Low dissolved Fe and the absence of diatom blooms in remote Pacific watersof the Southern Ocean

The remote waters of the Pacific region of the Southern Ocean are the furth est away from any upstream and upwind continental Fe sources. This prime ar ea for expecting Fe limitation of the plankton ecosystem was studied (March -April 1995) along a north-south transect at similar to 89 degrees W. At th e end of the austral summer the upper wind-mixed layers were in the order o f similar to 100 m deep, thus mixing the algae down into the dimly lit part of the euphotic zone where photosynthesis is severely restricted. The diss olved Fe was found at low concentrations ranging from 0.05 nM near the surf ace to 0.5 nM in deeper waters. Along the transect (52 degrees S-69 degrees S), the dissolved iron was enhanced in the Polar Front, as well as near th e Antarctic continental margin (0.6-1.0 nM). In between, the southern ACC b ranch was depleted with iron; here the concentrations in surface waters wer e quite uniform at about 0.21 nM. This is only somewhat lower than the 0.49 nM (October 1992) and 0.31 nM (November 1992) averages in early spring in the southern ACC part of Atlantic 6 degrees W sections [de Baar, H.J.W., de Jong, J.T.M., Bakker, D.C.E.. Loscher, B.M., Veth, C., Bathmann, U., Smeta cek, V., 1995. Importance of iron for phytoplankton spring blooms and CO2 d rawdown in the Southern Ocean. Nature 373, 412-415; Loscher, B.M., de Jong, J.T.M., de Baar, H.J.W., Veth, C., Dehairs, F., 1997. The distribution of iron in the Antarctic Circumpolar Current. Deep-Sea Research II 44, 143-188 .]. First, the lower similar to 0.21 nM in March-April 1995 may partly be d ue to continuation of the seasonal trend where the phytoplankton growth, al beit modest, was removing Fe from the surface waters. Secondly, the 89 degr ees W Pacific stations are further downstream continental or seafloor sourc es than the Atlantic 6 degrees W section. In the latter case, the ACC water had passed through the Drake Passage and also over the Sandwich Plateau. I ndeed for Drake Passage, intermediate Fe concentrations have been reported by others. The generally somewhat lower surface water Fe at the ACC and PF at 89 degrees W is consistent with the distance from sources and the late s ummer. It also would explain the very low abundance of phytoplankton (Chl a ) in the region and the conspicuous absence of plankton blooms. In the subA ntarctic waters north of the Polar Front there are no diatoms, let alone di atom blooms, due to low availability of silicate. Thus, it appears the biol ogical productivity is suppressed due to iron deficiency in combination wit h the severe seasonal effects of wind mixing on the light climate, as well as regional silicate limitation for diatoms. (C) 1999 Elsevier Science B.V. All rights reserved.