Mechanisms determining species succession and dominance during an iron-induced phytoplankton bloom in the Southern Ocean (LOHAFEX)

• Main Author: Schulz, Isabelle Katharina
• Other Authors: Smetacek, Victor, Assmy, Philipp, Bischof, Kai
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Universität Bremen 2013
• Subjects: iron-fertilization; LOHAFEX; Southern Ocean; plankton community; flagellates; diatoms; vertical flux; carbon export; biological carbon pump; grazing; 570; 570 Life sciences; biology; ddc:570; Copepods
• Online Access: https://media.suub.uni-bremen.de/handle/elib/581; https://nbn-resolving.org/urn:nbn:de:gbv:46-00103521-10

To improve the understanding of population dynamics of organisms, it is necessary to examine organisms under natural conditions. Only at such conditions, knowledge about their in situ use of resources, predator-prey interactions and loss rates can be obtained. It is possible to examine and understand these processes with controlled ocean iron fertilization experiments, which stimulate the growth of unicellular algae. This thesis provides a comprehensive description of the reactions and vertical distribution of the main components of the pelagic ecosystem and highlights the food-web interactions of individual organisms, which characterize this habitat. The response of the plankton community to iron addition was successfully observed during the Indo-German iron fertilisation experiment LOHAFEX (LOHA is hindi and means iron, FEX stands for fertilisation experiment) carried out in the Southern Ocean from January to March in 2009 lasting for 38 days. The iron-induced bloom was achieved in the closed core of a mesoscale eddy. The aim of the experiment was to study the growth and demise of the phytoplankton bloom and to examine whether the biomass is retained in the surface layers through recycling processes or whether biomass sinks out to the deep ocean. The fertilized patch was characterized by low silicic acid concentrations, which is an essential nutrient for diatoms, hence a flagellate-bloom developed with cells < 20 µm in size. The bloom remained stable over the course of the experiment. This was verified by microscopic analysis and molecular methods. The main reason for the lack of large scale biomass increase, was the strong grazing pressure by the large copepod population, consisting primarily of Calanus simillimus and Oithona similis. Incubation experiments proved that copepods increased their grazing rates and fecal pellet production within the patch. Neutrally buoyant PELAGRA traps were deployed to quantify the export fluxes. These contained a few diatoms and fecal pellets and were dominated by ...