JOURNAL TRANSCRIPT
Alfred-Wegener-Institut für Polar- und Meeresforschung Bremerhaven
PHYSIOLOGICAL ADAPTATIONS OF COPEPODS FROM THE NORTH SEA AND THE NORTH ATLANTIC TO CHANGING NUTRITIONAL CONDITIONS
Tobias Kreibich
2007
PHYSIOLOGICAL ADAPTATIONS OF COPEPODS FROM THE NORTH SEA AND THE NORTH ATLANTIC TO CHANGING NUTRITIONAL CONDITIONS
UNTERSUCHUNGEN ZUR ERNÄHRUNGSPHYSIOLOGIE DOMINANTER COPEPODENARTEN DER NORDSEE UND DES NORDATLANTIKS
Tobias Kreibich
Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.)
am Fachbereich 2 (Biologie/Chemie) der Universität Bremen
Dezember 2007
CONTENTS
CONTENTS Abstract
i
Zusammenfassung
iii
Outline of publications
vi
1 Introduction
1
1.1 Scientific background
1
1.2 Life strategies of calanoid copepods in different environments
2
1.2.1 North Sea
3
1.2.2 St. Lawrence estuary
5
1.3 Physiological and biochemical characteristics of copepods
7
1.3.1 Major organic components
8
1.3.2 Metabolic rates
9
1.3.3 Digestive activity
10
1.3.4 Fatty acids and their use as biomarkers
11
1.4 Objectives 2 Materials and methods 2.1 Field work
12 15 15
2.1.1 Feeding and starvation experiments
16
2.1.2 Egg production measurements
18
2.2 Analytical work
18
2.2.1 Dry mass
18
2.2.2 Water-soluble protein content
19
2.2.3 Lipid content and fatty acid analysis
19
2.2.4 Enzyme analysis
20
2.2.4.1 Digestive enzyme activities
20
2.2.4.2 Metabolic enzyme activities
24
2.2.5 Statistics
24
CONTENTS
3 Results and synoptic discussion 3.1 Implications of the physiological adaptive potential on life strategies 3.1.1 Biochemical and metabolic characteristics of copepods
26 26 26
3.1.1.1 Lipid and protein contents of copepods
27
3.1.1.2 Metabolic activity of copepods
29
3.1.1.2.1 Differences in specific metabolic activities
30
3.1.1.2.2 Response to changing nutritional conditions
33
3.1.2 Digestive activity of copepods
39
3.1.2.1 Differences in digestive potentials due to metabolic requirements
40
3.1.2.2 Digestive response to variable nutritional conditions
41
3.1.3 Conclusions 3.2 Use of fatty acid trophic biomarkers
48 48
3.2.1 Time-dependent accumulation of dietary fatty acids
50
3.2.2 Factors influencing the accumulation of dietary fatty acids
50
4 Perspectives
54
5 References
56
Publications Publication I
71
Physiological responses of Temora longicornis females (Crustacea, Copepoda) to changing nutritional conditions in the North Sea Publication II
101
Short term variation of nutritive and metabolic parameters in Temora longicornis females (Crustacea, Copepoda) as a response to diet shift and starvation Publication III Influence of physiological conditions and feeding history on digestion, metabolism and fatty acid composition of Temora longicornis females (Crustacea, Copepoda) under different nutritional conditions
121
CONTENTS
Publication IV
153
Feeding strategies, digestive activities and reproduction of Calanus finmarchicus and Metridia longa in the lower St. Lawrence estuary, Québec, Canada Publication V
187
Dietary shifts in the North Sea copepod Temora longicornis in spring 2005 – evidence from stable isotope signatures, fatty acid biomarkers and feeding experiments Acknowledgements
225
Erklärung
229
ABSTRACT
ABSTRACT 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 i
ABSTRACT
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. ii
ZUSAMMENFASSUNG
