Boersma, Maarten

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Boersma, Maarten

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Boersma, Maarten

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Zooplankton community responses to Ocean Alkalinity Enhancement
(2025-09-22)
Bhaumik, Amrita
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Boersma, Maarten
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Renz, Jasmin
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Boersma, Maarten
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Meunier, Cedric Leo
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.
Dissertation
74 34
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Surfing the heatwave: Understanding the influence of marine heatwaves in driving community dynamics and species performance in the North Sea
(2024-12-02)
Deschamps, Margot M F
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Harder, Tilmann
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Jenkins, Stuart
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Harzsch, Steffen
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Boersma, Maarten
As a consequence of anthropogenic climate change, the frequency, duration and magnitude of extreme weather events are increasing and expected to rise further in the coming decades. Among them, marine heatwaves (MHWs) are driving significant ecological changes worldwide, including mass mortalities, shifts in species distributions, and biodiversity loss. Growing concerns about MHW impacts have led to intensified research efforts to define, categorize, and assess their consequences across different levels of biological organisation. Despite this increasing attention, key knowledge gaps remain. The unpredictable nature of MHWs makes it challenging to quantify their effects on marine communities. Moreover, most studies on climate change focus on rising mean temperatures, often overlooking biological responses to thermal fluctuations, which can differ from those under constant conditions. Research has also largely concentrated on coral reefs and fish, with less attention to plankton, a key component of marine food webs. This thesis quantifies MHW effects on individuals, populations and communities using zooplankton as model organisms. It consists of two main blocks: (1) analysing historical time series data to assess MHW impacts on mesozooplankton in the North Sea and (2) conducting laboratory experiments to evaluate how present and future MHWs affect the survival, growth, and phenology of meroplankton larvae. Findings reveal that MHW impacts on zooplankton communities vary by season, emphasizing the need for season-specific studies. Copepods, a key zooplankton group, are unable to adjust their phenology in response to MHWs occurring before their bloom. The lack of a phenological shift, combined with rising MHW days and temperature increase rates, can lead to faster and more or less intense blooms, with potential repercussions for higher trophic levels. The thesis also shows that MHW effects on species performance differ from those of general warming, as MHW components interact in complex ways. Species resilient to warmer temperature or invasive species may benefit from MHWs, often at the expense of native species. As MHWs continue to grow stronger, more frequent and longer, understanding the link between their components and their ecological and socio-economic consequences will become increasingly crucial.
Dissertation
163 141
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Impact of global change on coastal plankton: a multiple environmental driver approach - from cellular processes to food webs
(2023-03-10)
Duarte Moreno, Hugo
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Scherer, Cordula
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Meunier, Cedric Leo
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Boersma, Maarten
Phytoplankton ist für ca. 50% der globalen Primärproduktion verantwortlich und bildet die Grundlage pelagischer Nahrungsnetze. Saisonale Phytoplanktonblüten in gemäßigten Küstenlebensräumen steigern die Produktivität in diesen Systemen, während die Dynamik der Blüten direkt von abiotischen Faktoren abhängt. Menschliche Aktivitäten haben zu globalen Veränderungen der Umweltbedingungen geführt, einschließlich höherer Temperatur, Partialdruck von Kohlendioxid (pCO2) und Konzentration gelöster Nährstoffe, die die Planktongemeinschaften unter Druck setzen. Solche Umweltveränderungen haben Fragen darüber aufgeworfen, wie diese Gemeinschaften auf zukünftige Bedingungen reagieren werden. In meiner Doktorarbeit habe ich einen Multi-Driver-Ansatz angewendet, um die Auswirkungen globaler Veränderungstreiber auf Phytoplanktonzellen sowie auf Planktongemeinschaften während saisonaler Blütenereignisse zu untersuchen. Unter Verwendung von Zukunftsszenarien des „Intergovernmental Panel on Climate Change“ (IPCC) zusammen mit vorhergesagten Verschiebungen der N:P-Verhältnisse im Küstensystem zielt meine