Southern Ocean Experiment (SOFeX): Primary/Bacterial Production and Taxon-Specific Growth and Photosynthetic Responses

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This is part of a project - the Southern Ocean Iron Experiment (SOFeX) - to seed a small area within the Antarctic Circumpolar Current with iron, to follow this patch of water by measuring the distribution of an inert tracer (SF6) added with the iron, and to assess the chemical and biological responses to this addition. Recent observations have indicated that iron plays a controlling role in the production and export of organic matter in all regions of the Southern Ocean. Furthermore, because the Southern Ocean contains the largest pool of inorganic macronutrients of the world ocean, it has the greatest potential to influence the concentration of atmospheric carbon dioxide by acting as a sink for C02- In order to predict the impact of natural variations in iron supply on carbon partitioning, it is imperative that we understand the complex biological interactions that occur upon iron enrichment. This investigation will measure primary productivity within and outside the iron-enriched patch of water to quantify the direct response of iron to the in situ phytoplankton assemblage on time scales of days. Productivity of three size fractions will be measured: 5gm, 5 but 20 gm, and 20 ~tm. The quantitative taxonomic composition of the three size fractions will be directly determined by high performance liquid chromatography (HPLC). The photosynthesis versus irradiance (P vs E) responses of the same fractions also will be measured for two reasons. First, the P vs E response is a sensitive measure of iron limitation. Secondly, the data will be used to investigate iron/irradiance interactions because it has been hypothesized that phytoplankton adapted to low light require more iron than do those adapted to higher iffadiances (a condition especially important in the Southern Ocean). The taxon-specific growth rates of phytoplankton will also be measured directly using a pigment-labeling technique. Finally, the project will use a new Fast Repetition Rate (FRR) fluorescence technique that will provide P vs E responses in near real-time, allowing the investigators to assess the spatial (both horizontal and vertical) responses of quantum efficiency and bulk photosynthesis along gradients across the enriched patch. Resolving the responses of diatoms relative to other taxa is critical for accurate modeling of the phytoplankton response to varying iron supply because diatom blooms play a pivotal role in the export of carbon to the deep sea.