Fig. 1. Rob Sherrell and postdoc Jess Fitzsimmons aboard the R/V LM Gould, preparing to deploy the trace metal CTD/Rosette for another successful sampling cast in the Southern Ocean along the western Antarctic Peninsula
The Southern Ocean surrounding Antarctica is the largest region of the ocean where the biological productivity is limited by the supply of the micronutrient iron (Fe). Along the western side of the Antarctic Peninsula (WAP), however, satellite observations show considerably higher densities of phytoplankton cells, suggesting that the proximity to the continent provides considerable Fe to the surface waters in this shelf region, helping to support a vibrant community of seals, penguins and whales, and removing CO2 from the atmosphere at the same time. Rob Sherrell (Professor, DMCS and EPS) has just completed his first NSF-funded cruise, joining the Long-Term Ecosystem Research (LTER) project and DMCS Professor Oscar Schofield on the R/V LM Gould, to make comprehensive observations of the distribution and dynamics of Fe and other micro-nutrient trace metals in the WAP shelf region. He was assisted by DMCS Postdoc Jessica Fitzsimmons, Rutgers Env. Sci. PhD student Philip Sontag, and MIT Postdoc Cheryl Zurbrick in this exhaustive and exhausting sampling effort.
Fig. 2. Cheryl Zurbrick (left), a postdoc at MIT, with Jess, heading off the ship in a Zodiac. Cheryl was a critical member of the trace metal team.
Sampling trace metals without contamination requires specialized equipment, and the group was initially stunned by the loss of the US Antarctic Program’s trace metal CTD/rosette, when a cable unexpectedly broke, just weeks before the LTER cruise. Fortunately, a replacement was hobbled together, assembled last minute on board, and made to work, saving the project. The central questions being investigated are: How does bioavailable Fe get delivered from the continent to the shelf surface waters? Are the melting glaciers along the peninsula an important source? Is soluble Fe from the sediments carried with warm offshore water that is known to upwell in certain regions along the coast? Is the seasonal melting of floating sea ice a substantial Fe input? What is the effect of the rapid climate change in this area on the supply of Fe to phytoplankton? Results of Sherrell’s previous work in this region suggest that, surprisingly, large swaths of the northern peninsula region have low Fe concentrations in the surface water, penetrating very close to shore in places, suggesting that phytoplankton may be Fe-limited at least regionally even in the shelf waters close to the continent. The southern WAP region appears to have much higher Fe concentrations. The new project sampled 21 stations from surface to bottom, and will provide the first 3-D distribution map of Fe in both dissolved and particulate forms, along with other metals like manganese, copper, and zinc, for this important shelf region. Measurements of the isotopic composition of Fe and of the continentally-supplied companion element neodymium will allow Sherrell and his group to identify the sources and to trace the distribution of Fe. More than twenty shipboard incubation experiments were also carried out, in which Fe was added to bottles of seawater and biological response was monitored, as a means of quantifying the degree of Fe limitation in the extant assemblage of cells. Thanks to collaborative effort by the Rutgers oceanography group and their colleagues on board the Gould, this was an extremely successful expedition. Stay tuned for results in a later Newsletter!
Fig. 3. Rob and Jess sampling in the trace metal "bubble" clean room constructed within the ship's lab