Collaborative Research: Southern Ocean Iron Experiment (SOFeX): Primary/Bacterial Production and Taxon-Specific Growth and Photosynthetic Response

College Of William & Mary Virginia Institute Of Marine Science, Gloucester Point VA

Investigators

Abstract

This project is a component of Southern Ocean Iron Experiment (SOFeX). SOFeX is an effort 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 CO2. 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 component will measure primary productivity within an outside the iron-enriched path 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: < 5um, > 5 but < 20 um, and > 20 um. 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 irradiances (a condition especially important in the Southern Ocean). The bacterial biomass and production both within and outside the patch will be measured to assess the heterotrophic response to iron addition. Experiments will assess whether the bacterial response is direct (a result of inorganic iron limitation removal) or indirect (a result of organic substrate addition due to the stimulation of phytoplankton photosynthesis and DOC production, or via increased grazing on the increased phytoplankton biomass generated by iron additions). The taxon-specific growth rates of phytoplankton will also be measured directly using a pigment-labeling technique. 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. The results of the SOFeX project will provide insights into the response of the biogeochemistry and ecology of the Southern Ocean to iron enrichment and produce a new understanding of the functioning of this poorly known, but critical oceanic region.