Doktorarbeit darauf ab, die für das Jahr 2100 erwarteten Umweltbedingungen realistisch zu simulieren. Angesichts der Bedeutung von Phytoplankton für die biologische Kohlenstoffpumpe untersuchte ich den Einfluss von Treibern des globalen Wandels auf den Kohlenstoffstoffwechsel und die antioxidative Kapazität der Diatomee Phaeodactylum tricornutum. Diese Phytoplankton Art wurde den Umweltbedingungen des RCP 8.5-Szenarios (+3°C und pCO2 1000 μatm) des IPCC sowie einem höheren N:P-Verhältnis gelöster Nährstoffe in einem vollfaktoriellen Design ausgesetzt. Die Ergebnisse dieses Experiments zeigen, dass die Temperatur der Haupttreiber hinter zellulären Prozessen ist. Die Erwärmung führte zu einer geringeren antioxidativen Kapazität, während sie die DOC-Exsudation und die Wachstumsrate positiv beeinflusste. Die Zellen waren bei der aktuellen Temperatur größtenteils unbeeinflusst von pCO2 und dem N:P-Verhältnis, und nur bei erhöhten Temperaturen wurden die Zellen anfällig für diese Umwelttreiber. Ein höherer pCO2 stimulierte die Primärproduktion unter wärmeren Bedingungen, während die Zellatmung gedämpft wurde. Ein höheres N:P-Verhältnis wirkte sich auch positiv auf die DOC-Exsudation aus. Als Ergebnis hatten die Zellen einen niedrigeren Kohlenstoffgehalt, wenn sie den Bedingungen des RCP 8.5-Szenarios ausgesetzt wurden. Antioxidative Enzyme waren unter Erwärmung weniger aktiv und die Konzentration der lichtschützenden Pigmente war geringer. Alternative Oxidase-Aktivität (AOX), sowie die Akkumulation von Malondialdehyd in den Zellen erhöhte sich unter Erwärmung, was auf zellulären oxidativen Stress hinweist. Um zu beurteilen, ob Veränderungen in der Zelldynamik des Phytoplanktons auch parallel zu Veränderungen in der Struktur und Zusammensetzung der Planktongemeinschaft verlaufen, wurden zwei Mesokosmenexperimente während einer Frühlings- und Herbstblüte mit natürlichen Plankton-Nahrungsnetzen durchgeführt. Die Planktongemeinschaft wurde einem integrierten Ansatz mit mehreren Treibern ausgesetzt. Es wurden zwei verschiedene Szenarien mit höherer Temperatur und pCO2, RCP 6.0 (+1,5 °C und pCO2 800 μatm) und RCP 8.5 (+3°C und pCO2 1000 μatm) gegen Umgebungsbedingungen getestet. Die Szenarien wurden erweitert (ERCP), um auch höhere N:P-Verhältnisse einzubeziehen. Diese Experimente zeigten die Widerstandsfähigkeit der Phytoplankton-Frühlingsblüte gegenüber Treibern des globalen Wandels, bei denen die Biomasse und die Zusammensetzung der Gemeinschaft von Phytoplankton, Mikrozooplankton und Bakterioplankton in allen Szenarien ähnlich blieben. Mesozooplankton, nämlich Ruderfußkrebse, zeigten jedoch im Frühling im ERCP 8.5 eine Zunahme der Abundanz, was zeigt, dass in diesem Szenario mehr Energie von Primärproduzenten zu höheren trophischen Ebenen aufstieg. Die höhere Abundanz von Ruderfußkrebsen fand trotz des höheren N:P- und C:P-Verhältnisses des Sestons unter den ERCP-Szenarien statt, was sich unter solchen Bedingungen als kein limitierender Faktor für das Wachstum von Mesozooplankton erwies. Im Herbst zeigten die Ergebnisse, dass die Phytoplanktongemeinschaft im Rahmen des ERCP 8.5-Szenarios umstrukturiert wurde und hauptsächlich von kleineren Arten auf Kosten großer Diatomeen dominiert wurde. Der Anstieg des Coccolithophoren Emiliania huxleyi in diesem Szenario deutet auch auf mögliche funktionelle Veränderungen in der biologischen Kohlenstoffpumpe aufgrund der Verkalkungskapazität dieser Art hin. Umgekehrt wurde Mesozooplankton durch die Bedingungen des ERCP 8.5-Szenarios negativ beeinflusst und teilweise durch Mikrozooplankton ersetzt. Die mikrobielle Schleife wurde im ERCP 8.5-Szenario verstärkt, was auf einen geringeren Energiefluss zu höheren trophischen Ebenen hindeutet. Nichtsdestotrotz erwies sich in beiden Experimenten das ERCP 6.0-Szenario im Vergleich zu ERCP 8.5 als ähnlicher zu den Umgebungsbedingungen. Schließlich wurden die Ergebnisse des Herbst-Mesokosmen-Experiments durch eine inverse Modellierung und Netzwerkanalyse neu bewertet, um Kohlenstoffflüsse und Wechselwirkungen zwischen den Nahrungsnetzbestandteilen zu quantifizieren. Diese Analyse zeigt, dass die Funktionsweise des Plankton-Nahrungsnetzes im Ambient- und ERCP 6.0-Szenario ähnlich war, während im ERCP 8.5 wesentliche Änderungen festgestellt wurden. Diese Ergebnisse zeigen die höheren Kohlenstoffflüsse durch die mikrobielle Schleife im ERCP 8.5-Szenario und damit eine höhere Kapazität zum Recycling von Kohlenstoff innerhalb des Systems. Mikrozooplankton zeigte in diesem Szenario auch einen höheren Herbivory-degree und übte einen höheren Fraßdruck auf das Phytoplankton aus als Mesozooplankton. Insgesamt präsentiert diese Arbeit ein umfassendes Bild der Auswirkungen globaler Veränderungstreiber auf die Phytoplanktonphysiologie und ihre Auswirkungen auf das Plankton-Nahrungsnetz sowie Hinweise auf strukturelle und funktionelle Veränderungen in Planktongemeinschaften unter Zukunftsszenarien. Aufgrund der Tatsache, dass Plankton-Nahrungsnetze im ERCP 6.0-Szenario und Ambient im Vergleich zum ERCP 8.5-Szenario ähnlich blieben, identifiziert diese Arbeit auch, dass das Worst-Case-ERCP-Szenario zu erheblichen Veränderungen in der Planktongemeischaft führen kann.
