The roles of MCDW and deep water iron supply in sustaining a recurrent phytoplankton bloom on central Pennell Bank (Ross Sea)

•Mid-summer elevated chl a on central Pennell Bank (CPB) appears to be recurrent.•All algal populations were Fe limited, but CPB stations had high net productivity.•Western flank (WF) MCDW is not rich in dissolved Fe, but is transported to CPB.•A portion of the Fe demand of CPB stations can be explained by vertical supply.•Local sources of Fe may enrich CPB deep water relative to WF MCDW.

During January–February 2011 standing stocks of phytoplankton (chl a) in the Pennell Bank region of the Ross Sea were variable over 10–100 km spatial scales. One area of elevated chl a on central Pennell Bank (CPB) appeared to be a recurrent mid-summer feature. The western flank (WF) of Pennell Bank had pronounced signatures of Modified Circumpolar Deep Water (MCDW). We evaluated the spatial extent of Fe limitation and net primary production and tested whether MCDW may provide elevated amounts of Fe to the CPB region, through a combination of in situ measurements, shipboard incubations and a horizontally resolved physical model. Regional fluxes of dissolved Fe from deep to surface waters were compared to calculated Fe demands. Low in situ variable to maximum fluorescence (Fv/Fm; 0.24–0.37) and surface water dissolved Fe concentrations (~0.12–0.21 nM) were suggestive of widespread limitation, corroborated by the consistent responses (Fv/Fm, growth, and nutrient removal ratios) of incubation treatments to Fe addition. MCDW from the WF region had lower dissolved Fe concentrations than that measured in CDW (Circumpolar Deep Water), which suggests on-shelf modification with Fe deplete surface waters and is consistent with the lack of stimulation due to incubation amendments with filtered MCDW. Model results and empirical data suggest MCDW from the WF region is further modified and mixed en route to the CPB region, leading to both the erosion of the canonical MCDW signature and an elevated dissolved Fe inventory of CPB region deep water. This suggests the addition of Fe possibly via diagenesis, as suggested by Marsay et al. (2014). Calculated deep water supply rates to the surface waters of CPB were ~0.18–0.43 m d−1, while calculated rates at the WF or northern Pennell Bank (NPB) regions were negative. The CPB populations exhibited ~4.5-fold higher net production rates compared to those in the WF and NPB regions and required 520–3200 nmol Fe m−2 d−1. The modeled vertical supply rates seem to provide ~2–15% of the estimated Fe requirement. Since this flux is based on subsurface dissolved Fe inventories, it does not account for any bioavailable Fe from deep water particulate sources or for Fe recycling in the upper water column. These data suggest the recurrent productivity hotspots at CPB are not fueled by Fe-rich MCDW but are partially supported by the delivery of Fe through vertical exchange processes.