Selected interactions between phytoplankton, zooplankton and the microbial food web: Microcosm experiments in marine and limnic habitats

The experiments presented in this thesis elucidate selected interactions between the phytoplankton, the zooplankton and the microbial food web in aquatic ecosystems. The objective is to provide a mechanistic understanding of classic general ecology topics including competition, predator-prey relations, food web structure, succession, and transfer of matter and energy. Special relevance is attributed to the role of mixotrophic organisms, marine cladocerans, and gelatinous mesozooplankton. Although they may contribute substantially to plankton composition they have thus far been neglected in common ecosystem models. All experiments were based on enrichment with nutrients and organic compounds. Enrichment with nutrients and organic compounds that influence overall system productivity is one of the most pervasive human alterations of the environment and profoundly affects species composition, food web structure, and ecosystem functioning. In order to predict the consequences of such enrichment, a better understanding of the impact that trophic structure has on community dynamics and ecosystem processes is required. The presented thesis consists of two studies. The first study includes three experiments in which I investigated the role copepods, cladocerans and doliolids play in plankton interactions. Copepods, cladocerans and doliolids are major mesozooplankton groups in marine systems. The first experiment (Katechakis et al. 2004) showed that copepods, cladocerans and doliolids have different food size spectra and different assimilation efficiencies. According to my experiment, copepods actively select for larger food items, whereas cladocerans and doliolids passively filter medium-sized and small food items, respectively, with doliolids being the only group that feeds efficiently on bacteria and picoplankton. The results illustrate that food niche separation enables copepods, cladocerans and doliolids to coexist. In addition, they emphasize the fact that doliolids are favored in low nutrient environments, characterized by small food items, whereas cladocerans and copepods have competitive advantages at moderate and high nutrient supplies, respectively. Furthermore, copepods obviously utilize ingested food best, gauged in terms of produced biomass, followed by cladocerans and doliolids, which suggests that the different mesozooplankton have different impacts on energy transfer efficiency within the food web. In the second experiment (Katechakis et al. 2002), I investigated how copepods, cladocerans and doliolids directly influence the phytoplankton and the microbial food web over a longer period of time by grazing. Furthermore, I investigated how they indirectly influence the system's nutrient dynamics through "sloppy feeding" and their excretions. According to my experiment, in the long run, doliolids and cladocerans promote the growth of large algae whereas copepods shift the size spectrum towards small sizes with different consequences for food chain length. Doliolids, cladocerans and copepods also affect the microbial food web in different ways. Size-selective grazing may lead to differences in the nanoplankton concentrations. These in turn can affect bacterial concentrations in a trophic cascade. My findings offered the first experimental evidence for the occurrence of top-down effects in marine systems. Although top-down explanations of phytoplankton size structure had been acknowledged for limnic systems before, they had not been attempted for marine systems. In the last experiment of this series (Katechakis and Stibor 2004) I sought to complement the knowledge about the feeding behavior of marine cladocerans. Marine cladocerans are difficult to cultivate in the laboratory. Therefore, the three cladoceran genera found in marine systems, Penilia, Podon and Evadne, had never before been compared under similar conditions. Existing studies with single cladoceran genera were to some extent contradictory. My experiments indicate similar feeding characteristics for Penilia, Podon and Evadne, that is to say, similar food size spectra, clearance and ingestion rates. However, Evadne obviously has problems feeding on motile prey organisms. The results generated by my first study have been summarized and their importance has been hypothetically extended to ecosystem level by Sommer et al. (2002) and by Sommer and Stibor (2002). My second study includes two experiments that refer to the ecological role of mixotrophs in aquatic systems. Mixotrophic organisms combine phototrophic and phagotrophic production dependent on the availability of light and nutrients. Although they are common in aquatic systems, their function for nutrient cycling and as a link to higher trophic levels has never before been examined. In my first experiment (Katechakis et al. 2005) I investigated if mixotrophs influence energy transfer efficiency to higher trophic levels differently than predicted for purely phototrophic organisms. My results indicate that compared to phototrophic specialists mixotrophs may enhance transfer efficiency towards herbivores at low light conditions and in situations when limiting nutrients are linked to bacteria and to the picoplankton. Additionally, the results suggest that mixotrophs may have a stabilizing effect on variations in trophic cascade strength caused by perturbations to light and nutrient supply ratios. My second experiment (Katechakis and Stibor 2005a) served as a first step towards analyzing if the results gained from the first experiment have any ecological relevance in situ, that is, if mixotrophs in nature-like communities can gain enough importance to relevantly influence transfer efficiency and system stability. Competition experiments revealed that mixotrophs may invade and suppress plankton communities that consist of purely phototrophic and purely phagotrophic specialists at low nutrient conditions while high nutrient supplies prevent mixotrophs from successfully invading such communities. In systems where mixotrophs suppressed their specialist competitors they indeed had a habitat-ameliorating effect for higher trophic levels, gauged in terms of plankton food quality

