The relative contribution of deep and shallow benthic sources to iron supply in the Ross sea, with specific emphasis on Ross Bank

Abstract

Summer primary productivity in the Ross Sea is limited by the availability of dissolved iron (DFe) in the euphotic zone. Previous studies have suggested that benthic sources dominate the supply of DFe to the upper water column in the southern Ross Sea polynya. The purpose of this study is to investigate whether these benthic sources are derived from shallow banks or deeper areas of the continental shelf. Our study combines analysis of in situ observations near Ross Bank with a physical ocean model simulation in an overlapping summertime period to investigate the vertical supply of DFe. Two passive tracer dyes were employed in the ocean model with the first (second) being input in the bottom model layer over seabed depths of less (greater) than 400 m. The tracer dyes provide temporal (March 2011–January 2012) and spatial estimates of the advection, mixing and diffusion throughout the Ross Sea model domain, and enable estimates of the relative contributions of shallow and deep benthic iron sources to the upper ocean DFe inventory. A comparison of the model output with a ship-based survey and a mooring deployed on Ross Bank in January 2012 provides confidence that the model simulations produce a sufficiently accurate representation of ocean conditions to support estimates of benthic DFe supply to surface waters. The results demonstrate that for Ross Bank the local (on-bank) benthic supply is important during the early winter period, whereas for most of the year the deep (off-bank) benthic iron is the predominant source supplying the upper ocean over this shallow region.