than on the long-term. The resource imbalance had no direct link with productivity in the long-term. The long-term invasive species success and its impact on productivity is favored by successive cold years with high resource imbalance which rise the number of small species and its bloom. I finally discussed on the methodological improvements, the potential use of the trait-based approach, and possible experimental set-ups to support the thesis results.

Proefschrift Rijksuniversiteit Groningen. / Met lit.opg. - Met samenvatting in het Nederlands.

Much of the world’s aquatic food webs and nutritional relationships have been blurred by the ever-increasing evidence that many phytoplankton are not exclusively heterotrophic or autotrophic, but instead mixotrophic. Mixotrophy is a continuum of different energy and carbon-acquisition mechanisms utilizing both autotrophy and heterotrophy which distorts the concept of single trophic tier modality. This makes mixotrophs flexible to adapt to environmental pressures and is becoming more the rule than the exception in many aquatic ecosystems. One unique environmental setting where mixotrophy could be highly beneficial to food web stability is in seasonally occurring ephemeral pools – aka vernal pools. Mid-Atlantic vernal pools are biodiverse biogeochemical hotspots and critical breeding habitats for a diverse number of endemic taxa including many endangered amphibian species. Vernal pools are not permanent standing bodies of water and have fluxes in hydrology, temperatures, nutrients, and irradiance to name a few. These extremes make vernal pools an ideal setting for mixotrophic phytoplankton, however it’s never been investigated. Our survey found mixotrophy in every vernal pool sampled, as well as elevated grazing rates in pools experiencing nontypical seasonal conditions. From these small-scale forest pools to the world’s oceans mixotrophy is a widespread nutritional strategy. The Southern Ocean is essential for powering worldwide ocean circulation, regional biogeochemical cycles, and global climate. One of the major hurdles with understanding mixotrophy is identifying the phytoplankton capable of shifting nutritional strategies. While many Southern Ocean plankters have been properly identified as mixotrophic, one such keystone species has gone mislabeled until now. Phaeocystis antarctica is a well-studied Haptophyte algae that plays major roles in the global carbon and sulfur cycles. This species has been historically labeled as an obligate phototroph, but contradictory survives the long dark Antarctic winter without any known evidence of encystment. We suspect that this highly abundant species is in fact mixotrophic, capable of phagocytosis to supplement the irradiance shortcomings of the Antarctic dark. We experimented with varying degrees of light and nutrient limitations to determine possible triggers for P. antarctica grazing. Our results showed P. antarctica ingesting in every treatment, but its highest grazing rates corresponded with limitations to its primary photosynthetic mode. Apart from the newly realized complexity P. antarctica brings to the Southern Ocean food web, it is an environment that suffers from microplastic pollution that can impede these mixotrophic species. Mismanaged plastic waste around the world, especially microfiber discharge from laundered synthetic textiles, escape into the natural environment, and eventually concentrate in the oceans. The Southern Ocean can become disproportionately polluted in regions due to microfibers becoming sequestered once crossing the Antarctic circumpolar current and even becoming trapped in sea-ice formations. While it is easy to see the devastation plastic waste has on megafauna (i.e. turtles, fish, birds, and whales), its microscopic devastation is less obvious. Plastic waste comes in many forms and one less researched form is buoyant polyester microfibers <1mm that interact with colony forming algae. We utilized different concentrations of polyester microfibers and mixing speeds to determine if microfiber interactions with colony formations increases or decreases overall colony buoyancy. Smaller concentrations of polyester microfibers can impart a positively buoyant effect onto P. antarctica colonies regardless of mixing speed, however larger concentrations negatively affected colony buoyancy regardless of mixing speed. / Biology

The retention by protists of foreign plastids and other organelles obtained from algal prey is an ecologically important example of mixotrophy and also represents a potential pathway for the symbiogenetic evolution of novel permanent plastids. A gymnodinoid dinoflagellate isolated from the Ross Sea, Antarctica (RSD) retains plastids from its haptophyte prey Phaeocystis antarctica. It is a member of the Kareniaceae, a dinoflagellate family whose other members all contain permanent tertiary plastids of haptophyte origin. A subset of its cells also contain foreign nuclei. The following chapters describe experiments that indicate the RSD's selectivity for P. antarctica in feeding and plastid uptake, when compared to other potential prey; and observations that demonstrate survival of plastid-retaining RSD for over two years in the absence of its prey. Further experiments assess the resilience of P. antarctica and the RSD in response to the prolonged darkness of the austral winter. / Biology