ZUSAMMENFASSUNG Marine calanoide Copepoden bilden einen Hauptbestandteil des pelagischen Nahrungsnetzes und stellen ein Bindeglied zwischen dem Phyto-/Mikrozooplankton und höheren trophischen Ebenen dar. In marinen Lebensräumen sind Copepoden darauf angewiesen, sich schnell an verändernde Umwelt- und Nahrungsbedingungen anzupassen, um die Nahrungsbestandteile effizient für Wachstum und Reproduktion nutzen zu können. Zahlreiche Studien haben sich mit dem Fraßverhalten calanoider Copepoden, ihrer Reproduktion sowie ihren physiologischen Reaktionen auf sich ändernde Nahrungsbedingungen beschäftigt. Jedoch existieren bislang keine Studien, die sich gleichzeitig sowohl mit den physiologischen Anpassungen mariner Copepoden an sich verändernde Nahrungsbedingungen als auch den Auswirkungen dieser Veränderungen auf die Reproduktion der Copepoden befassen. Inter-disziplinäre Untersuchungen über das Fraßverhalten calanoider Copepoden, ihren physiologischen Reaktionen auf sich verändernde Nahrungsbedingungen, wie z.B. Verdauungsaktivitäten, metabolische Aktivitäten und ihr Reproduktionsverhalten sind von großer Bedeutung, um den Einfluss der Nahrung auf das Wachstum und Reproduktion von Copepoden besser verstehen zu können. Das Projekt "Trophische Interaktionen in pelagischen Ökosystemen: Die Rolle des Zooplanktons" des Alfred-Wegener-Institutes für Polar- und Meeresforschung, Bremerhaven, in Zusammenarbeit mit der Arbeitsgruppe Marine Zoologie an der Universität Bremen, untersucht daher übergreifend sowohl das Fraßverhalten als auch die physiologische Anpassung und Reproduktion von Copepoden an sich verändernde Umweltbedingungen. Ziel des Projektes ist es, ein genaues Bild der Prozesse zu erhalten, welche die Populationsdynamik von Copepoden beeinflussen. Die vorliegende Ar-beit, als Teil des Projektes, beschäftigt sich mit der physiologischen Anpassung verschiedener calanoider Copepodenarten an sich verändernde Nahrungsbedingungen. Dabei wird der Einfluss der physiologischen Anpassung auf die Lebensstrategien, metabolische Anforderungen und biochemische Eigenschaften der Copepoden, wie z. B. Lipid- und Proteingehalte, untersucht. Im Rahmen dieser Arbeit wurden Copepodenarten mit unterschiedlichen Lebensstrategien aus der südlichen Nordsee (Temora longicornis, Acartia clausi und Centropages typicus) und des St. Lorenz Ästuars (Ostküste Kanada; Calanus finmarchicus und Metridia longa) während mehreren Phytoplanktonblüten 2005 und 2006 analysiert. In dieser Arbeit wird ein umfassender Datensatz über verdauungsspezifische und metabolische Reaktionen sowie Reproduktionsverhalten der Copepoden auf sich verändernde Nahrungsbedingungen vorgestellt und in Hinblick auf die verschiedenen Lebensstrategien der Copepoden diskutiert. Die veränderten Nahrungsbedingungen wurden anhand trophischer Fettsäuremarker ermittelt. Temora longicornis, Acartia clausi und Centropages typicus sind durch kurze Lebensdauer und mehrere Generationen im Jahr charakterisiert. Während der Phytoplanktonblüten im Frühjahr 2005 (T. longicornis, A. clausi) und im Herbst 2006 (C. typicus) waren diese Arten unterschiedlichen Nahrungsbedingungen ausgesetzt. Sie zeichneten sich durch niedrige Lipidiii
ZUSAMMENFASSUNG
gehalte (2-9% der Trockenmasse (TM)), hohe spezifische metabolische Aktivitäten und Verdauungspotenziale aus. Unter günstigen Nahrungsbedingungen wurden in erster Linie Proteine als Energiereserven akkumuliert, während Lipide in geringerem Maße gespeichert wurden. T. longicornis zeigte eine hohe physiologische Anpassungsfähigkeit an wechselnde Nahrungsbedingungen, indem verschiedene Nahrungskomponenten effizient verdaut wurden und erfolgreich für reproduktive Prozesse genutzt werden konnten. Zu Beginn des Frühjahrs war die Phytoplanktonabundanz gering und T. longicornis Weibchen ernährten sich omnivor. Zu diesem Zeitpunkt bestand ihre Nahrung hauptsächlich aus lipidarmen und wahrscheinlich kohlenhydratreichen Bacillariophyceen sowie aus heterotrophen Beuteorganismen wie Copepodeneiern und Nauplien. Im Verlauf der Phytoplanktonblüte veränderte sich die Nahrungszusammensetzung hin zu hauptsächlich autotrophen, lipidreichen Organismen. Diese Veränderung in der Nahrungszusammensetzung spiegelte sich auch in den Verdauungsaktivitäten der Copepoden wider. Zu Beginn der Zeitreihe waren die Aktivitäten der Verdauungsenzyme Proteinase und Amylase hoch, dies weist auf protein- und kohlenhydratreiche Nahrungspartikel hin. Die Veränderung im Nahrungsangebot