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Steel support structures for offshore wind turbines operate in a harsh chloride-containing marine environment, which can lead to surface degradation due to the formation of corrosion pits. Depending on, amongst others, the applied potential, the corrosion kinetics can either be in activation-, migration- or diffusion-controlled regime. The main aim of this work, which is part of the MAXWind project, is to identify the potential values corresponding to each of these regimes for structural steel S355 in an environment representative of the North Sea. Hereto, the PRISMS-PF open-source phase-field modelling framework is used. Potentiodynamic polarization tests are performed for the electrochemical characterization of this material in artificial seawater. The corrosion potential and current density values obtained are -693 mV vs. Ag/AgCl and 0.005813 mA/〖cm〗^2, respectively. Open circuit potential (OCP) measurements revealed a similar result for the corrosion potential, i.e. -670 mV vs. Ag/AgCl. Besides, the effect of the applied potential on geometrical parameters (pit width and depth) and electrochemical parameters associated with the pit growth rate is studied. For an applied potential of -600 mV (vs. Ag/AgCl) and lower, the corrosion process stays in the activation-controlled regime throughout the simulation time (1000s) and a pit will thus not change in size. Applied potentials of -550 to -400 mV (vs. Ag/AgCl) take the system to the migration-controlled regime, and above -350 mV (vs. Ag/AgCl) the system is in the diffusion-controlled regime. The higher the applied potential (towards zero), the more pitting corrosion is accelerated until it reaches a threshold where any additional increase in applied potential will not further change the pit growth rate. Numerical results are validated with experimental observations of pit depth and width on corroded specimens under temperature-controlled conditions throughout a potentiostat test. Simulating the autonomous growth of a pit for long-term exposure using the phase-field technique is computationally expensive. Based on the preliminary results of this work, it can be assumed that the normal velocity of the pit surface will remain constant in the long term because the applied potential in the real application is lower than -600 mV (vs. Ag/AgCl), negligibly small being close to corrosion potential (no external source of current). A more simple model of pit growth can therefore be used for long-term exposure. The authors acknowledge the financial support of the Belgian Federal Government through its Energy Transition Fund.

Title : Compendium voor Kust en Zee = Compendium for Coast and Sea. Vlaams Instituut voor de Zee (VLIZ): Oostende. ISSN 2983-5526; e-ISSN 2983-5534 Volume : 2023 Issue : Pagination : 1-25

Phytoplankton stoichiometry and cell size could result from both phenology and environmental change. Zooplankton graze on primary producers, and this drives both the balance of the ecosystem and the biogeochemical cycles. In this study, we performed incubations with copepods and coccolithophores including different prey sizes and particulate carbon contents by considering phytoplankton biovolume concentration instead of chlorophyll a level (Chl a) as is usually performed in such studies. The egestion of fecal pellet and ingestion rates were estimated based on a gut fluorescence method. The latter was calibrated through the relationship between prey Chl a level and the biovolume of the cell. Chl a/biovolume ratio in phytopkanton has to be considered in the copepod gut fluorescent content method. Both coccolithophore biovolume and particulate inorganic/organic carbon ratios affect the food foraging by copepods. Finally, we observed a non-linear relationship between ingestion rates and fecal pellet egestion, due to the presence of calcite inside the copepod's gut. These results illustrate that both prey size and stoichiometry need to be considered in copepod feeding dynamics, specifically regarding the process leading to the formation of fecal pellets. International audience

