Research Objectives:
The Shackleton Fracture Zone (SFZ) in the Drake Passage marks a boundary between low- and high-phytoplankton waters. West of the passage, waters have very low concentrations of surface chlorophyll, and east of the SFZ, mesoscale eddy kinetic energy and chlorophyll are higher than they are west of it. Data from a 10-year survey confirm the existence of a strong hydrographic and chlorophyll gradient in the region. We hypothesize that bathymetry, including the 2,000-meter-deep SFZ, influences mesoscale circulation and transport of iron, leading to the differences in phytoplankton patterns.
To test this hypothesis, we will examine phytoplankton and bacterial physiological states (including responses to iron enrichment) and the structure of plankton communities from virus to zooplankton; the concentration and distribution of iron, manganese, and aluminum; and mesoscale flow patterns near the SFZ. We will examine relationships between iron concentrations and phytoplankton in the context of the mesoscale transport of trace nutrients to determine how much of the variability in biomass can be attributed to iron supply, as well as the most important sources of iron east of the Drake Passage. Our goal is to better understand how plankton productivity and community structure in the Southern Ocean are affected by bathymetry, mesoscale circulation, and nutrient distributions.
We will perform rapid surface surveys of chemical, plankton, and hydrographic properties, complemented by a mesoscale station grid for vertical profiles, water sampling, and bottle incubation enrichment experiments. Manganese and aluminum distributions will help distinguish aeolian, continental shelf, and upwelling sources of iron. We will monitor the physiological state of the phytoplankton by active fluorescence methods sensitive to iron limitation. Concentrations of pigment, carbon, and nitrogen will be obtained by analysis of filtered samples, cell size distributions by flow cytometry, and species identification by microscopy. We will measure primary production and photosynthesis parameters (absorption, quantum yields, variable fluorescence) with depth profiles, surface surveys, and bulk samples from enrichment experiments. We will also determine the abundance of viruses and bacteria and ascertain whether bacterial production is limited by iron or organic carbon sources.
We aim to improve scientific understanding of processes controlling iron distribution and the response of plankton communities in the Southern Ocean. Moreover, we will have an undergraduate and teacher outreach component and plan to create an Web site and K–12 curricular modules.