bewirkte eine Abnahme der Enzymaktivitäten, insbesondere der Amylase. Dies lässt auf eine nahrungsinduzierte Veränderung im enzymatischen Muster der Copepoden schließen. Gleichzeitig erhöhte sich der Lipidgehalt der Weibchen, hauptsächlich induziert durch die Akkumulation spezifischer Fettsäuren für Bacillariophyceen und Dinoflagellaten, wie 16:1(n-7) und 18:4(n-3). Die hohe Anpassungsfähigkeit des Verdauungssystems von T. longicornis wurde parallel zur Zeitreihe in mehreren Versuchen bestätigt. T. longicornis Weibchen reagierten innerhalb von 24 h auf veränderte Nahrungsbedingungen, indem sie ihre Verdauungsaktivität anpassten. In Abwesenheit von Nahrung erfolgte eine starke Reduzierung der Verdauungsaktivität. Weibchen, die mit Nahrungsüberschuss inkubiert wurden, zeigten sowohl eine Zu- als auch eine Abnahme ihrer Verdauungsaktivität, abhängig von ihrer Aktivität im Freiland. Des weiteren konnte eine nahrungsinduzierte Sekretion spezifischer Lipasen beobachtet werden, was das hohe Anpassungspotenzial von T. longicornis an neue Nahrungsbedingungen unterstreicht. Dieses hohe Anpassungspotenzial resultierte in einem deutlichen Anstieg nahrungsspezifischer Fettsäuren in den Lipiden der Copepoden innerhalb von drei Tagen. Des weiteren stieg die Eiproduktionsrate in den gefütterten Weibchen innerhalb von 24 h deutlich an. Dies zeigt, dass die Nahrung schnell und erfolgreich für die Produktion von Eiern genutzt werden konnte. Calanus finmarchicus und Metridia longa sind an die Phytoplanktonblüten und saisonalen Bedingungen der höheren Breitengrade angepasst. Ihre Generationszeiten liegen zwischen mehreren Monaten und mehr als einem Jahr. C. finmarchicus und M. longa waren im Jahr 2006 im St. Lorenz Ästuar einem schwankendem Nahrungsangebot ausgesetzt. Beide Arten waren durch höhere Lipidgehalte (7-30% der TM), geringere spezifische metabolische Aktivitäten und Verdauungspotenziale als die Copepoden aus der Nordsee gekennzeichnet. Proteine spielten als Energiereserve in C. finmarchicus und M. longa eine untergeordnete Rolle. iv
ZUSAMMENFASSUNG
Die Verdauungsaktivitäten waren bereits vor der Phytoplanktonblüte, welche von Bacillariophyceen und Dinoflagellaten dominiert war, erhöht. Während der Blüte wurden keine Veränderungen in den Verdauungsaktivitäten beider Arten auf das veränderte Nahrungs-angebot nachgewiesen. C. finmarchicus ernährte sich hauptsächlich von Bacillariophyceen und Dinoflagellaten, während M. longa zusätzlich Calanus Eier oder Nauplien als Nahrung nutzte. Dies wurde anhand trophischer Fettsäurebiomarker gezeigt. Auch unter extremen experimentellen Bedingungen, d.h. Nahrungsüberschuss und -limitierung, wurde bei C. finmarchicus keine Veränderungen in der Verdauungsaktivität nachgewiesen. Die Ergebnisse deuten darauf hin, dass diese beiden Copepodenarten nicht auf kurzzeitige Veränderungen des Nahrungsangebots reagieren. Vielmehr scheinen deutliche und langanhaltende Veränderungen Auswirkungen auf die Verdauungsaktivität der Arten zu haben. Die Ergebnisse der vorliegenden Arbeit zeigen, dass das adaptive Verdauungspotenzial der Copepoden Auswirkungen auf deren Lebensstrategien hat. Ein höheres Potenzial zur Anpassung wurde bei Copepoden mit geringen Energiereserven, kurzer Lebensdauer und kurzen Generationszeiten festgestellt. Copepodenarten mit einer Lebensdauer von mehr als einem Jahr, die starken saisonalen Schwankungen im Nahrungsangebot ausgesetzt sind, zeichnen sich durch ein geringeres Anpassungspotenzial aus. Der integrative Ansatz der vorliegenden Arbeit führt zu einem tieferen Verständnis der physiologischen Anpassungen von Copepoden an sich verändernde Nahrungsbedingungen und deren Auswirkungen auf die Lebensstrategien. Zudem wurde gezeigt, dass die Veränderungen der Aktivitäten der Verdauungsenzyme abhängig von früheren Fraßereignissen und den metabolischen Anforderungen der untersuchten Arten ist.
