Do tidal sand waves always regenerate after dredging?

Tidal sand waves are rhythmic bedforms found on sandy continental shelves that pose a threat to offshore activities. While emphasis is placed on studying their natural morphodynamic evolution, little is known about if and how fast sand waves recover after dredging. This work presents an analysis of multibeam echosounder data collected at three former sand extraction sites on the Belgian continental shelf. At one of the sites, sand waves seemed to reappear approximately 5 years after dredging had stopped, which did not happen at the other two sites during the measurement period (5 and 9 years). The lack of recovery in those sites is likely the result of larger depths and smaller local sediment availability compared with the site where recovery occurred. Furthermore, these data reveal that in the latter site sand wave recovery was established mainly through local sediment redistribution. • Tidal sand waves are isolated from bathymetric data of the Belgian continental shelf. • At only one of the three sites, sand waves seemed to regenerate after dredging. • Possible explanations are differences in water depth and local sediment availability. • The regenerating tidal sand waves do so as a result of local redistribution of sand

Bottom-up and top-down controls of diatoms in the Belgian coastal zone, Southern North Sea: combining plankton functional type modelling and trait-based approaches

Les diatomées sont une composante majeure des écosystèmes marins et sont caractérisées par une grande diversité. L’objectif général de cette thèse est d’étudier les facteurs de contrôle des diatomées dans la Zone Côtière Belge (ZCB). Pour ce faire, la modélisation par groupes fonctionnels et l’approche basée sur les traits sont combinées :la première fournit un cadre écosystémique utile pour étudier les interactions des diatomées avec les compartiments biotiques et abiotiques de l’écosystème, et la seconde permet de représenter de façon efficace leur diversité fonctionnelle.Dans un premier temps, puisque la taille est reconnue comme un trait majeur affectant de nombreux aspects de la valeur sélective du phytoplancton, la dépendance au biovolume de différents traits des diatomées a été établie sur base d’observations disponibles dans la littérature. Cette revue a montré qu’un compromis sépare les diatomées sur base de leur biovolume :les petites espèces sont meilleures compétitrices pour l’acquisition des ressources mais plus sensibles au broutage, et inversement. Sur cette base, un ré-analyse d’un jeu de données incluant des comptages et mesures de biométrie avec l’identification de 68 taxons dans la ZCB entre 1992 et 2000 a été réalisée, avec une attention particulière pour la structure en termes de taille de la communauté de diatomées. Un regroupement des diatomées en fonction de leur biovolume a été réalisé sur base de leur évolution saisonnière typique. L’analyse de leur évolution temporelle suggère que la communauté des diatomées est composée d’assemblages avec une évolution saisonnière et interannuelle distinctes qui répondent vraisemblablement différemment aux conditions environnementales.Afin d’étudier plus en profondeur les facteurs environnementaux contrôlant la structure en termes de taille de la communauté des diatomées dans la ZCB, le module diatomées du modèle MIRO, qui représente l’écosystème planctonique de la ZCP, a été modifié afin d’inclure les dépendances à la taille de quatre traits des diatomées. Cet outil reproduit correctement l’évolution saisonnière du biovolume moyen de la communauté, qui est caractérisé par de plus petites diatomées au printemps qui maximisent l’acquisition de ressources tandis que la prévalence du broutage en été induit une transition vers des espèces plus grandes. Le modèle a également été exploité pour étudier la gamme de tailles viables dans la ZCB en fonction des conditions environnementales. Les résultats du modèle basés sur les traits ont ensuite été analysés sur la période 1992-2000. Les simulations ont mis en évidence les interactions complexes entre les processus physiques, de contrôles par les ressources et par le broutage, qui sont susceptibles de déterminer la variabilité interannuelle de la structure en termes de taille des diatomées dans la ZCB. La comparaison de ce modèle adaptatif avec un modèle plus conventionnel qui ne représente pas de diversité interne aux diatomées suggère que le premier produit une réponse légèrement plus flexible que le dernier, mais que la rigidité de la réponse simulée demeure un problème avec le modèle adaptatif qui reste incapable de représenter des évènements extrêmes de biomasse ou de structure de la communauté. Cela suggère que des développements supplémentaires du modèle sont nécessaires, en particulier du module zooplancton.Finalement, une fonction écologique particulière est abordée dans une dernière section de cette thèse :la production d’une neurotoxine par certaines diatomées du genre Pseudo-nitzschia. Cette fonction a été incluse comme un métabolisme secondaire dans un modèle idéalisé de la croissance de Pseudo-nitzschia afin d’étudier les facteurs contrôlant la production de la toxine. Il est notamment montré que l’environnement lumineux est déterminant lorsque les conditions menant à la production de toxine sont rencontrées. /Diatoms are a key component of marine