Do marine phytoplankton follow Bergmann's rule sensu lato?

Global warming has revitalized interest in the relationship between body size and temperature, proposed by Bergmann's rule 150 years ago, one of the oldest manifestations of a ‘biogeography of traits’. We review biogeographic evidence, results from clonal cultures and recent micro- and mesocosm experiments with naturally mixed phytoplankton communities regarding the response of phytoplankton body size to temperature, either as a single factor or in combination with other factors such as grazing, nutrient limitation, and ocean acidification. Where possible, we also focus on the comparison between intraspecific size shifts and size shifts resulting from changes in species composition. Taken together, biogeographic evidence, community-level experiments and single-species experiments indicate that phytoplankton average cell sizes tend to become smaller in warmer waters, although temperature is not necessarily the proximate environmental factor driving size shifts. Indirect effects via nutrient supply and grazing are important and often dominate. In a substantial proportion of field studies, resource availability is seen as the only factor of relevance. Interspecific size effects are greater than intraspecific effects. Direct temperature effects tend to be exacerbated by indirect ones, if warming leads to intensified nutrient limitation or copepod grazing while ocean acidification tends to counteract the temperature effect on cell size in non-calcifying phytoplankton. We discuss the implications of the temperature-related size trends in a global-warming context, based on known functional traits associated with phytoplankton size. These are a higher affinity for nutrients of smaller cells, highest maximal growth rates of moderately small phytoplankton (ca. 102 µm3), size-related sensitivities for different types of grazers, and impacts on sinking rates. For a phytoplankton community increasingly dominated by smaller algae we predict that: (i) a higher proportion of primary production will be respired within the microbial food web; (ii) a smaller share of primary production will be channeled to the classic phytoplankton – crustacean zooplankton – fish food chain, thus leading to decreased ecological efficiency from a fish-production point of view; (iii) a smaller share of primary production will be exported through sedimentation, thus leading to decreased efficiency of the biological carbon pump

Vegetation transitions drive the autotrophy-heterotrophy balance in Arctic lakes

“Arctic greening” will alter vegetation quantity and quality in northern watersheds, with possible consequences for lake metabolic balance. We used paleolimnology from six Arctic lakes in Greenland, Norway, and Alaska to develop a conceptual model describing how climate-driven shifts in terrestrial vegetation (spanning herb to boreal forest) influence lake autotrophic biomass (as chlorophyll and carotenoid pigments). Major autotrophic transitions occurred, including (1) optimal production of siliceous algae and cyanobacteria/chlorophytes at intermediate vegetation cover (dwarf shrub and Betula; dissolved organic carbon (DOC) range of 2–4 mg L21 ), below and above which UVR exposure (DOC; 4 mgL21 ), respectively limit algal biomass, (2) an increase in potentially mixotrophic cryptophytes with higher forest cover and allochthonous carbon supply. Vegetation cover appears to influence lake autotrophs by changing influx of (colored) dissolved organic matter which has multiple interacting roles—as a photoprotectant—in light attenuation and in macronutrient (carbon, nitrogen) supply

Effect of food concentration and type of diet on Acartia survival and naupliar development

We have performed life table experiments to investigate the effects of different food types and concentrations on the larval development and survival up to adulthood of Acartia tonsa. The food species offered comprised a wide taxonomic spectrum: the pigmented flagellates Isochrysis galbana, Emiliania huxleyi, Rhodomonas sp., Prorocentrum minimum, the diatom Thalassiosira weissflogii, grown on medium offering enriched macronutrient concentrations and the ciliate Euplotes sp. initially cultured on Rhodomonas. For the ciliate species, also the functional response was studied. In order to avoid limitation by mineral nutrients, food algae have been taken from the exponential growth phase of the nutrient replete cultures. The suitability of Rhodomonas as a food source throughout the entire life cycle was not a surprise. However, in contrast to much of the recent literature about the inadequacy or even toxicity of diatoms, we found that also Thalassiosira could support Acartia-development through the entire life cycle. On the other hand, Acartia could not complete its life cycle when fed with the other food items, Prorocentrum having adverse effects even when mixed with Rhodomonas and Thalassiosira. Isochrysis well supported naupliar survival and development, but was insufficient to support further development until reproduction. With Emiliania and Euplotes, nauplii died off before most of them could reach the first copepodite stages. Acartia-nauplii showed a behavioral preference for Euplotes-feeding over diatom feeding, but nevertheless Euplotes was an insufficient diet to complete development beyond the naupliar stages

Diet influence on egg production of the copepod Acartia tonsa (Dana, 1896)

Egg production in the copepod Acartia tonsa was evaluated using different densities of the microalgae Thalassiosira weissflogii, Chaetoceros muelleri and Isochrysis galbana. Male and female were kept under controlled conditions (salinity 30, 20°C, photoperiod 12L:12D), acclimated to the experimental conditions and left over a period of 24 h to allow copulation. Algal densities tested were equivalent in biovolume and corresponded to 0, 2.5, 5, 10, 20, 40 and 60.10³ cells.mL-1 of T weissflogii. Ten acclimated female were separated, transferred to glass bottles and exposed for further 24 h to the corresponding experimental medium. After this period, the eggs were fixed and counted. Copepod egg production reached a threshold value when T weissflogii, C. muelleri and I. galbana were supplied at 10.10³, 140.10³ and 640.10³ cells.mL-1, respectively. Mean egg production corresponded to 28.0 ± 0.5, 20.1 ± 1.0 and 22.0 ± 3.5 eggs.female-1 .day-1, respectively. Copepods fed T weissflogii showed the highest mean egg production while those fed I. galbana reached a maximum egg production when the algae were supplied at a density two- to fourfold higher, considering the biovolume of T weissflogii and C. muelleri. These differences are explained considering the different sizes of the microalgae used to feed the copepods.A produção de ovos do copépode Acartia tonsa foi avaliada utilizando diferentes densidades das microalgas Thalassiosira weissflogii, Chaetoceros muelleri e Isochrysis galbana. Machos e fêmeas foram colocados sob condições controladas (salinidade 30, 20°C, fotoperíodo 12L:12D), aclimatados às condições experimentais e mantidos juntos por 24 h para permitir a copula. As densidades de algas foram equivalentes em biovolume e corresponderam a 0, 2,5, 5, 10, 20, 40 e 60,10³ células.mL-1 de T. weissflogii. Dez fêmeas aclimatadas foram separadas, transferidas para frascos de vidro e expostas por mais 24 h ao meio experimental correspondente. Após este período, os ovos foram fixados e contados. A produção de ovos alcançou um valor limiar quando T. weissflogii, C. muelleri e I galbana foram oferecidas a concentrações de 10,10³,140,10³ e 640,10³ células.mL-1, respectivamente. A média de produção de ovos correspondeu a 28,0 ± 0,5, 20,1 ± 1,0 e 22,0 ± 3,5 ovos.fêmea-1.dia-1, respectivamente. Copépodes alimentados com T. weissflogii mostraram a maior produção de ovos média enquanto os alimentados com I. galbana alcançaram uma produção de ovo máxima quando as algas foram providas a uma densidade de duas a quatro vezes maior, considerando o biovolume de T. weissflogii e C. muelleri. Estas diferenças podem ser explicadas considerando os diferentes tamanhos das microalgas utilizadas para alimentar os copépodes

Le chemin vers un jour véritablement historique

„Migration“ wird regelmäßig als die drängendste Sorge der Europäerinnen und Europäer identifiziert. Endlos jedoch ist und bleibt der Streit darüber, was die europäische Antwort auf diese Sorge sein mag. Während gesetzgeberische Lösungsversuche spätestens bei der Umsetzung scheitern und sich die Akteure sämtlicher europäischer Entscheidungsebenen im Verhandlungsprozess für eine Reform laufend gegenseitig das Vertrauen entziehen, gewinnen rechtspopulistische Parteien nationale, europapolitisch relevante Wahlen und nutzen die Forderung nach asylpolitischer Abschottung, um den „Rechtsruck“ zum allgemeinen Kurs der Union zu erheben. Was kann das politische Zentrum in Europa dem entgegensetzen? Wie hat es die Reform des Gemeinsamen Europäischen Asylsystems als legislativen Kern der Migrationssteuerung gegen den Rechtsruck der letzten Jahre immunisiert und ausverhandelt? Die vorliegende Arbeit analysiert den Verhandlungsprozess mithilfe des Multiple-Streams-Ansatzes von John W. Kingdon. Vorbereitet durch eine juristische Kontextualisierung, eröffnen sich Probleme, Policies und Politics der Reform der europäischen Migrationssteuerung als Dimensionen („Ströme“) der weiteren Untersuchung. Zudem werden Politik-Unternehmer identifizierbar, ebenso wie das Ausbleiben von Allianzen oder Widerständen von Akteuren mit Veto-Position. Einen Testfall der Befunde dieser Arbeit bildet die Verortung der Grünen, die als Fraktion im Europäischen Parlament ganz mehrheitlich gegen die Reform gestimmt, als Teil der deutschen Bundesregierung jedoch letztlich die Zustimmung im Rat mitgetragen haben.C’est « l’immigration » qui représente régulièrement selon les sondages la préoccupation principale des Européens et des Européennes. Les débats sur ce que soit la juste réponse européenne à cette préoccupation ne semblent jamais finir. Des mesures législatives échouent souvent déjà pendant leur application et partout en Europe les acteurs responsables n’arrivent pas à se faire la confiance nécessaire pour négocier avec succès une reforme européenne. Cependant, les partis de la droite extrême gagnent, élection par élection, en influence au sein des institutions de l’Union européenne en demandant une politique migratoire encore plus ferme. – Qu’est-ce que le centre pro-européen peut y opposer ? Comment a-t’ il réussi de protéger le cœur législatif de la gestion des migrations, la réforme du système européen commun d’asile, contre la montée de la droite extrême pendant les négociations des dernières années ? L’analyse présente part du contexte juridique pour utiliser l’approche des courants multiples de John W. Kingdon afin de préciser les problèmes, les solutions possibles et les processus politiques comme dimensions (« courants ») à enquêter. De plus, des entrepreneurs politiques sont identifiables ainsi que l’absence d’alliances ou de résistances d’acteurs avec une position de véto. Les résultats de cet travail peuvent être testés sur le cas des Verts, dont le groupe dans le Parlement européen a majoritairement voté contre la réforme pendant que les Verts faisant partie du gouvernement allemand ont finalement soutenu sa position favorable au sein du Conseil.„Immigration“ is regularly mentioned as the Europeans’ most pressing political concern. There is a seemingly endless disagreement over the European response to it. Legislative measures fail soon with their application; European actors at every decision-making level distrust each other even in the midst of negotiating the way to any reform. Meanwhile, right-wing populist parties win relevant elections by demanding ever tougher measures against immigration and use their growing influence to shift the EU politically further right in general. – Can the pro-European centre counter this? How has it eventually negotiated the reform of the Common European Asylum System and how has it protected this legislative core of migration governance against the populist rise in recent years? This thesis analyses the reform negotiations beginning with the legal context. By using John W. Kingdon’s Multiple Streams approach, the dimensions (“streams”) of the European migration governance reform – its problems, policies and politics – are laid out as subjects of further research. Important political entrepreneurs are identified, the absence of resistance from actors with a potential veto position is investigated. This leads by itself to an examination of the Greens’ position, whose group in the European parliament voted overwhelmingly against the reform, while the Greens as part of the German government finally supported its approval in the Council