v
OUTLINE OF PUBLICATIONS
OUTLINE OF PUBLICATIONS The following overview outlines the five publications included in this thesis and my contributions to the respective publications.
PUBLICATION I Kreibich T, Gentsch E, Hansen B, Hagen W, Niehoff B Physiological responses of Temora longicornis females (Crustacea, Copepoda) to changing nutritional conditions in the North Sea I was involved in the field work and performed the methodological development for the measurements of digestive enzyme activities. The measurements for digestive and metabolic enzyme activities, protein contents, dry mass, lipid contents and fatty acid composition were done by myself. I wrote the manuscript with scientific and editorial advice of B Niehoff and W Hagen. E Gentsch and B Hansen were involved in the field work, additionally E Gentsch provided important ideas to the manuscript and B Hansen contributed the egg production rate data.
The manuscript was submitted to Marine Biology.
PUBLICATION II Kreibich T, Saborowski R, Hagen W, Niehoff B Short term variation of nutritive and metabolic parameters in Temora longicornis females (Crustacea, Copepoda) as a response to diet shift and starvation
I developed the idea of this experimental work and performed the experiments. The analytical work like measurements of enzyme activities and fatty acid compositions was done by myself. R Saborowski was involved in writing the manuscript and provided important scientific ideas. W Hagen was involved by giving scientific and editorial advice. B Niehoff was involved in writing the manuscript and provided important ideas and scientific advice during the experiment.
The manuscript was submitted to Helgoland Marine Research.
vi
OUTLINE OF PUBLICATIONS
PUBLICATION III Kreibich T, Saborowski R, Hagen W, Niehoff B Influence of physiological conditions and feeding history on digestion, metabolism and fatty acid composition of Temora longicornis females (Crustacea, Copepoda) under different nutritional conditions
I developed the idea of this experimental work together with B Niehoff. The experiments were performed by myself, as well as the analytical work like measurements of enzyme activities and fatty acid compositions. R Saborowski was involved in developing the method for the characterisation of lipase/esterase patterns by SDS-PAGE and gave scientific advice for writing the manuscript. W Hagen and B Niehoff were involved by giving scientific and editorial advice. The manuscript will be submitted to Journal of Experimental and Marine Ecology.
PUBLICATION IV Kreibich T, Plourde S, Joly P, Starr M, Auel H, Niehoff B Feeding strategies, digestive activities and reproduction of Calanus finmarchicus and Metridia longa in the lower St. Lawrence estuary, Québec, Canada
I was involved in the field work and performed the measurements for digestive and metabolic enzyme activities, protein contents, dry mass, lipid contents and fatty acid compositions. The stable isotope data were evaluated by myself. S Plourde and P Joly were involved in the field work and provided the egg production rate data. M Starr provided the data for chlorophyll a and the phytoplankton composition. H Auel gave scientific advice for writing the manuscript. B Niehoff was involved in the field work and in writing the manuscript by giving scientific and editorial advice.
The manuscript will be submitted to Polar Biology.
vii
OUTLINE OF PUBLICATIONS
PUBLICATION V Gentsch E, Kreibich T, Hansen B, Hagen W, Niehoff B Dietary shifts in the North Sea copepod Temora longicornis in spring 2005 – evidence from stable isotope signatures, fatty acid biomarkers and feeding experiments
I shared the field work with the first author and B Hansen. I performed all fatty acid analyses and evaluated the data, and provided important ideas to the manuscript.