The Belgian Coastal Zone (BCZ) in the Southern Bight of the North Sea is a highly dynamic and optically complex area. This is due to high non-algal particles (NAP) and coloured dissolved organic matter (CDOM) content which in spring adds together with undesirable blooms of the haptophyte Phaeocystis globosa. There is a need for improving the algorithms for chlorophyll a (chl a) retrieval in these highly turbid waters and for developing algorithms for species detection in order to attempt to create an early warning bloom system. This information will contribute to the knowledge of the extent and magnitude of the P. globosa bloom in Belgian waters. In this study, pure cultures of the main taxa present in the BCZ, diatoms and P. globosa, were combined with field measurements of light absorption of total particles, phytoplankton and dissolved material, pigment determination and phytoplankton counts to address the main objectives. Sampling was performed during 8 cruises covering winter, spring, summer and late summer, and along nearshore-offshore gradients from 2003 to 2006. The area is characterised by a high spatio-temporal variability of IOPs due to the high dynamics of the area in terms of currents, salinity gradients and biological production. During spring the presence of P. globosa modulates the IOPs across all the area, the particle absorption is significantly higher than summer and there is no significant coast-offshore variability for phytoplankton and CDOM. The design of chl a retrieval algorithms assumes negligible absorption of NAP and CDOM in the near infrared (NIR) and the use of a fixed value of specific phytoplankton absorption. It is shown that neglecting the NAP and CDOM absorption in the NIR will have a significant overestimation impact in retrieval of chl a. On the other hand, the specific phytoplankton absorption was found to be highly variable (0.015 „b 0.011 m2 mg chl a-1). Both results will affect directly the retrieved chl a. The spatial variability of CDOM was significant varying between 0.20-1.31 m-1 in the marine area and between 1.81-4.29 m-1 in the Scheldt estuary. CDOM was found to be related to salinity with conservative mixing within the Scheldt estuary and during some seasons in the BCZ, however deviations from conservative mixing suggest other inputs to the CDOM pool. Analyses of the spectral slope of the CDOM absorption curve revealed two main CDOM pools in the area, an allochthonous one delivered by the Scheldt estuary and an autochthonous one associated with the phytoplankton spring bloom decomposition. Algorithms for CDOM retrieval will be affected if the variability in the relation between CDOM and salinity is not taken into account. The optical characterisation of diatoms and P. globosa from the BCZ in pure cultures revealed that the main differences in the phytoplankton absorption spectra were found at 467 and 500 nm corresponding to the absorption of the pigments chlorophyll c3 (chl c3) characteristic of P. globosa and fucoxanthin, respectively. Accordingly, both the absorption at 467 nm and the ratio 500/467 nm were successfully used to discriminate the two taxa in cultures and field samples. This latter indicator was not preserved in the reflectance signal due to degradation of the signal when passing from absorption to reflectance, and thus could not be used for algorithm development. The spectral feature at 467 nm was later used as the basis for the development of a flag-type algorithm to detect chl c3 using either absorption or water-leaving reflectance data. Also, the correlation between the algorithm¡¦s retrieved chl c3 and P. globosa cell number allowed the quantification of the bloom. The main findings of this thesis highlight the importance of the IOPs characterisation for the improvement and development of ocean colour retrieval algorithms in these highly complex waters.

Ce travail porte sur la dynamique bactérienne (abondances totale, vivante et active, productions de cellules et de biomasse, activités de dégradation exo-enzymatique) en Manche orientale, depuis une approche écosystémique à l'application d'outils microbiens spécifiques. Il permet de mieux comprendre le rôle des bactéries, jusqu'alors peu étudiées en Manche orientale, dans les flux de matière et d'énergie, particulièrement en période de floraison printanière.<br />Le premier objectif consistait à déterminer la réponse du bactérioplancton à la variabilité spatio-temporelle du milieu, à l'échelle (i) de l'écosystème au cours de deux années consécutives (2003/2004), (ii) saisonnière sur le site de Wimereux et (iii) journalière au cours de suivis Lagrangiens de masse d'eau. Cette étude montre que les années 2003 et 2004 ont été très différentes en termes de fonctionnement trophique du réseau microbien. Par ailleurs, la diversité des paramètres de contrôle de la dynamique bactérienne (température, disponibilité en matières organiques, pression de prédation) conditionne son caractère saisonnier. Sur un cycle journalier, l'application de deux méthodes de mesure de la production a permis une analyse plus précise et plus fine des conditions de croissance bactérienne.<br />Afin d'évaluer le rôle trophique du bactérioplancton, particulièrement lors des blooms massifs de Phaeocystis globosa, les fractions cellulaires viables et actives du pool bactérien, responsables de l'activité mesurée, ont été estimées. Les abondances de ces cellules augmentent significativement en réponse aux matières organiques accumulées pendant et après le bloom phytoplanctonique. Des mesures d'activité exo-enzymatique montrent une dégradation bactérienne significative des matières issues du bloom, et soulignent l'existence d'une étroite relation « trophique » entre bactéries et P. globosa.