ecosystems and are characterized by an important diversity. The general objective of this thesis is to investigate the bottom-up and top-down controls of diatoms in the Belgian Coastal Zone (BCZ). This is done through the integration of their functional diversity into a coherent framework. In order to achieve this, Plankton Functional Type modelling and trait-based approaches are combined: the former provides an ecosystem context convenient to study the interactions of diatoms with biotic and abiotic compartments, and the latter allows an efficient representation of their functional diversity. As a first step, since size is recognized as a master trait shaping many aspects of phytoplankton fitness, the cell volume dependences of diatom functional traits were compiled from observations available in the literature. This review showed that a trade-off distinguishes diatoms on the basis of their cell volume: smaller species are better competitor for resource acquisition but sensitive to grazing, and inversely. On this basis, a re-analysis of a dataset including diatom cell counts and biometry with the identification of 68 taxa in the BCZ over the 1992-2000 period was carried out, with a focus on the size structure of the diatom community. A clustering of diatoms according to their cell volume was realized on the basis of their average seasonality. Investigation of their time evolution suggested that the diatom community in the area is composed of assemblages with distinct seasonal and interannual evolution that likely respond differently to environmental conditions.In order to further investigate the bottom-up and top-down constrains on the size structure of the diatom community in the BCZ, the diatom module of the MIRO model, which represents the planktonic ecosystem of the BCZ, was modified in order to accommodate size-dependences of four diatom functional traits. This tool adequately reproduced the seasonal evolution of the mean cell volume of the diatom community, which is characterized by smaller diatoms in spring that maximize resource acquisition while the prevalence of grazing pressures in summer induces a shift towards larger species. The model was also used to investigate the range of viable diatom sizes in response to the bottom-up and top-down pressures in the BCZ.Results of the trait-based adaptive model were then analysed over the 1992-2000 period. Simulations evidenced the complex interactions between physical, bottom-up and top-down processes that are likely to govern the interannual variability in the size structure of diatom in the area. Comparison of the adaptive model with a more conventional model resolving no diversity within diatoms suggested that the former produces a slightly more flexible response than the latter, but showed that the rigidity of the simulated response persists with the adaptive model which remains unable to catch extreme biomass and community structure events. This calls for further developments of other compartments of the model, more particularly zooplankton.Finally, a peculiar ecological function is addressed on its own in the last section of this thesis: the toxigenicity of some species of the genus Pseudo-nitzschia. This function was added as a secondary metabolism in an idealized model of Pseudo-nitzschia growth in order to investigate the factors affecting toxin production. It notably showed that light was determinant under conditions leading to its production.Doctorat en Sciences agronomiques et ingénierie biologiqueinfo:eu-repo/semantics/nonPublishe

IMPACT OF ANTHROPIC AND CLIMATE CHANGES ON PHYTOPLANKTON DIVERSITY OF THE COASTAL OCEAN AND THEIR EFFECT ON CARBON CYCLE AND FEEDBACKS ON CLIMATE: A MODELLING STUDY

participantThis research addresses global change issues and aims to study the effect of anthropogenic and climate changes on the diversity of phytoplankton in the coastal ocean and analyse their impact on ecosystem goods (or damages) and services (atmospheric CO2 uptake). The methodology includes the further development of the existing MIRO-CO2 biogeochemical model for constructing a generic model able to describe the dynamics of different phytoplankton functional types (PFTs) and to identify the environmental factors governing their spatio-temporal distribution. This model will be iteratively developed and validated in different areas (Bay of Biscay, Bay of Seine, English Channel and Southern Bight of the North Sea), which represent from the Southwest to the Northeast a successive dominance of different PFTs: calcifying organisms (Coccolithophoridae) in the Bay of Biscay, toxic algae (Dinophysis, dinoflagellates and diatoms like Pseudo-nitzschia) in the Bay of Seine and Phaeocystis colonies in the Channel and Southern Bight of the North Sea. A module corresponding to each of these PFTs will be developed/improved based on a review of the literature and tested on each site. Once validated, the model will be used to identify the environmental factors determining the spatio-temporal distribution of PFTs and their specific effect on the absorption of atmospheric CO2 or damage on the ecosystem. We also plan to use the validated model to predict possible changes of PFTs in response to expected anthropogenic or climate changes (acidification, surface water warming, reverted-eutrophication). The model construction and first achievements are presented here