Dissertation
409 893
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Impacts of Arctic permafrost erosion on nearshore planktonic food webs
(2025-10-28)
Juma, Gabriel Akoko
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Boersma, Maarten
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Havermans, Charlotte
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Boersma, Maarten
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Meunier, Cedric Leo
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.
Dissertation
52 51
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Responses of early-life stages of coastal marine invertebrates to different environmental variables
(2023-11-30)
Espinosa-Novo, Noé
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Zimmer, Martin
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Boersma, Maarten
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Boersma, Maarten
Quantifying species responses to the effects of changing environmental conditions is critical for a better understanding of how climate change affects invasion, expansion, and contraction of marine coastal species. Climate change is leading to modifications in the marine coastal environment, to conditions not experienced before; climate change results in that marine organisms experience simultaneous changes in several environmental variables (=drivers: e.g. temperature, salinity, food). How simultaneous changes in multiple drivers are experienced depend on species-specific traits (e.g. physiological tolerance, developmental time); for instance, co-occurring native and non-native species may experience and respond to climate change in different ways. In addition, within species, responses to multiple drivers may vary across populations and environmental gradients. The general objective of this thesis was to quantify the effects of environmental drivers (temperature, salinity and food limitation) on performance of native and non-native species with focus on larval stages and using crabs as model systems. There were two main objectives, first to compare native and non-native species in the responses to multiple environmental drivers and to quantify larval responses to temperature across their distribution range. I focused on larvae because they play a critical role in population dynamics: larvae are important for the dispersion and connectivity of populations, and are more sensitive to changes in environmental conditions than adults. I used three ecologically relevant species of coastal areas of the North Sea and North Atlantic Ocean as models: Hemigrapsus sanguineus, Carcinus maenas and Hemigrapsus takanoi. C. maenas is native to Europe; Hemigrapsus spp. are both non-native species in the European coast, where they coexist with C. maenas as juveniles and adults in the benthos. I used factorial experiments rearing larvae from hatching to megalopae at different combinations of temperature and other environmental drivers (salinity, food limitation). Larval performance was quantified as survival, duration of development, and growth. The first series of result show that both non-native (Hemigrapsus spp) species had higher performance (high survival, shorter duration of development and high growth rates) than the native C. maenas at higher temperatures and at moderately low salinities (18 – 24 °C, 20 – 25 ‰). These results are comparable to another non-native species in Europe, the Chinese mitten crab Eriocheir sinensis. In H. sanguineus, larvae show moderate level of tolerance to limited access to food at high temperature, which contrasted to the low tolerance shown in native C. maenas. Experiments and modelling show that the nature of the multiple driver response depends strongly on the metric used to measure time, where my emphasis is on biological time (time to metamorphosis). The results from the populations comparisons showed species and gradient-specific responses. For H. takanoi, distributed over a salinity gradient (North Sea -Baltic Sea), larvae from the North Sea populations always showed higher survival and faster development compared with those from the Baltic Sea. The population near the limit of the distribution showed very low survival, suggesting that subsidies or complex ontogenetic migration patterns are needed for population persistence. Results did not show genetic differentiation among the studied populations in the mitochondrial cytochrome c oxidase subunit one gene (COI) suggesting that there is high connectivity among populations. For C. maenas distributed across a latitudinal gradient (South: Vigo, Spain; North: Bergen and Trondheim, Norway) and reared under different temperatures (range 6 to 27 °C in steps of 3 °C), there was little variation in survival and growth among populations. However, larvae from the Norwegian populations had a slightly shorter duration of