Aquatic food production plays a pivotal role in providing food, nutrition, employment, recreation, trade and economic well-being for people throughout the world, for present and future generations. It offers a life-changing opportunity for the hundreds of millions of undernourished people around the world, particularly in low- and middle-income countries. Moreover, aquatic food production is one of the candidate sectors recently foreseen as fortification of nutrient-sensitive agriculture. Aquaculture is continuously increasing worldwide to meet the global market demand for fish and fishery products, driven by the increasing human population and over-exploitation of wild capture fisheries. Though overexploitation of wild capture fisheries has been reported, still it dominates the fish supply in Africa, and aquaculture production in the continent contributes an insignificant percentage. To sustain aquatic food production, there would be a diversifying option of fish species and parts used for human consumption (Chapter 1). People have never consumed as much fish nor depended so much on the aquatic food products for their livelihoods. As a matter of fact, micronutrient deficiency is affecting most developing countries. In such condition, diet shift towards diverse fish species could be a sensible remedy for micronutrient deficient nations. In Ethiopia, both capture and culture fisheries are focusing on a relatively narrow diversity of fish species, which likely underutilizes the nation’s aquatic resources for food and nutrition provision, because fish species vary considerably in their nutrient composition and density. Given the wide diversity of aquatic life, fish nutritive value, especially mineral concentrations could be influenced by various factors, such as species, habitat, climate, and tissue characteristics (metabolic activity and homeostasis). Therefore, investigating the impact of these factors in nutritive value, essential and non-essential mineral concentrations in different fish species and tissues is paramount in the aquaculture sector development. The general aim of this PhD thesis thus was to investigate the impact of species, tissues and ecosystem on the nutrient metabolism and nutritive value of fish as part of a nutrition- sensitive agriculture (Chapter 2). Bayissa T.N. 2021 179 Fish nutrient deficiency should not occur when diets have been formulated and prepared based on the species’ requirement. However, some commercially available diets for another species may sometimes be used in the absence of a suitable formulation, resulting in deficiencies. Chapter 3 compared micromineral (Fe, Zn and Cu) homeostasis across ten ornamental fish species. Ten different species of live ornamental fish were randomly sampled from one big aquarium in a pet store in Belgium. All fish samples were dissected manually for the collection of targeted tissues and analyzed for the above mentioned microminerals. In general, muscle tissue showed the lowest concentrations for each of the three microminerals in all species, still with important variation among species. Fe was associated with Cu in muscle tissue (p < 0.05), but neither of them were associated with Zn in the muscle. However, the three micromineral concentrations were correlated in the heart (p < 0.05). Similarly, all of them were correlated in the liver (p < 0.05), but none of them showed a significant association in the tail fin. Excess deposition of minerals in heart tissue is a new observation, and it is not known if this is meant as storage or rather the fish heart has a high requirement for microminerals. Storage in the tail fin should be interpreted as a sign of permanent deposition as a tool to dispose of toxic excess. The lack of correlation between the muscular concentrations of Zn on the one hand, and those of Fe and Cu on the other hand, further suggests that fish species distinctly differ in their micromineral metabolism. Although this exploratory study still leaves many questions unanswered, it points to the large diversity in micromineral metabolism among fish species. Consequently, Chapter 4, aimed to evaluate the macronutrient and mineral composition of whole fish (Labeobarbus intermedius, Garra quadrimaculata) and fillet (Oreochromis niloticus, Labeobarbus intermedius) from the same water body. A total of 64 fish samples were collected from Gilgel Gibe reservoir, Ethiopia, and analyzed for its macronutrient and mineral composition. The proximate composition and mineral contents of fillets and whole body samples were determined. The whole fish showed a much higher fat and ash percentage than the fillets (p<0.05). The fillets contained a Bayissa T.N. 2021 180 much higher protein concentration than the whole fish (p<0.05). The higher Ca: P ratios in whole fish compared to fillet in our study confirms the importance for a healthy human skeletal development, especially in diets where Ca is typically lacking. Whole Garra appeared to be containing important trace elements such as zinc and iron, a feature that was not found to the same extent in the whole Labeobarbus. These differences may find its origin in the feeding pattern of these fish species in the reservoir. The advantage of benthic species such as Garra to enrich the human diet with essential minerals may, however, coincide with the accumulation of toxic heavy metals as a potential result of soil erosion. In natural conditions, the distribution of minerals can vary with the local activity (farming, industrial, or urban activity) that limits the resealing rate of effluents into the nearby aquatic environment. The accumulation and distribution of beneficial and toxic trace elements in fish tissues can be affected by the mineral load in its environment and diet, which, in turn, will be a reflection of soil composition and geological events, such as soil erosion. In Chapter 5, a study was aimed to evaluate the differences in mineral and toxic trace element concentrations of Nile tilapia tissues from three aquatic ecosystems in Ethiopia namely, Lake Ziway, Lake Langano, and Gilgel Gibe reservoir with a focus on edible (fillet) and discarded (digestive tract, gills, skin, and liver) parts. From each lake, twenty Nile tilapia samples were collected, dissected for the targeted tissues and analyzed by inductively coupled plasma mass spectrometry. All elements varied markedly among tissues and between the lakes. Some differences in element concentrations were attributed to differences in nutrient load in the ecosystems and the function of the tissues. For instance, the calcium concentrations in skin and gill were distinctly higher in fish from calcium-rich Lake Langano. The discarded parts were richer in essential trace elements, showing an opportunity to promote their use in human nutrition to increase the intake of important minerals. However, the accumulation of elements toxic to humans, such as aluminum, should be monitored and controlled when rearing these fish in aquaculture. Bayissa T.N. 2021 181 A natural aquatic ecosystem harbors a diversity of factors, making it hard to identify the most limiting factors for fish development and body composition. In Chapter 6, study was conducted to explore how metabolite analysis can mechanistically explain differences in tissue composition and size of Nile tilapia from different habitats. Dried blood spot (DBS) samples of Nile tilapia were collected from three Ethiopian lakes (Gilgel Gibe, Ziway, and Langano) and analysed for the carnitine esters and free amino acids. Metabolite ratios were calculated from relevant biochemical pathways that could identify relative changes in nutrient metabolism. Marked differences were observed in Nile tilapia metabolic activity between the lakes. For instance, the lower body weight and body condition of the fish in Lake Langano coincided with several metabolite ratios pointing to a low flow of glucogenic substrate to the citric acid cycle. In the Gilgel Gibe fish, the metabolic markers for lipogenesis and metabolic rate could explain the high fat concentration in several parts of the body. The nutrition-related blood metabolite ratios are useful to understand the underlying metabolic events leading to the habitatdependent differences in growth of Nile tilapia, and by extension, other species. In conclusion (Chapter 7), this thesis demonstrates the potential for targeted enrichment of the human diet with minerals and other nutrients through fish. Apart from the fish diet and environment, the choice of the fish species and which parts are eaten are available strategies to that end. Whole blood metabolic profiling in fish showed to be a valuable tool to facilitate and speed up the evaluation of these strategies.