Meunier, Cedric Leo
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Meunier, Cedric Leo
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Meunier, Cedric Leo
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Cedric Meunier
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Item type:Publication, Zooplankton community responses to Ocean Alkalinity Enhancement(2025-09-22); ; ; Reducing atmospheric CO2 concentrations while limiting further ocean acidification has increased interest in marine carbon dioxide removal (mCDR) approaches. Ocean Alkalinity Enhancement (OAE) aims to increase seawater total alkalinity (TA), lower surface-water pCO2, and thereby enhance air-to-sea CO2 uptake and storage predominantly as bicarbonate. Despite its theoretical potential, the ecological consequences of OAE remain insufficiently constrained, particularly for zooplankton that shape marine food webs and mediate carbon transfer to depth. This thesis quantifies how OAE-driven shifts in carbonate chemistry affect zooplankton performance and community dynamics, with explicit attention to non-CO2-equilibrated conditions and to the relative importance of direct physiological stress versus indirect, food-web mediated effects. Three experimental studies combined mesocosm deployments with controlled laboratory incubations. First, two mineral-based OAE approaches were evaluated in coastal post-bloom waters using slaked lime and olivine across a ΔTA range up to 600 µmol kg-1. The appendicularian Oikopleura dioica showed no detectable changes in abundance, feeding performance, or house production across treatments, suggesting resilience of this gelatinous zooplankton and limited risk to larvacean-mediated particle export within this alkalinity range. Second, a spring bloom mesocosm experiment applied a wider ΔTA gradient reaching 1250 µmol kg-1 and compared immediate versus delayed mixing. Zooplankton recruitment, abundance, and diversity increased under moderate alkalinity additions but declined when ΔTA exceeded approximately 750 µmol kg-1. The decline coincided with delayed phytoplankton bloom development and reduced availability of suitable prey during critical larval stages, consistent with a trophic mismatch mechanism rather than solely direct chemical stress. Third, laboratory incubations with the copepod Temora longicornis showed dose-dependent metabolic impairment with increasing ΔTA, consistent with elevated energetic costs associated with acid-base regulation. At the same time, OAE altered prey elemental properties, improving aspects of food quality and partially buffering copepod performance when nutritional conditions were favorable. Together, these findings support a precautionary ecological threshold near ΔTA of about 750 µmol kg-1, beyond which risks of disrupted recruitment, altered community composition, and cascading food-web effects may increase. The thesis provides mechanistic evidence and experimentally derived constraints that can inform environmentally responsible OAE design, including guidance for deployment intensity, mixing considerations, and monitoring strategies aligned with future monitoring, reporting, and verification frameworks.doctoral thesis74 34 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Impacts of Arctic permafrost erosion on nearshore planktonic food webs(2025-10-28); ; ; Arctic planktonic communities form the foundation of Arctic marine food webs and play a crucial role in the biological carbon pump. Global warming is increasing the thawing and erosion of permafrost coasts in the Arctic. This leads to the discharge of substantial amounts of sediment, carbon, and nutrients into the Arctic Ocean’s nearshore zone, changing the ecosystem conditions. Questions have arisen about how planktonic communities in the nearshore zone are affected by such changes in the environmental conditions. In my thesis, I applied a multiple-study approach to investigate the effects of Arctic coastal erosion and the associated changes in turbidity, carbon, and nutrient levels on planktonic community dynamics, biomass, and interactions within the nearshore zone. I decided on the shallow nearshore due to the fact that these zones represent 20% of the Arctic shelves and 7.5% of the Arctic Ocean, a proportion substantially greater than that of the nearshore zones of other oceans. In Chapter 2, the manuscript, “Future Arctic: How will increasing coastal erosion shape nearshore planktonic food webs?” sets the scene. In this chapter, I assessed how coastal erosion impacts carbon, nutrients, and light regimes in the nearshore zone, and what we can expect for the future. Additionally, I assessed the potential effects on planktonic community structure and food web dynamics. I used published literature and a formal review of our current state of knowledge. The literature data showed that sediment discharge increases turbidity and reduces light penetration into the water column. This darkening is expected to reduce phytoplankton productivity, while additional carbon will support bacterial production and shift the balance between autotrophic and heterotrophic production at the base of the food web. Given the lower energy transfer efficiency in the heterotrophic pathway, its dominance might lower zooplankton biomass with potential negative consequences for higher trophic levels. Drawing some of the testable hypotheses from the in-depth literature synthesis, I investigated the influence of terrigenous input on planktonic community dynamics around Herschel Island-Qikiqtaruk. Located in the Western Canadian Arctic, the permafrost coast around Herschel Island-Qikiqtaruk is one of the highly eroding sites in the Arctic. The results in the manuscript, “Eroding permafrost coasts lead to lower productivity in the Arctic nearshore zone,” in Chapter 3, show that permafrost thaw and erosion impact planktonic biomass. Relative to stable sites, actively eroding sites exhibited higher turbidity, resulting in a 45% reduction in phytoplankton biomass. Moreover, the very nearshore stations zone showed higher heterotrophic dinoflagellates and microzooplankton biomass than the offshore stations, suggesting that the nearshore stations were dominated by heterotrophy, while the offshore stations were dominated by autotrophic energy