The eutrophication of the Southern Bight of the North Sea has been benefitting to the prymnesiophyte Phaeocystis globosa (P. globosa). This species is a known high dimethylsulfoniopropionate (DMSP) producer whose bloom accounts for 95% of spring phytoplankton biomass. An increase in DMS(P) and its oxidation product dimethylsulfoxide (DMSO) cellular contents have been frequently observed in cellular stress conditions. To test this, we have first analysed the natural distribution of DMS(P,O) cellular contents in the North Sea. Secondly, we have measured DMS(P,O) cellular contents in monospecific cultures of several key species of the North Sea and their responses to salinity variations. Our main working hypothesis is that DMSP acts as an osmoregulator and/or as an antioxidant, depending on the species. The DMS(P,O) annual cycle in the Southern Bight of the North Sea revealed a seasonality linked to the spring phytoplankton communities succession: (1) colonial diatoms (reappearing in autumn), (2) Chaetoceros spp. (3) P. globosa, (4) large-size summer diatoms (mainly Guinardia spp.), and (5) dinoflagellates. Spatial gradients of DMS(P) were related to those of phytoplankton biomass, itself related to the inputs of nutrients from the Scheldt estuary. It also discharges suspended matter in which DMSO may have been produced by anaerobic oxidation of DMS. Laboratory measurements confirmed a large variability in DMSP cellular contents between the six studied diatoms (Nitzschia closterium, Skeletonema costatum, Thalassiosira rotula, Chaetoceros socialis, Chaetoceros debilis, and Guinardia delicatula), low producers in comparison with P. globosa and even more with Heterocapsa triquetra (Dinoflagellate). In particular, communities 2 and 4 have lower DMSP cellular contents than community 1 (N. closterium, S. costatum and T. rotula). Senescence induces a decrease in DMSP/DMSO suggesting an oxidative stress caused by nutrients and/or light limitation in DMSP producers. In S. costatum, DMSP seems to play an osmoregulatory role and is oxidised into DMSO in hyposaline conditions. In P. globosa and H. triquetra, an oxidative stress appears in hypo- and hypersaline conditions diverging from their salinity optimum. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

The Belgian Coastal Zone (BCZ) in the Southern Bight of the North Sea is a highly dynamic and optically complex area. This is due to high non-algal particles (NAP) and coloured dissolved organic matter (CDOM) content which in spring adds together with undesirable blooms of the haptophyte Phaeocystis globosa. There is a need for improving the algorithms for chlorophyll a (chl a) retrieval in these highly turbid waters and for developing algorithms for species detection in order to attempt to create an early warning bloom system. This information will contribute to the knowledge of the extent and magnitude of the P. globosa bloom in Belgian waters. In this study, pure cultures of the main taxa present in the BCZ, diatoms and P. globosa, were combined with field measurements of light absorption of total particles, phytoplankton and dissolved material, pigment determination and phytoplankton counts to address the main objectives. Sampling was performed during 8 cruises covering winter, spring, summer and late summer, and along nearshore-offshore gradients from 2003 to 2006. <p>The area is characterised by a high spatio-temporal variability of IOPs due to the high dynamics of the area in terms of currents, salinity gradients and biological production. During spring the presence of P. globosa modulates the IOPs across all the area, the particle absorption is significantly higher than summer and there is no significant coast-offshore variability for phytoplankton and CDOM. <p>The design of chl a retrieval algorithms assumes negligible absorption of NAP and CDOM in the near infrared (NIR) and the use of a fixed value of specific phytoplankton absorption. It is shown that neglecting the NAP and CDOM absorption in the NIR will have a significant overestimation impact in retrieval of chl a. On the other hand, the specific phytoplankton absorption was found to be highly variable (0.015 „b 0.011 m2 mg chl a-1). Both results will affect directly the retrieved chl a. The spatial variability of CDOM was significant varying between 0.20-1.31 m-1 in the marine area and between 1.81-4.29 m-1 in the Scheldt estuary. CDOM was found to be related to salinity with conservative mixing within the Scheldt estuary and during some seasons in the BCZ, however deviations from conservative mixing suggest other inputs to the CDOM pool. Analyses of the spectral slope of the CDOM absorption curve revealed two main CDOM pools in the area, an allochthonous one delivered by the Scheldt estuary and an autochthonous one associated with the phytoplankton spring bloom decomposition. Algorithms for CDOM retrieval will be affected if the variability in the relation between CDOM and salinity is not taken into account. <p>The optical characterisation of diatoms and P. globosa from the BCZ in pure cultures revealed that the main differences in the phytoplankton absorption spectra were found at 467 and 500 nm corresponding to the absorption of the pigments chlorophyll c3 (chl c3) characteristic of P. globosa and fucoxanthin, respectively. Accordingly, both the absorption at 467 nm and the ratio 500/467 nm were successfully used to discriminate the two taxa in cultures and field samples. This latter indicator was not preserved in the reflectance signal due to degradation of the signal when passing from absorption to reflectance, and thus could not be used for algorithm development. The spectral feature at 467 nm was later used as the basis for the development of a flag-type algorithm to detect chl c3 using either absorption or water-leaving reflectance data. Also, the correlation between the algorithm¡¦s retrieved chl c3 and P. globosa cell number allowed the quantification of the bloom. The main findings of this thesis highlight the importance of the IOPs characterisation for the improvement and development of ocean colour retrieval algorithms in these highly complex waters.<p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished