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

摘要

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.