The manuscript was submitted to Marine Ecology Progress Series.
viii
INTRODUCTION
1 INTRODUCTION 1.1 SCIENTIFIC BACKGROUND Calanoid copepods are abundant in the marine plankton communities worldwide and they constitute a major component of the pelagic food web (Longhurst 1985). Herbivorous copepods are often the most important primary consumers and link phytoplankton and higher trophic levels (e.g. Rae and Rees 1947, Hickel 1975, Fransz et al. 1991). On the other hand, they are a major food source for e.g. fish larvae (Last 1978, Støttrup et al. 1986), krill (Atkinson and Snÿder 1997, Lass et al. 2001) and whales (Beardsley et al. 1996), and the copepod faecal pellets, sinking from surface to deep-water layers, are fed upon by mesopelagic organisms (e.g. Fransz et al. 1991, Longhurst 1991, Ducklow et al. 2001).
Due to their key role in pelagic ecosystems calanoid copepods were subject to numerous studies during the last century, focusing on e.g. morphology, taxonomy, phylogeny, distribution, life strategies, reproduction or feeding behaviour (extended reviews by e.g. Huys and Boxshall 1991, Mauchline 1998). Copepods, like all heterotrophic organisms, rely on feeding on other organisms in order to cover their energetic demands for growth and to reproduce successfully. Studies on feeding physiology of copepods are therefore of major importance for the understanding of the adaptation to changing nutritional conditions and the functional responses to these changes. As a consequence, this information will expand our knowledge on trophic interactions which lead to a better understanding of ecosystem functioning in the marine environment. Numerous studies focused on the influence of changing nutritional conditions on functional responses, such as egg production rate and hatching success of copepods. The results indicated strong relations to food diversity (Roman 1984, Støttrup and Jensen 1990), quality (Jones et al. 2002) and quantity (e.g. Checkley 1980). Food quality can be indicated by the content of different dietary components, such as essential fatty acids (Støttrup and Jensen 1990, Jónasdóttir 1994, Klein Breteler et al. 1999, Broglio et al. 2003), sterols (Ederington et al. 1995), proteins (Kleppel and Hazzard 2000) or essential amino acids (Kleppel et al. 1998). However, food quality is also defined by the digestibility of the diet. Digestion links the two processes of ingestion and assimilation, and is therefore a physiological key process. The energy required for different physiological processes is mainly provided through the hydrolisation of lipids and proteins, and these processes are catalysed by specific enzymes. Therefore, studies focused on the influence of external factors on digestive enzyme activities of copepods (e.g. Mayzaud and Conover 1975, Hirche 1981, Hassett and Landry 1983). However, those studies often led to controversial conclusions. Some authors suggested that digestive activity in copepods increases with increasing food supply, whereas others showed that the digestive activity decreases under surplus food or remains unaltered (e.g. Mayzaud and Conover 1975, 1
INTRODUCTION
Hirche 1981, Hassett and Landry 1983, Head et al. 1984, Harris et al. 1986). Hassett and Landry (1983) discuss that the activity may differ with the life cycle strategies of copepods, however, data are lacking. Moreover, most studies disregarded metabolic requirements and feeding histories of copepods, although it is possible that digestive activity is controlled by feedback mechanisms, which operate to meet the metabolic requirements of the individual (Roche-Mayzaud et al. 1991, Mayzaud et al. 1992). Hence, many questions still remain unanswered concerning physiological adaptive processes in copepods and the factors influencing these processes.
Combining detailed knowledge of the dietary composition (input), the digestive responses to the diet and the fuelling of metabolic activity, growth and reproduction (output), is the next essential step to better understand how diet affects physiological responses, 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 (AWI), Bremerhaven, in cooperation with the Marine Zoology at the University Bremen, aimed therefore to investigate feeding behaviour, physiological and functional responses of copepods to changing environmental conditions in detail in order to obtain a more precise picture of processes influencing population dynamics. The present thesis, embedded within this project, elucidates the physiological adaptations of calanoid copepod species to changing nutritional conditions, comparing species with different life strategies from the German Bight, southern North Sea, and the lower St. Lawrence estuary, eastern Canada.
1.2 LIFE STRATEGIES OF CALANOID COPEPODS IN DIFFERENT ENVIRONMENTS In the following an overview is given on calanoid copepod life cycle strategies in different environments, focussing on the North Sea and the lower St. Lawrence estuary and the respective species, which were studied within this thesis.
Marine copepods inhabit areas with different environmental conditions, from tropical and temperate regions to high latitudes. These areas cover a wide range of abiotic and biotic conditions. E.g. salinity ranges between >40 in hypersaline environments and