Changes in zooplankton diversity and distribution pattern under varying precipitation regimes in a southern temperate estuary

http://www.sciencedirect.com/science/article/B6WDV-4VGF3YJ-5/2/0532a272b92cfc2de9a24d92edc5265

An indoor mesocosm system to study the effect of climate change on the late winter and spring succession of Baltic Sea phyto- and zooplankton

An indoor mesocosm system was set up to study the response of phytoplankton and zooplankton spring succession to winter and spring warming of sea surface temperatures. The experimental temperature regimes consisted of the decadal average of the Kiel Bight, Baltic Sea, and three elevated regimes with 2°C, 4°C, and 6°C temperature difference from that at baseline. While the peak of the phytoplankton spring bloom was accelerated only weakly by increasing temperatures (1.4 days per degree Celsius), the subsequent biomass minimum of phytoplankton was accelerated more strongly (4.25 days per degree Celsius). Phytoplankton size structure showed a pronounced response to warming, with large phytoplankton being more dominant in the cooler mesocosms. The first seasonal ciliate peak was accelerated by 2.1 days per degree Celsius and the second one by 2.0 days per degree Celsius. The over-wintering copepod populations declined faster in the warmer mesocosm, and the appearance of nauplii was strongly accelerated by temperature (9.2 days per degree Celsius). The strong difference between the acceleration of the phytoplankton peak and the acceleration of the nauplii could be one of the “Achilles heels” of pelagic systems subject to climate change, because nauplii are the most starvation-sensitive life cycle stage of copepods and the most important food item of first-feeding fish larvae

Feeding selectivities and food niche separation of Acartia clausi, Penilia avirostris (Crustacea) and Doliolum denticulatum (Thaliacea) in Blanes Bay (Catalan Sea, NW Mediterranean)

Selectivity-size spectra, clearance and ingestion rates and assimilation efficiencies of Acartia clausi (Copepoda), Penilia avirostris (Cladocera) and Doliolum denticulatum (Doliolida) from Blanes Bay (Catalan Sea, NW Mediterranean) were evaluated in grazing experiments over a wide range of food concentrations (0.02–8.8 mm3 L−1 plankton assemblages from Blanes Bay, grown in mesocosms at different nutrient levels). Acartia clausi reached the highest grazing coefficients for large algae >70 μm (longest linear extension), P. avirostris for intermediate food sizes between 15 and 70 μm, and D. denticulatum for small sizes from 2.5 to 15 μm. Penilia avirostris and D. denticulatum acted as passive filter-feeders. Acartia clausi gave some evidence for a supplementary raptorial feeding mode. Effective food concentration (EFC) decreased linearly with increasing nutrient enrichment for D. denticulatum and followed domed curves for A. clausi and for P. avirostris with maximum values at intermediate and high enrichment levels, respectively. Clearance rates of crustacean species showed curvilinear responses with narrow modal ranges to increasing food concentration. Clearance rates of D. denticulatum increased abruptly and levelled into a plateau at low food concentrations. Mean clearance rates were 13.9, 25.5 and 64.1 mL ind.−1 day−1, respectively. No clearance could be detected for A. clausi at food concentrations <0.1 mm3 L−1 and for P. avirostris at food concentrations ≤0.02 mm3 L−1. Ingestion rates indicate a rectilinear functional response for A. clausi and for P. avirostris and showed a sigmoidal curve for D. denticulatum. Mean ingestion rates were 1.3, 2.8 and 7.7 μg C μg Cind.−1 day−1, respectively. Conversion of ingested carbon to tissue was 30–80% for the investigated crustaceans and 20–50% for doliolids. Food niche calculations suggest that food niche separation may explain the coexistence of the three species in summer in Blanes Bay