Bottom-up and top-down controls of diatoms in the Belgian coastal zone, Southern North Sea: combining plankton functional type modelling and trait-based approaches

Les diatomées sont une composante majeure des écosystèmes marins et sont caractérisées par une grande diversité. L’objectif général de cette thèse est d’étudier les facteurs de contrôle des diatomées dans la Zone Côtière Belge (ZCB). Pour ce faire, la modélisation par groupes fonctionnels et l’approche basée sur les traits sont combinées :la première fournit un cadre écosystémique utile pour étudier les interactions des diatomées avec les compartiments biotiques et abiotiques de l’écosystème, et la seconde permet de représenter de façon efficace leur diversité fonctionnelle.Dans un premier temps, puisque la taille est reconnue comme un trait majeur affectant de nombreux aspects de la valeur sélective du phytoplancton, la dépendance au biovolume de différents traits des diatomées a été établie sur base d’observations disponibles dans la littérature. Cette revue a montré qu’un compromis sépare les diatomées sur base de leur biovolume :les petites espèces sont meilleures compétitrices pour l’acquisition des ressources mais plus sensibles au broutage, et inversement. Sur cette base, un ré-analyse d’un jeu de données incluant des comptages et mesures de biométrie avec l’identification de 68 taxons dans la ZCB entre 1992 et 2000 a été réalisée, avec une attention particulière pour la structure en termes de taille de la communauté de diatomées. Un regroupement des diatomées en fonction de leur biovolume a été réalisé sur base de leur évolution saisonnière typique. L’analyse de leur évolution temporelle suggère que la communauté des diatomées est composée d’assemblages avec une évolution saisonnière et interannuelle distinctes qui répondent vraisemblablement différemment aux conditions environnementales.Afin d’étudier plus en profondeur les facteurs environnementaux contrôlant la structure en termes de taille de la communauté des diatomées dans la ZCB, le module diatomées du modèle MIRO, qui représente l’écosystème planctonique de la ZCP, a été modifié afin d’inclure les dépendances à la taille de quatre traits des diatomées. Cet outil reproduit correctement l’évolution saisonnière du biovolume moyen de la communauté, qui est caractérisé par de plus petites diatomées au printemps qui maximisent l’acquisition de ressources tandis que la prévalence du broutage en été induit une transition vers des espèces plus grandes. Le modèle a également été exploité pour étudier la gamme de tailles viables dans la ZCB en fonction des conditions environnementales. Les résultats du modèle basés sur les traits ont ensuite été analysés sur la période 1992-2000. Les simulations ont mis en évidence les interactions complexes entre les processus physiques, de contrôles par les ressources et par le broutage, qui sont susceptibles de déterminer la variabilité interannuelle de la structure en termes de taille des diatomées dans la ZCB. La comparaison de ce modèle adaptatif avec un modèle plus conventionnel qui ne représente pas de diversité interne aux diatomées suggère que le premier produit une réponse légèrement plus flexible que le dernier, mais que la rigidité de la réponse simulée demeure un problème avec le modèle adaptatif qui reste incapable de représenter des évènements extrêmes de biomasse ou de structure de la communauté. Cela suggère que des développements supplémentaires du modèle sont nécessaires, en particulier du module zooplancton.Finalement, une fonction écologique particulière est abordée dans une dernière section de cette thèse :la production d’une neurotoxine par certaines diatomées du genre Pseudo-nitzschia. Cette fonction a été incluse comme un métabolisme secondaire dans un modèle idéalisé de la croissance de Pseudo-nitzschia afin d’étudier les facteurs contrôlant la production de la toxine. Il est notamment montré que l’environnement lumineux est déterminant lorsque les conditions menant à la production de toxine sont rencontrées. /Diatoms are a key component of marine ecosystems and are characterized by an important diversity. The general objective of this thesis is to investigate the bottom-up and top-down controls of diatoms in the Belgian Coastal Zone (BCZ). This is done through the integration of their functional diversity into a coherent framework. In order to achieve this, Plankton Functional Type modelling and trait-based approaches are combined: the former provides an ecosystem context convenient to study the interactions of diatoms with biotic and abiotic compartments, and the latter allows an efficient representation of their functional diversity. As a first step, since size is recognized as a master trait shaping many aspects of phytoplankton fitness, the cell volume dependences of diatom functional traits were compiled from observations available in the literature. This review showed that a trade-off distinguishes diatoms on the basis of their cell volume: smaller species are better competitor for resource acquisition but sensitive to grazing, and inversely. On this basis, a re-analysis of a dataset including diatom cell counts and biometry with the identification of 68 taxa in the BCZ over the 1992-2000 period was carried out, with a focus on the size structure of the diatom community. A clustering of diatoms according to their cell volume was realized on the basis of their average seasonality. Investigation of their time evolution suggested that the diatom community in the area is composed of assemblages with distinct seasonal and interannual evolution that likely respond differently to environmental conditions.In order to further investigate the bottom-up and top-down constrains on the size structure of the diatom community in the BCZ, the diatom module of the MIRO model, which represents the planktonic ecosystem of the BCZ, was modified in order to accommodate size-dependences of four diatom functional traits. This tool adequately reproduced the seasonal evolution of the mean cell volume of the diatom community, which is characterized by smaller diatoms in spring that maximize resource acquisition while the prevalence of grazing pressures in summer induces a shift towards larger species. The model was also used to investigate the range of viable diatom sizes in response to the bottom-up and top-down pressures in the BCZ.Results of the trait-based adaptive model were then analysed over the 1992-2000 period. Simulations evidenced the complex interactions between physical, bottom-up and top-down processes that are likely to govern the interannual variability in the size structure of diatom in the area. Comparison of the adaptive model with a more conventional model resolving no diversity within diatoms suggested that the former produces a slightly more flexible response than the latter, but showed that the rigidity of the simulated response persists with the adaptive model which remains unable to catch extreme biomass and community structure events. This calls for further developments of other compartments of the model, more particularly zooplankton.Finally, a peculiar ecological function is addressed on its own in the last section of this thesis: the toxigenicity of some species of the genus Pseudo-nitzschia. This function was added as a secondary metabolism in an idealized model of Pseudo-nitzschia growth in order to investigate the factors affecting toxin production. It notably showed that light was determinant under conditions leading to its production.Doctorat en Sciences agronomiques et ingénierie biologiqueinfo:eu-repo/semantics/nonPublishe

