On the copepod response to iron induced phytoplankton blooms in the Southern Ocean [Elektronische Ressource] / vorgelegt von Sören Krägefsky

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• Publié par: universitat_bremen
• Publié le: 01 janvier 2008
• Nombre de lectures: 46
• Langue: Deutsch
• Poids de l'ouvrage: 4 Mo

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Alfred-Wegener-Institut für Polar- und Meeresforschung Bremerhaven
On the copepod response to iron-induced
phytoplankton blooms in the Southern Ocean
Dissertation
zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
-Dr. rer. nat.-
Fachbereich 2 (Biologie/Chemie)
Universität Bremen
vorgelegt von
Sören Krägefsky
November 20081. Gutachter: Prof. Dr. Ulrich Bathmann
Alfred-Wegener-Institut für Polar- und Meeresforschung, Bremerhaven u.
Universität Bremen
2. Gutachter: Prof. Dr. Dieter Wolf-Gladrow
Alfred-Wegener-Institut für Polar- und Meeresforschung, Bremerhaven u.
Universität BremenDanksagung
Ich danke Prof. Dr. Ulrich Bathmann, daß er mir die Möglichkeit zur Promotion
gab und mich sehr unterstützt hat, sowie Prof. Dr. Dieter Wolf-Gladrow, dem
zweiten Betreuer dieser Promotion.
Danken möchte ich Freunden und Kollegen für die Bereitstellung von Daten, für
Diskussionen und die viele kleinen und großen Hilfen. Besonders nennen möchte ich:
Phillip Assmy, Boris Ciesewski, Gaby David, Corinna Dubischar, Lars Friedrichs,
Bettina Fach, Lena v. Harbou, Christian Hamm, Joachim Henjes, Sandra Jansen,
Christine Klaas, Karoline Klinck, Gerald Langer, Harry Leach, Barbara Niehoff,
Helga Schwarz und Volker Strass. Prof. Dr. Victor Smetacek danke ich für die
begeisternden Diskussionen während meines Studiums der Meeresbiologie. Mein
Dank gilt den Kapitänen und Mannschaften aller Polarsternexpeditionen.
Danken möchte ich meiner Familie für ihre Unterstützung und Geduld..Contents
1 Summary 1
2 Zusammenfassung 5
3 General Introduction 9
3.1 Shortintroductiontocopepods......................... 9
3.2 TheSouthernOcean............................... 12
3.3 Outlineofthisthesis .............................. 16
4 List of Manuscripts 19
4.1 ListofManuscripts................................ 19
4.2 Contributionsoftheauthors........................... 20
5 Manuscript 1 21
6 Manuscript 2 63
7 Manuscript 3 105
8 Manuscript 4 149
9 Manuscript 5 157
10 Manuscript 6 185
11 General Discussion 215
11.1 Vertical migration behaviour as a food finding strategy . . . . . . . . . . . . 215
11.2 Effects of copepod grazing on the prey community and on carbon and nu-
trientfluxes....................................220
1.3Perspectivesforfutureresearch.........................25
..1 Summary
This thesis investigates the response of copepods to increased phytoplankton abun-
dance during two mesoscale iron-fertilisation experiments in the Southern Ocean
(EisenEx in austral spring 2001 and EIFEX in austral autumn 2004), focussing on
the copepods vertical migration behaviour.
The horizontal and vertical distribution of zooplankton was surveyed by acoustic
methods (multi-frequency echosounder Simrad EK60), which generated data with
high spatial and temporal resolution. In addition to the acoustic measurements and
to identify acoustic scatterer, net samples were taken and analysed. Mathematical
models which take into account the copepod behaviour and the prevailing current
regimes were developed and used to interpret the population dynamics during the
experiments. To assess trophic interaction within the pelagic ecosystem, ingestion
rates of the dominant copepod species were measured during both experiments.
During EIFEX, faecal pellet production, egg production and gut content of copepods
were investigated in addition. The combination of these diverse methods provided
new insights in the behaviour of copepods.
Diatom dominated phytoplankton blooms developed during both experiments
within the fertilised patches. In response to the increased phytoplankton concentra-
tion, the distribution patterns of the copepods changed as follows: During EisenEx,
the abundance of Oithona similis, a small, non diurnal migrating copepod species,
increased strongly within the food-rich upper layer in the central area of the fer-
tilised patch. The corresponding mathematical simulation study showed that most
of this increase can be explained by the vertical migration behaviour of O. similis:
Within the fertilised area during EisenEx, O. similis shifted its vertical distribu-
tion towards the phytoplankton enriched surface layer, while the population showed
a deeper vertical distribution in the non-fertilised areas characterized by low food
concentration. The deep-dwelling behaviour of O. similis enhances the drift out of
unproductive regions and increases the chance of getting advected from a low to a
high food environment. In contrast, during EIFEX, no significant change in vertical1 Summary 2
migration of O. similis was observed. Here, in contrast to EisenEx, diatoms with
long spines (Chaetoceros spp.) and heavily silicified diatoms (Fragilariopsis kergue-
lensis) dominated within the phytoplankton community. Previous studies indicate
that small copepods such as O. similis have difficulties to feed on such diatoms.
Diel vertical migration was observed for larger copepods, such as Ctenocalanus
citer, Calanus simillimus, Rhincalanus gigas and Metridia lucens. During both
experiments, EisenEx and EIFEX, the acoustic survey showed that diel migrating
copepods and maybe other zooplankton decreased their daytime residence inside
the bloom areas. Shallow migrating species reduced their amplitude of diel vertical
migration (DVM) by tens and deep migrating species by up to more than 150 metres.
Similar to the effects observed in Oithona similis, the simulation study showed that
the changes in migration behaviour of diel migrating species led to a retention of
individuals inside phytoplankton-rich areas, and can largely explain the observed
increase in abundance in the fertilised area, especially during EIFEX.
Further simulation results showed, that improvements in food acquisition, which
could be achieved by changes in migratory behaviour, can be very substantial. The
nutritional benefit gained inside the phytoplankton bloom was also reflected by data
from egg production experiments. The percentage of adult females of Rhincalanus
gigas producing eggs increased during the development of the bloom from 0 to
90%. Accordingly, faecal pellet production increased significantly during bloom
development. The observed migration behaviour can be considered as a food finding
strategy to cope with the strongly seasonal and horizontal patchy food supply.
Zooplankton can strongly influence fluxes of carbon and nutrients after iron fer-
tilisation in the Southern Ocean. The mechanisms mentioned above caused both an
accumulation of copepods and increased feeding pressure during the experiments.
However, copepod abundances did not increase suddenly, but gradually. Accord-
ingly, feeding pressure increased only gradually and allowed bloom formation of the
phytoplankton, which encountered substantial feeding pressure, but could, due to
higher growth rates, increase its biomass. Gut content analyses showed that strongly
armoured diatom frustules (Fragilariopsis kerguelensis) or species having long spines
(Chaetoceros spp.), were readily ingested by larger copepods during EIFEX, in con-
trast, O. similis seemed not to be able to feed on these diatoms. The faecal pellet
concentration within the surface layer was not substantially higher than daily pro-
duction of faecal pellets, thus, no significant accumulation of faecal pellets occurred.
This observation suggests in the context with further field data that organic faecal1 Summary 3
matter was largely degraded and respired in the surface layer.
The results of the present thesis show, that a combination of classical methods
of zooplankton research with high-resolution acoustic methods and mathematical
simulations can contribute significantly to a better understanding of pelagic ecosys-
tems..