development at low temperatures than those from Vigo, this response has an adaptive value in that it could sustain survival in scenarios of reduced temperature, by shortening the larval phase, when mortality rates are high. Besides, results from this experiment (as well as for the mentioned above) showed high intrapopulation variability in larval performance which has a potential to affect range expansion of the above-mentioned species. Variation in the responses of larval stages to the effects of different environmental drivers highlights the importance of using physiological descriptors to quantify the performance of marine invertebrates to changing environments. Larval responses vary in rates of survival but also in the duration of time to achieve metamorphosis, as well as the rate at which the organisms grow, with concomitant effects on post-metamorphic success, which in seasonal habitats may strongly depend on temperature. The results from the thesis highlight the importance of quantifying the responses of marine invertebrates to changing environmental conditions, considering different species and species distributed across different gradients as well as variations among and within species.
Dissertation
306 232
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Plankton in an acidifying ocean : from individual responses to community changes and the potential of adaptation
(2018-06-25)
Langer, Julia A. F.
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Schwenk, Klaus
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Boersma, Maarten
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Boersma, Maarten
Ocean acidification has strong direct (decreased seawater pH) and indirect (altered food regimes) effects on the performance of marine organisms, their trophic interactions and, consequently, whole ecosystems. Therefore, the aim of this thesis was to investigate potential direct and indirect effects of elevated pCO2 on marine planktonic organisms and their community composition, with a main emphasise on copepods and their adaptational potential. The development of high throughput sequencing technologies has provided scientists with an efficient tool to assess the biodiversity of marine communities, particularly with the recent advances in community barcoding technologies using universal primers. Another aim of this thesis was therefore, to test whether community barcoding is suitable for the assessment of marine planktonic communities, and allows the detection of compositional changes, which would probably remain unnoticed using classical morphological approaches.
Dissertation
393 357
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Physical and Physiological Growth Constraints of Key, North Sea Gelatinous Zooplankton
(2017-05-05)
Lesniowski, Thomas
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Peck, Myron
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Boersma, Maarten
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Boersma, Maarten
The aim of this thesis was to investigate the physical and physiological growth constraints of gelatinous zooplankton, focusing on under-studied juvenile stages of key North Sea gelatinous zooplankton: the scyphozoan benthic polyp; and the planktonic ephyra stage. The thesis focused on climate-driven factors that are predicted to change due to global warming. We conducted tri-trophic food chain experiments to research the direct and indirect effects of OA and changes of nutrient availability. Polyp growth and carbon content was not affected by CO2, but was significantly negatively affected by P-limitation of the food. The conclusion is that phosphorus can be a limiting factor influencing the fitness of scyphozoan polyps and that phosphorus limited food is of poor nutritional quality. The next step was to investigate the effect of temperature and different levels of CO2. The growth and carbon content of polyps and ephyrae were significantly affected by temperature, but there was no effect either by CO2 or by the interaction between temperature and CO2. To investigate the effect of current, polyps on plates were tested in an annular flow channel under different current velocities. Additionally, two field experiments were conducted to verify the laboratory results and survey the influence of water current and shelter on the survival of scyphozoan polyps. The laboratory and field results demonstrate that current velocity has a negative effect and protection a positive effect on the survival of scyphozoan polyps. The conclusion of this thesis is that the juvenile stages of the species used for this work have a wide range of tolerance for the investigated abiotic factors. This makes them more adaptable to the predicted shift of climate.
Dissertation
369 219
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Balance of power: Dissolved nitrogen-to-phosphorus ratios and phytoplankton growth rate determine the balance between bottom-up and top-down processes in planktonic food webs
(2023-05-31)
Klip, Helena C. L.
