Selected interactions between phytoplankton, zooplankton and the microbial food web [Elektronische Ressource] : microcosm experiments in marine and limnic habitats / by Alexis Katechakis

Alexis Katechakis

Sujets

• Biologie

Informations

• Publié par: ludwig-maximilians-universitat_munchen
• Publié le: 01 janvier 2005
• Nombre de lectures: 14
• Langue: English
• Poids de l'ouvrage: 10 Mo

Extrait

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






Dissertation
zur Erlangung des Doktorgrades der Naturwissenschaften
Dr. rer. nat.
der Fakultät für Biologie der Ludwig-Maximilians-Universität München



by
Alexis Katechakis





München 2005



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






Dissertation
zur Erlangung des Doktorgrades der Naturwissenschaften
Dr. rer. nat.
der Fakultät für Biologie der Ludwig-Maximilians-Universität München



by
Alexis Katechakis





Eingereicht am: 24. November 2005
1. Gutachter: PD Dr. Herwig Stibor
2. Gutachter: Prof. Dr. Sebastian Diehl
Tag der mündlichen Prüfung: 17. März 2006







A L'ANNA
CONTENTS

Summary / Key words …………………………………………………………………………………1


1. Introduction ……….……….………………………………………………………………………4
1.1. Thesis objectives, approach and outline……………………………………………………….5
1.2. The pelagic food web ………………………………………………………………………….7
1.3. Bottom-up vs. top-down control ……………………9
1.4. Natural vs. cultural enrichment ……………………………………………………10

2. Study backgrounds ………………………………………………………………………………11
2.1. Backgrounds Study A – Copepods, cladocerans, doliolids…………………………………..11
2.2. Backgrounds Study B – Mixotrophs …………………………………………………………13

3. Paper summaries …………………………………………………………………………………15
A1 Feeding selectivities and food niche separation of Acartia clausi, Penilia avirostris
(Crustacea) and Doliolum denticulatum (Thaliacea) in Blanes Bay (Catalan Sea,
NW Mediterranean). Journal of Plankton Research (2004) 26:589–603 ….………………15

A2 Changes in the phytoplankton community and microbial food web of Blanes Bay
(Catalan Sea, NW Mediterranean) under prolonged grazing pressure by doliolids
(Tunicata), cladocerans or copepods (Crustacea). Marine Ecology Progress Series
(2002) 234:55–69 ………………………………………….…………………………………17

A3 Feeding selectivities of the marine cladocerans Penilia avirostris, Podon
intermedius and Evadne nordmanni. Marine Biology (2004) 145:529–539l ………………..19

B1 Mixotrophic vs. obligately autotrophic algae as food for zooplankton – the
light:nutrient hypothesis might not hold for mixotrophs. Limnology and
Oceanography (2005) 50:1290–1299 ………………………….……………………………21

B2 The mixotroph Ochromonas tuberculata may invade and suppress specialist phago-
and phototroph plankton communities depending on nutrient conditions. Oecologia
(2005), submitted ………………………………………………………………………….…24

4. Conclusions …………………………………………………………………………………….…27

5. Research outlook ……………….……………….………………………………………………28

6. References …………………………….……………………….…………………………………31


Attachments
● Paper reprints
● Personal notes
Curriculum vitae – Publication list / Grants – Acknowledgements – Declaration



1 · Summary
SUMMARY

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 Summary · 2
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 pr