Trophic interactions in the pelagic Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Susanna Knotz Kiel 2006 Referent: Professor Dr. U. Sommer Koreferent: PD Dr. M. Boersma Tag der mündlichen Prüfung: 6. Juli 2006 Zum Druck genehmigt: 4. Januar 2007 der Dekan: gez. J. Grotemeyer 2 Content Chapter 1: Community structure of marine pelagic food webs and trophic interactions................................................................................................................ 5 1.1. Introductory remarks................................................................................................... 5 1.2. The North Sea plankton communities off Helgoland ...............................................6 Phytoplankton................................................................................................................................ 6 Zooplankton................................................................................................................................... 7 Ichthyoplankton.............................................................................................................................. 8 Microbial food web......................................................................................................................... 9 1.3. Factors influencing plankton communities.............................................................10 Hydrography of the German Bight (Otto et al. 1990).................................................................... 10 Irradiance, temperature and salinity............................................................................................. 11 Nutrients and nutrient limitation – food quantity and quality......................................................... 11 Algal toxins and chemical defence............................................................................................... 14 Anthropogenic impacts – excess nutrients, toxins, fishing, climate change................................. 15 1.4. Internal factors influencing zooplankton communities ........................................ 16 1.5. Overview of the following chapters..........................................................................17 Chapter 2: Microassays for a set of enzymes in individual small marine copepods ................................................................................................................. 19 2.1. Introduction................................................................................................................ 19 2.2. Materials and methods.............................................................................................. 20 2.3. Results and discussion............................................................................................. 24 Chapter 3: Sublethal physiological effects of food limitation in copepods ...... 28 3.1. Introduction................................................................................................................ 28 3.2. Material and Methods................................................................................................ 30 Laboratory experiments............................................................................................................... 30 Digestive enzyme activities.......................................................................................................... 30 Water soluble protein content of individuals................................................................................. 30 Protease zymograms................................................................................................................... 31 Statistics ...................................................................................................................................... 32 3.3. Results........................................................................................................................ 32 Mortality and survival after starvation........................................................................................... 32 Hunger experiments – digestive enzyme activities and protein contents..................................... 33 Hunger experiments – protease zymograms................................................................................ 34 Species comparison – protease zymograms ............................................................................... 35 3.4. Discussion.................................................................................................................. 36 Time scales of reactions.............................................................................................................. 37 Role of enzymes.......................................................................................................................... 39 Zymograms.................................................................................................................................. 40 Species specificity........................................................................................................................ 41 Sexual differences........................................................................................................................ 41 Conclusion .................................................................................................................................. 41 Chapter 4: Influence of food quality and quantity on egg production of Acartia clausi and Temora longicornis during a spring phytoplankton bloom.............. 43 4.1. Introduction................................................................................................................ 43 4.2. Material and Methods................................................................................................ 45 Sampling – animals and surface seawater................................................................................... 45 3 Abiotic environmental data........................................................................................................... 46 Phytoplankton determination........................................................................................................ 46 Dry mass, C, N and P determination............................................................................................ 46 Copepod egg production.............................................................................................................. 47 Copepod grazing selectivity......................................................................................................... 48 Statistics....................................................................................................................................... 50 4.3. Results........................................................................................................................ 50 Abiotic factors............................................................................................................................... 50 Prey community structure influencing egg production.................................................................. 51 Stoichiometric factors influencing egg production........................................................................ 57 Grazing behaviour........................................................................................................................ 58 4.4. Discussion.................................................................................................................. 64 Conclusion................................................................................................................................... 68 Chapter 5: Stable isotopes as predictors of copepod reproductive success....69 5.1. Introduction ............................................................................................................... 69 5.2. Material and Methods................................................................................................ 71 Seasonal Study............................................................................................................................ 71 Sampling – animals and surface seawater................................................................................... 71 Copepod egg production ............................................................................................................. 72 Stable isotope determination........................................................................................................ 72 Protein content and enzyme activities.......................................................................................... 72 Spatial study................................................................................................................................. 73 C, N and P determination............................................................................................................. 74 5.3. Results........................................................................................................................ 74 Seasonal Study............................................................................................................................ 74 Protein and digestive enzyme activities....................................................................................... 81 Spatial study................................................................................................................................