Award: ANT-0944165

The Ross Sea continental shelf is the single most productive area in the Southern Ocean, and may comprise a significant but unaccounted for oceanic CO2 sink, largely driven by phytoplankton production. However, the processes that control the magnitude of primary production in this region are not well understood. During summer, an observed abundance of macronutrients and scarcity of dissolved iron are consistent with iron limitation of phytoplankton growth in the Ross Sea polynya, as is further suggested by shipboard bioassay experiments. Field observations and model simulations indicate four potential sources of dissolved iron to surface waters of the Ross Sea: (1) circumpolar deep water intruding from the shelf edge; (2) sediments on shallow banks and nearshore areas; (3) melting sea ice around the perimeter of the polynya; and (4) glacial meltwater from the Ross Ice Shelf. Our project achieved what we believe to be the most quantitative regional estimates of iron supply and demand in an Antarctic plankton ecosystem to date. Our results from the Ross Sea show that supply and demand estimates agree to within their associated uncertainties—which is a significant finding unto itself. Moreover, the combination of observations and models facilitate assessment of the relative importance of various iron sources. Specifically, detailed profile data reveal a significant benthic source of iron, a process that has been inferred in other areas but seldom documented with near-bottom measurements. The benthic source is on par with that of sea ice, which is the other major contributor in the Ross Sea. This is in marked contrast to recent data from the Amundsen Sea, where glacial meltwater is the dominant source of iron. These findings highlight the marked heterogeneity of iron supply in Antarctic shelf ecosystems—an aspect which must be explicitly accounted for if we are to understand and predict how these systems will respond to climate change. Last Modified: 09/28/2016 Submitted by: Dennis J Mcgillicuddy