Evaluating the river de-eutrophication gain on the magnitude of Phaeocystis blooms in the Southern North Sea between 1985 and 2005: a model study

info:eu-repo/semantics/publishe

Factors controlling the production of domoic acid by Pseudo-nitzschia (Bacillariophyceae): A model study

A mechanistic model has been developed to explore the factors controlling the production of domoic acid (DA) by the pennate diatom Pseudo-nitzschia. The idealized model allows consideration of the uncoupling between photosynthesis and growth, while DA production has been set as a secondary metabolism sharing common precursors with growth. Under growth limitation, these precursors can accumulate, resulting in an increased DA production. The model was first evaluated based on its ability to simulate the observed DA production by either silicon (Si) or phosphorus (P) limited batch cultures of Pseudo-nitzschia available in the literature. Sensitivity tests were further performed to explore how the ambient nutrients and the light regime (intensity and photoperiod length) are possibly directing the Pseudo-nitzschia toxicity. The general pattern that emerged is that excess light, in combination with Si or P limitation, favours DA production, provided nitrogen (N) is sufficient. Model simulations with varying nutrient stocks supporting Pseudo-nitzschia blooms under non-limiting light suggest two potential ways for nutrients to control DA production. First, N excess in comparison to available Si and P relieves DA production from its limitation by N, an absolute requirement of the DA molecule. Second, increased nutrient stocks amplify the DA production phase of the blooms (in addition to enhancing Pseudo-nitzschia biomass) which leads to an even more toxigenic bloom. Simulations investigating the light regime suggest a light threshold below which an important delay in DA production could be expected in Pseudo-nitzschia cultures. In the natural environment, the monitoring of light conditions during Pseudo-nitzschia blooms might help to anticipate the magnitude of the toxic event. Pseudo-nitzschia toxicity is indeed linked to the excess of primary carbon that accumulates during photosynthesis under growth limitation by nutrients.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Trait-based representation of diatom diversity in a Plankton Functional Type model