mobilization. Mesozooplankton abundance decreased by 26% from the nearshore towards offshore stations, suggesting potential utilization of both marine and terrestrial OC sources. In the third manuscript, “Impact of permafrost coastal erosion on Arctic marine food webs”, I investigated the sources, age, and utilization of marine versus terrigenous organic carbon. The results showed that although permafrost erosion discharges a substantial amount of OC into the nearshore zone, only 6% of the old permafrost OC ends up in the planktonic food web. Planktonic consumers are mainly supported by marine production, and the additional terrigenous OC carbon utilized by nearshore consumers largely comes from the active layer, representing modern terrestrial carbon. Overall, this study highlights that Arctic permafrost thaw and erosion influence planktonic community structure by reducing phytoplankton biomass and shifting the balance between autotrophs and heterotrophs in the nearshore zone. These processes might weaken the Arctic Ocean’s capacity as a CO2 sink, and potentially turn it into a net CO2 source.doctoral thesis52 51 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Exploring the factors behind the expansion of the harmful dinoflagellate Alexandrium pseudogonyaulax in Northern European waters(2024-09-25); ; ; ; This Phd-thesis investigated factors driving the expansion of the harmful dinoflagellate Alexandrium pseudogonyaulax in Northern European waters. It includes an ecophysiological study investigating the influence of dissolved nitrogen and light availability on growth and toxin content of A. pseudogonyaulax, a food web study addressing the effects of this HAB species on multiple trophic levels ranging from phytoplankton to fish, as well as a time series analysis analysing the apparent expansion of A. pseudogonyaulax across Northern European waters. The first study, published in Limnology & Oceanography, indicates that urea is not an important driving factor of the expansion of A. pseudogonyaulax and shows that this species has high intraspecific variability and resilience to light. The second study, published in Harmful Algae, demonstrates deleterious effects of A. pseudogonyaulax on other planktonic organisms which suffer adverse fitness effects ultimately resulting in lysis and mortality. This study also demonstrates for the first time ichthyotoxic characteristics of A. pseudogonyaulax, which are likely driven by uncharacterized bioactive extracellular compounds (BECs) and not by goniodomins. The thesis contains a third first-author manuscript, envisioned for future publication, which demonstrates that the frequency of A. pseudogonyaulax occurrences has increased in Northern Europe. In addition, it indicates that temperature may have partly driven increasing occurrences and that A. pseudogonyaulax benefits from high carbon-to-chlorophyll a ratios.doctoral thesis310 251 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Phytoplankton functional traits under the impact of environmental change: from single cells to communities(2022-06-13); ; ; Phytoplankton are responsible for half of the total oxygen production on Earth and play an important role in ocean productivity as the base of most aquatic food webs. However, human activities are leading to changes in marine abiotic parameters, and phytoplankton are facing simultaneous changes in temperature, partial pressure of carbon dioxide (pCO2), and dissolved nutrient concentrations, as well as more frequent and intense extreme weather events. Trait-based approaches were used to investigate these effects in several multiple driver experiments at different ecological scales, from individual phytoplankton cells to populations to entire phytoplankton communities. This thesis indicates that temperature was the most important driver affecting phytoplankton growth and other functional traits. For phytoplankton species currently growing below their thermal limits, growth rates will likely increase with projected temperature rises but the magnitude of this growth enhancement may be dampened by simultaneous increases in pCO2 and N:P ratios. Furthermore, increasing growth rates within cultures of the ubiquitous diatom species Thalassiosira weissflogii, as well as increasing temperatures and pCO2, caused a decline in cell-to-cell variability, which could reduce the potential for phytoplankton to cope with changes in environmental conditions. The impact of abrupt changes in temperature due to marine heatwaves was investigated using the diatom Phaeodactylum tricornutum. Alternative oxidase (AOX) activity was examined as a stress marker but no increase in AOX activity due to the abrupt increase in temperature was observed. Most changes in cellular traits were rather observed after acclimation to higher temperatures. These results indicate that phytoplankton cells might have the potential to buffer environmental fluctuations such as marine heatwaves yet undergo significant changes in their traits. The potential impact of increased river discharge was investigated on natural North Sea phytoplankton communities. Pulses of nutrient-rich freshwater caused an increase in community growth rates but also decreased phytoplankton cell size within the different phytoplankton groups and a decrease in cellular N:P ratio. At the same time, no strong negative effect of decreasing salinity was observed, indicating a large salinity tolerance of North Sea phytoplankton communities. Overall, responses of phytoplankton communities depended more on the initial community structure at each sampling site than on the origin of the communities. Still, an abrupt increase in river discharge might have the potential to restructure phytoplankton communities and modulate short-term responses in higher trophic levels. This thesis provides a comprehensive picture of the effects of different types of environmental changes on the functional traits of phytoplankton, from single cells to phytoplankton communities. Long-term environmental changes, as well as short-term extreme weather events, can lead to substantial changes in growth rates, cell size, and the biochemical composition of phytoplankton, which has the potential to alter marine food webs and biogeochemical cycling.doctoral thesis422 412