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Striebel, Maren
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Boersma, Maarten
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Boersma, Maarten
Phytoplankton are globally responsible for ~50% of the global oxygen production via primary production, fuelling food webs, and can alter biogeochemical cycles. Grazing forms a massive loss factor of phytoplankton standing stocks. Since it can be challenging to measure variation in these relatively tiny organisms, most studies on planktonic predator-prey interactions overlook variation within groups and populations. It has recently become evident that neither prey populations nor predator populations can be viewed as homogeneous entities. Numerous physiological and behavioural differences can influence how predator-prey interactions act out in both phytoplankton and herbivorous zooplankton. Community organization may be influenced by variation in nutrient stoichiometry, cell quotas, and nutritional requirements and, particularly intraspecific (within-population) variation. I therefore concentrated on nutritional stoichiometry as the changeable trait among populations since the processes driving variation in this trait within populations are different in primary and secondary producers, which can result in mismatch phenomena. A series of grazing dilution experiments using field samples, a growth medium literature review, chemostat experiments, grazing experiments and trait-based approaches were used within this dissertation to investigate on different organisational levels the role of dissolved nitrogen-to-phosphorus ratios and phytoplanktonic growth on the balance between bottom-up and top-down processes in planktonic food webs. The dilution experiments indicated that zooplankton protists (i) actively select between and within phytoplankton and bacterioplankton prey populations, (ii) shift their grazing pressure depending on their nutritional requirements, as both prey items are plastic in their composition, and (iii) play roles in termination of spring phytoplankton and bacterioplankton blooms. The chemostat experiment showed changing stoichiometry as well as lower intercellular trait variability in the faster-growing isogenic phytoplankton populations. As there are multiple ways to grow slowly for phytoplankton, but only one way to grow fast, the recommendation for medium optimisation is a Redfield ratio-correction of the nitrogen-to-phosphorus ratio in future use growth media. The grazing experiments demonstrated that the secondary consumers get impacted by fluctuations in dissolved nitrogen-to-phosphorus ratios (resource quality) and growth rates of their prey. This dissertation provides ultimately more insights in biogeochemical cycling and planktonic trophodynamics with fine-tuning implications for food web models.
Dissertation
405 377
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Fish community dynamics in a cold temperate tidal lagoon in the Wadden Sea, in relation to changes in environmental conditions
(2024-09-25)
Odongo Otieno, Victor
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Dahlke, Flemming
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Boersma, Maarten
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Boersma, Maarten
The thesis investigated the effects of long-term and seasonal changes in environmental conditions on fish communities in the Wadden Sea. Using a monthly fish monitoring dataset from 2007 to 2019, a total of 55 fish species were recorded, including 8 new observations of Lusitanian, boreal, and Atlantic species, while four species from earlier surveys (1989-1995) were no longer present. The fish community was dominated by 22 species, which accounted for over 95% of the total abundance. These species exhibited three distinct trends: dome-shaped, increasing, or decreasing, influenced by winter temperatures which impacted recruitment. The analysis revealed seasonal groupings based on species’ temperature preferences, affecting their migration patterns. Warming autumn temperatures delayed cod (Gadus morhua) immigration and whiting (Merlangius merlangus) emigration into the Wadden Sea. Environmental factors, primarily water temperature, salinity, and chlorophyll a, explained 18% of fish community variation on a seasonal basis and 10% on an inter-annual basis. Species richness was higher at 5°C and 15°C, reflecting seasonal immigration and emigration patterns. However, species diversity was negatively correlated with water temperature, as Atlantic herring (Clupea harengus) and small sand eel (Ammodytes tobianus) dominated the summer, and whiting dominated autumn. Species richness declined with increasing water depth, as more species migrated to intertidal habitats during high tide. Evenness increased with water depth near seagrass meadows and bivalve beds, while pelagic species entered deep tidal channels during high tide. Fifteen young-of-the-year YOY fish species were observed at various developmental stages, with most supplied from the North Sea. Environmental factors significantly explained 39% of YOY fish variations. Clupea harengus had two cohorts (autumn and spring hatched), while Nilsson’s pipefish (Syngnathus rostellatus) had one cohort spawned within the Wadden Sea.