info:eu-repo/semantics/nonPublishe

The transition between coastal and offshore areas in the North Sea unraveled by suspended particle composition

Highlights • SPM concentration and organic fractions are analyzed in coastal-offshore gradients • Diagnostic model of SPM allows separating fresh, labile from less reactive PON • Analysis of PON fractions reveals a characteristic area, the transition zone • There, particle settling is enhanced, fostering their transport back to the coast, which controls the fate of organic matter • The transition zone is generally confined to water depths below 20 m Abstract Identifying the mechanisms that contribute to the variability of suspended particulate matter concentrations in coastal areas is important but difficult, especially due to the complexity of physical and biogeochemical interactions involved. Our study addresses this complexity and investigates changes in the horizontal spread and composition of particles, focusing on cross-coastal gradients in the southern North Sea and the English Channel. A semi-empirical model is applied on in situ data of SPM and its organic fraction to resolve the relationship between organic and inorganic suspended particles. The derived equations are applied onto remote sensing products of SPM concentration, which provide monthly synoptic maps of particulate organic matter concentrations (here, particulate organic nitrogen) at the surface together with their labile and less reactive fractions. Comparing these fractions of particulate organic matter reveals their characteristic features along the coastal-offshore gradient, with an area of increased settling rate for particles generally observed between 5 and 30 km from the coast. We identify this area as the transition zone between coastal and offshore waters with respect to particle dynamics. Presumably, in that area, the turbulence range and particle composition favor particle settling, while hydrodynamic processes tend to transport particles of the seabed back towards the coast. Bathymetry plays an important role in controlling the range of turbulent dissipation energy values in the water column, and we observe that the transition zone in the southern North Sea is generally confined to water depths below 20 m. Seasonal variations in suspended particle dynamics are linked to biological processes enhancing particle flocculation, which do not affect the location of the transition zone. We identify the criteria that allow a transition zone and discuss the cases where it is not observed in the domain. The impact of these particle dynamics on coastal carbon storage and export is discussed

Seafloor change detection using multibeam echosounder backscatter: case study on the Belgian part of the North Sea

To characterize seafloor substrate type, seabed mapping and particularly multibeam echosounding are increasingly used. Yet, the utilisation of repetitive MBES-borne backscatter surveys to monitor the environmental status of the seafloor remains limited. Often methodological frameworks are missing, and should comprise of a suite of change detection procedures, similarly to those developed in the terrestrial sciences. In this study, pre-, ensemble and post-classification approaches were tested on an eight km2 study site within a Habitat Directive Area in the Belgian part of the North Sea. In this area, gravel beds with epifaunal assemblages were observed. Flourishing of the fauna is constrained by overtopping with sand or increased turbidity levels, which could result from anthropogenic activities. Monitoring of the gravel to sand ratio was hence put forward as an indicator of good environmental status. Seven acoustic surveys were undertaken from 2004 to 2015. The methods allowed quantifying temporal trends and patterns of change of the main substrate classes identified in the study area; namely fine to medium homogenous sand, medium sand with bioclastic detritus and medium to coarse sand with gravel. Results indicated that by considering the entire study area and the entire time series, the gravel to sand ratio fluctuated, but was overall stable. Nonetheless, when only the biodiversity hotspots were considered, net losses and a gradual trend, indicative of potential smothering, was captured by ensemble and post-classification approaches respectively. Additionally, a two-dimensional morphological analysis, based on the bathymetric data, suggested a loss of profile complexity from 2004 to 2015. Causal relationships with natural and anthropogenic stressors are yet to be established. The methodologies presented and discussed are repeatable and can be applied to broad-scale geographical extents given that broad-scale time series datasets become available