Kreibich, T. (2008). Physiological adaptations of copepods from the North Sea and the North Atlantic to changing nutritional conditions. Ph.D. Thesis, Alfred-Wegener-Institut für Polar- und Meeresforschung Bremerhaven & Universität Bremen, Germany. viii + 229 pp. [In English and German].
562362
Kreibich, T.
2008
Physiological adaptations of copepods from the North Sea and the North Atlantic to changing nutritional conditions.
Ph.D. Thesis, Alfred-Wegener-Institut für Polar- und Meeresforschung Bremerhaven & Universität Bremen, Germany.
Marine calanoid copepods constitute a major component of the pelagic food web. They are an important link between phyto-/microzooplankton and higher trophic levels. In the marine realm, copepods have to adapt to changing environmental conditions in order to efficiently use dietary components for growth and reproduction. There are numerous studies on feeding behaviour and functional responses of calanoid copepods, as well as physiological responses to changing nutritional conditions. However, no combined information is available on functional and physiological responses in copepods to changing nutritional conditions. Detailed knowledge on their feeding behaviour, their physiological responses to nutritional conditions, i.e. digestive and metabolic activities, and their functional responses, i.e. reproduction, is of major importance in order to better understand how diet affects growth and reproduction of copepods. The project “Trophic interactions in pelagic ecosystems: the role of zooplankton” at the Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, in cooperation with the Marine Zoology at the University Bremen, aims therefore at comprehensively investigating feeding behaviour, physiological and functional responses of copepods to changing environmental conditions in order to obtain a more precise picture of processes influencing population dynamics. The present thesis, embedded within this project, aims at elucidating physiological adaptations of calanoid copepod species to changing nutritional conditions and how these are related to different life cycle strategies, metabolic requirements and biochemical properties, i.e. lipid and protein contents. For this purpose, species with different life strategies from the southern North Sea, i.e. Temora longicornis, Acartia clausi and Centropages typicus, and the lower St. Lawrence estuary (eastern Canada), i.e. Calanus finmarchicus and Metridia longa, were investigated during several phytoplankton blooms in 2005 and 2006. A comprehensive data set on digestive, metabolic and functional responses to changing nutritional conditions, traced via fatty acids trophic biomarkers, is presented and the data are discussed with respect to different life strategies.
The multi-voltine species with short life spans Temora longicornis, Acartia clausi and Centropages typicus were exposed to variable nutritional conditions during the phytoplankton blooms in spring 2005 (T. longicornis, A. clausi) and autumn 2006 (C. typicus). They were characterised by low lipid contents (2-9% of dry mass (DM)), high specific metabolic activities and high specific digestive potentials. Primarily proteins were accumulated during favourable feeding conditions, whereas lipid build-up followed secondarily. T. longicornis is well adapted to changing nutritional conditions by digesting different dietary components efficiently, thus enabling successful reproduction. Early in the season, when phytoplankton abundance was low, T. longicornis females fed omnivorously; the diet consisted of lipid-poor and potentially carbohydrate-rich diatoms, as well as of heterotrophic prey items like copepod eggs and nauplii. Later in the season, the diet changed towards mainly autotrophic, lipid-rich particles. According to these changes in diet, the digestive response of T. longicornis changed. At the.beginning of the time series the activities of the digestive enzymes proteinase and amylase were high, indicating feeding on protein- and carbohydrate-rich particles. When the diet changed, the digestive activity decreased, particularly amylase activity, suggesting a shift in the enzymatic pattern. At the same time lipid content increased in females indicating accumulation of dietary fatty acids, mainly diatom- and dinoflagellate-specific fatty acids such as 16:1(n-7) and 18:4(n-3). This high plasticity of digestive enzymes was confirmed in several experiments. Feeding and starvation experiments revealed that T. longicornis adapts its digestive activity within 24 h to changing nutritional conditions by strong reduction of digestive activities under starvation, increasing or decreasing activities under surplus food conditions, depending on their feeding history, and by diet-induced secretion of specific lipases when feeding on different diets. This high adaptive potential resulted in strong increases of specific dietary fatty acids within three days of incubation. In addition, due to the high adaptive potential egg production rates of T. longicornis females increased strongly within 24 h under surplus food conditions indicating that food was transformed successfully into egg material.
Calanus finmarchicus and Metridia longa are well adapted to the seasonal phytoplankton cycle in higher latitudes and generation times range between several months and more than one year. The species encountered changing nutritional conditions during the spring phytoplankton bloom in the St. Lawrence estuary in 2006. Both species were characterised by higher lipid contents (7-30% DM), lower specific metabolic activities and specific digestive potentials than the copepods from the North Sea. Proteins play a minor role as energy reserves in these species. Digestive activities were already elevated prior to the main phytoplankton bloom event, dominated by diatoms and dinoflagellates, and did not respond to changing nutritional conditions during the field campaign in both species, in spite different feeding behaviours. C. finmarchicus mainly fed on diatoms and dinoflagellates, whereas M. longa additionally fed on Calanus eggs or nauplii, indicated by fatty acid trophic biomarkers. Even under extreme feeding conditions, i.e. under surplus food as well as under starvation, C. finmarchicus showed no alterations in digestive activity. It can be suggested that these copepods do not respond to short-term changes but rather to distinct and long-lasting changes in food supply.
In conclusion, the results indicate that the adaptive digestive potential of copepods has implications on the life strategies. Higher adaptive potentials are found in copepods characterised by low energy reserves, short life spans and several generations per year, whereas copepod species with life spans of one year and more, which are well adapted to periodic food supply, are characterised by lower adaptive potentials.
The integrative approach applied in this study lead to a deeper understanding of the physiological adaptations of copepods to changing nutritional conditions and their implications for life-cycle strategies. Furthermore, it was demonstrated that the digestive response to different dietary conditions strongly depends on the feeding history and metabolic requirements of the investigated specimens.
