The ocean plays a critical role in sequestering CO2 by exporting fixed carbon to the deep ocean through the biological pump. There is a pressing need to understand the systematics of carbon export in the Southern Ocean in the context of global warming because of the sensitivity of this region to climate change, already manifested as significant temperature increases. Numerous studies have indicated that Fe supply is a primary control on phytoplankton biomass and productivity in the Southern Ocean. The results from previous cruises in Feb-Mar 2004 and Jul-Aug 2006 have revealed the major natural Fe fertilization from Fe-rich shelf waters to the Fe-limited high nutrient low chlorophyll (HNLC) Antarctic Circumpolar Current Surface Water (ASW) in the southern Drake Passage, producing a series of phytoplankton blooms. Remaining questions include: How is natural Fe transported to the euphotic zone through small-meso-large scale horizontal-vertical transport and mixing in different HNLC ACC areas? How does plankton community structure evolve in response to a natural Fe addition, how does Fe speciation respond to biogeochemical processes, and how is Fe recycled to determine the longevity of phytoplankton blooms? How does the export of POC evolve as a function of upwelling-mixing, Fe addition-recycling and bacteria-plankton structure? This synthesis proposal will address these fundamental questions using a unique dataset combining multiyear physical, Fe and biogeochemical data collected between 2004 and 2006 from 2 NSF-funded Fe fertilization experiment cruises and 3 Antarctic Marine Living Resource (AMLR) cruises in the southern Drake Passage and southwestern Scotia Sea through collaboration with scientists in the AMLR program and US Southern Ocean GLOBEC projects. All investigators involved in this study are engaged in graduate and undergraduate instruction, and mentoring of postdoctoral researchers. Each P.I. will incorporate key elements of the proposed syntheses in our lectures, problem sets and group projects. The project includes support to convene a 4-5 day international workshop on natural Fe fertilization at Woods Hole Oceanographic Institution. The workshop will include scientists from United Kingdom, France and Germany who have conducted natural Fe fertilization experiments, and Korea and China who are planning to conduct natural Fe fertilization experiments. The participation of graduate students and postdoctoral scholars will be especially encouraged. The results will be published in a Deep-Sea Research II special issue.
The current modeling and synthesis project builds upon prior funding entitled "Collaborative Research: Plankton Community Structure and Iron Distribution in the Southern Drake Passage and Scotia Sea" with the former award 0444134. Earlier funding supported two major field campaigns, LMG0402 and NBP0606 as well as fundamental integration with several field seasons of the NOAA Antarctic Marine Living Resources (AMLR) program and also with separate funding to B. Greg Mitchell from NASA for development of ocean color satellite algorithms and models for chlorophyll, primary production and export production. The project was conceived, designed and implemented as a truly interdisciplinary project integrating the interaction of fluid dynamics with solid Earth bathymetry, transport and mixing, chemistry and the response of plankton communities. The hypotheses and objectives of our project are detailed below: Hypotheses and objectives Our previously funded field expeditions to the Southern Drake Passage (summer 2004; winter 2006) provided us with a wealth of data on this important region of natural Fe fertilization. Although our studies have revealed the basic connection between Fe supply and chlorophyll gradients in this area, questions remain pertaining to the mode of Fe transport, the importance of Fe recycling, and the efficiency of Fe-mediated carbon export flux. We now report on our interdisciplinary data synthesis coupled with analysis of numerical modeling that addressed the following hypotheses and objectives: Hypothesis 1: Horizontal advection delivers Fe-enriched shelf waters from the shelf to the off shelf regions north of Elephant Island and then downstream throughout the summer-winter seasons to support a series of blooms at a broad scale of 100-1000 km in the southern Scotia Sea. Hypothesis 2: As the surface Fe is depleted in the late summer, Fe is supplied to the surface from subsurface waters by vertical mixing or upwelling associated with mesoscale eddy dynamics. Hypothesis 3: In addition to processes which supply "new" Fe to the euphotic zone, biological recycling of Fe and production of Fe-binding ligands also serve to enhance retention of Fe in surface waters and sustain the late-season bloom. Hypothesis 4: Fe addition to ACC surface waters, Fe transformation and subsequent depletion lead to an evolving plankton community structure that affects carbon flow between trophic levels and in turn affects carbon export from surface waters. Evaluating these hypotheses required the synthesis of physical processes such as currents, eddies, stratification and mixing, biological and trace metal data collected in 2004 and 2006 by our team and also from other programs whose data is in the literature or in public archives. Based on this synthesis and data / process analysis our goal was to develop a numerical model including biogeochemical processes such as phytoplankton physiology, bacterial carbon demand, and in situ Fe speciation to pursue these objectives: 1) Simulating the circulation fields between 2004 and 2006 using a 2-15 km regional hydrodynamic model already developed from the eastern Bellingshausen Sea to Weddell Sea, and validating results using all available data collected during NSF and AMLR cruises 2) Quantitatively estimating Fe distributions, transport and mixing fluxes from combining data synthesis and modeling in the southern Drake Passage, Southern Scotia Sea, western Weddell Sea and Bransfield Strait 3) Developing process models and quantitative relationships between Fe-complexing ligands, Fe recycling and plankton functional groups based on data synthesis and hypothesis testing 4) Comparing differences in physical and biogeochemical processes in different Southern Ocean regions, and applicability of our findings to other areas in the Southern Ocean 5) Publish the results of interdisciplinary observations, synthesis and models in Deep-Sea Research II. The past year, the final year of this award, we continued to build upon fundamental findings and discoveries as represented in the new publications reported with this award as well as findings derived from the original field program. This report represents the overall integration and synthesis of the data sets, major findings and discoveries not only from our group, but the entire team and the literature to achieve an interdisciplinary understanding published in Deep Sea Research II.
