Feeding of calanoid copepods and analyses of their faecal pellets

Abstract

Observations on the feeding behaviour of calanoid copepods, microscopical analysis of their faecal pellets and the possible ecological importance of copepod grazing will be addressed in this thesis. Laboratory as well as field studies were applied to investigate the responses of different copepod species to different food environments, covering the three main phytoplankton groups - Diatoms, Dinoflagellates and Coccolithophores. Since many of the species belonging to these groups are surrounded by mineral skeletons, qualitative observations can be revealed by the use of scanning electron microscopy (SEM).The importance of calcifying algae in the marine carbon and calcium cycles is widely known, but the role of copepods for the coccolith calcite dissolution is vague. Dissolution experiments were performed using the calcifying algae Calcidiscus leptoporus. SEM observations of the morphological changes of the coccoliths were used for content interpretation of freshly produced faecal pellets in feeding experiments with the common North Sea copepods Calanus helgolandicus and Temora longicornis. Coccolith calcite dissolution during copepod gut passage was found to be minor and can therefore not contribute significantly to the generally observed calcite dissolution in the upper ocean.Field experiments conducted during a dinoflagellate bloom in the North Sea clearly showed that some copepod species can feed very efficiently on the dinoflagellate species Dinophysis norvegica. SEM observations showed that faecal pellets produced by C. helgolandicus were almost exclusively filled with intact cells of this toxic dinoflagellate species. Faecal pellets, containing intact toxic phytoplankton species may be an important vehicle and new pathway transferring toxins within the pelagic and to the benthic community. The viability of D. norvegica cells within the faecal pellets was not investigated, but lead to the idea to observe the ability of different phytoplankton species to survive the copepod gut passage. Therefore a new staining method was applied. Viable cells of the dinoflagellate Scrippsiella trochoida and the diatom Fragilariopsis kerguelensis were detected within the faecal pellets of different copepod species. F. kerguelensis may protect itself with its stable frustules and also the formation of chains seemed to be advantageous for the survival of single cells when they are grazed. Whereas the long spines of the diatom Chaetoceos dichaeta provided no protection and was reduced to small pieces within the faecal pellets of all copepods investigated. Studies of species specific interactions among copepods and their prey can greatly improve the current understanding of the benefit of certain phytoplankton cell structures. In an additional experiment conducted with the two copepods Acartia clausi and T. longicornis the ability to graze on the largest centric diatom commonly found in the North Sea plankton was under investigation. A. clausi was not able to graze on Coscinodiscus wailesii but T. longicornis showed a very skilful handling of individual cells. The copepod did not ingest the whole cells, but bite a piece off the frustules to reach the protoplasma. After ingestion of the cell content, the empty diatom frustules were rejected. Results showed that traditional methods to determine grazing may sometimes be unsuitable and that new methods need to be developed. The new promising approach of genetical gut content analysis to qualify and quantify the grazing impact of copepods was therefore tested in a study and provided preliminary, but promising results.The general importance of food for copepods can be reflected in their energy requirements for egg production. To observe if and how food concentration is linked to egg production in a common Antarctic copepod species, the reproductive response of Rhincalanus gigas was studied during the build up of a diatom dominated phytoplankton bloom in the Southern Ocean. Results from egg production experiments as well as the maturation of their gonads revealed that R. gigas was able to react directly to enhanced food concentrations.