Dissertation
243 224
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Zooplankton community responses to Ocean Acidification
(2017-06-16)
Algueró Muniz, Maria
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Niehoff, Barbara
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Boersma, Maarten
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Boersma, Maarten
Ocean acidification is affecting marine ecosystems directly through changes in pH, as well as indirectly, via trophic pathways. Thus, to evaluate impacts of ocean acidification on marine communities it is necessary to consider the potential pCO2 effects on population dynamics as well as community trophic interactions. Within the framework of the BIOACID II project (Biological Impacts of Ocean ACIDification), the overarching goal of this thesis was to study the effects of ocean acidification on zooplankton, focusing on copepods and jellyfish. The main results are described in four chapters (CHAPTER I to IV), each of which corresponds to a manuscript. The first part of this thesis evaluated pCO2 effects on natural mesozooplankton communities from a boreal fjord (CHAPTER I) and the subtropical Northeast Atlantic (CHAPTER II). Large-scale pelagic mesocosm units (a Kiel Off-Shore Mesocosms for Future Ocean Simulations : KOSMOS) were artificially enriched in CO2 to simulate future ocean conditions. In both experiments, we detected species-specific sensitivities to ocean acidification in copepods, as well as positive pCO2 effect on total mesozooplankton abundances under high-CO2 bloom conditions, caused by a bottom-up effect. During the Gullmar Fjord KOSMOS2013 experiment (CHAPTER I) species-specific sensitivities to CO2 were detected in copepods, as well as in hydromedusae. However, these effects on single species were not translated into the structure or the diversity of the community, likely due to the overwhelmingly dominance of Pseudocalanus acuspes, which resulted to be more abundant under acidic conditions, especially the younger (copepodite) life stage. In the Gran Canaria KOSMOS2014 study (CHAPTER II) a significant effect of pCO2 on phytoplankton succession was detected, ultimately affecting the development of the plankton community only after a simulated bloom event. The zooplankton community responded to the phytoplankton bloom in all mesocosms, although the response was delayed under high pCO2 conditions. The most abundant mesozooplankters were calanoid copepods, which did not respond to CO2 treatments during the pre-bloom phase of the experiment. However calanoids were more abundant under elevated pCO2 conditions than in low- pCO2 levels in the post-bloom phase. Bottom-up effects of CO2-driven increases in phyto- and microzooplankton standing stocks would explain the increase in copepod abundance during both experiments. These results suggest that, under realistic end-of-century scenarios, the above-mentioned ocean acidification effects detected on copepods could potentially affect biomass transfer to higher trophic levels. As in community experiments it is not possible to separate out the pCO2 direct and indirect effects, mesocosms studies were combined with laboratory experiments in the second part of this thesis work. The aim was to evaluate direct and indirect effects of global change conditions on the two main groups of interest for this thesis: copepods and jellyfish. Apart from direct acidification effects, the increasing carbon availability in the marine environment will likely change primary production and the quality of phytoplankton as food for higher trophic levels, showing higher C:nutrient ratios as CO2 availability increases. Hence, a change in biochemical composition when culturing algae (Rhodomonas salina) in elevated pCO2 levels caused a change in food quality, affecting zooplankton by decreased growth and development. Indirect negative pCO2 effects were observed on the dinoflagellate Oxyrrhis marina and nauplii and copepodite stages of the copepod Acartia tonsa. Direct pH effects on these consumers seem to be of lesser importance than the indirect effects caused by a CO2-associated decrease in algal quality when having only a food source (CHAPTER III), unlike the positive CO2-effect observed in copepods when feeding on natural plankton communities. Direct pH effects on zooplankton, however, must be placed in a global change context, considering that ocean acidification in future oceans will not act alone but in combination with other climate factors such as warming and deoxygenation. The direct effects of these three stressors in conjunction were thus studied on 1-day-old ephyrae of the moon jellyfish (Aurelia aurita) from a North Sea subpopulation off Helgoland Island (Germany). The results obtained during this experiment point that end-of-century pCO2 scenarios will not affect these ephyrae in a substantial way. However, A. aurita may not be robust to larger changes in ocean pH, warming and deoxygenation, especially if simultaneous increases in atmospheric pCO2 levels and seawater temperature occur (CHAPTER IV). A. aurita is an ecologically and economically relevant species due to its interactions with commercially important fish species, hence the tolerance or resilience of this jellyfish to climate change might be detrimental for future fisheries.
Dissertation
533 188

Boersma, Maarten

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