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

Abstract

The remote waters of the Pacific region of the Southern Ocean are the furthest away from any upstream and upwind Ž continental Fe sources. This prime area for expecting Fe limitation of the plankton ecosystem was studied March-April . 1995 along a north-south transect at ; 898W. At the end of the austral summer the upper wind-mixed layers were in the order of ; 100 m deep, thus mixing the algae down into the dimly lit part of the euphotic zone where photosynthesis is severely restricted. The dissolved Fe was found at low concentrations ranging from 0.05 nM near the surface to 0.5 nM in Ž . deeper waters. Along the transect 528S-698S , the dissolved iron was enhanced in the Polar Front, as well as near the Ž . Antarctic continental margin 0.6-1.0 nM . In between, the southern ACC branch was depleted with iron; here the concentrations in surface waters were quite uniform at about 0.21 nM. This is only somewhat lower than the 0.49 nM Ž . Ž . October 1992 and 0.31 nM November 1992 averages in early spring in the southern ACC part of Atlantic 68W sections wde Baar, H.J.W., de Jong, J.T.M., Bakker, D.C.E.. Loscher, B.M., Veth, C., Bathmann, U., Smetacek, V., 1995. Importancë of iron for phytoplankton spring blooms and CO drawdown in the Southern Ocean. Nature 373, 412-415; Loscher, B.M., 2 de Jong, J.T.M., de Baar, H.J.W., Veth, C., Dehairs, F., 1997. The distribution of iron in the Antarctic Circumpolar Current.x Deep-Sea Research II 44, 143-188. . First, the lower ; 0.21 nM in March-April 1995 may partly be due to continuation of the seasonal trend where the phytoplankton growth, albeit modest, was removing Fe from the surface waters. Secondly, the 898W Pacific stations are further downstream continental or seafloor sources than the Atlantic 68W section. In the latter case, the ACC water had passed through the Drake Passage and also over the Sandwich Plateau. Indeed for Drake Passage, intermediate Fe concentrations have been reported by others. The generally somewhat lower surface water Fe at the ACC and PF at 898W is consistent with the distance from sources and the late summer. It also would explain the very low Ž . abundance of phytoplankton Chl a in the region and the conspicuous absence of plankton blooms. In the subAntarctic waters north of the Polar Front there are no diatoms, let alone diatom blooms, due to low availability of silicate. Thus, it Baar et al.r Marine Chemistry 66 1999 1-34 2 appears the biological productivity is suppressed due to iron deficiency, in combination with the severe seasonal effects of wind mixing on the light climate, as well as regional silicate limitation for diatoms. q

Interactions of the iron and phosphorus cycles: A three‐dimensional model study

“…Owing to the silicic acid requirement of the diatom class, it dominates the ecosystem south of 50°S. To the north of the Antarctic Circumpolar Current, small phytoplankton dominate (consistent with observations [de Baar et al, 1999]). The modeled equatorial Pacific is richer in diatoms than observations suggest [Gregg et al, 2003], consistent with the upwelling of waters from too great a depth to the surface of this region (as discussed above).…”

An iron budget during the natural iron fertilisation experiment KEOPS (Kerguelen Islands, Southern Ocean)

“…The higher values determined by Löscher et al (1997) in the Atlantic sector of the Southern Ocean are explained by the upwelling of iron rich Antarctic waters Apparent particulate iron (Fe app ) concentrations, calculated by subtracting DFe values measured during the cruise (Blain et al, 2008) from TDFe, are reported in Table 1. Fe app is defined as the iron adsorbed on lithogenic or biogenic particles, the biogenic iron and the labile lithogenic particulate iron (de Baar et al, 1999) plus the refractory colloids included in the dissolved fraction and that were not measured on board after a few days of acid storage. Fe app represents between 82 and 99% of the TDFe (mean value 95±5%) above the plateau and between 83 and 93% (mean value 86±3%) outside the plateau with no difference between surface and deep water (see Table 1).…”

“…That fraction represents the sum of the dissolved iron (i.e. the labile dissolved iron which is the fraction that is released after a few days of acidification) plus the refractory colloidal iron (i.e., that fraction that dissolved after more than a few days of acidification) plus the fraction of particulate material that dissolves during extended (>6 months) acid storage (iron adsorbed on lithogenic or biogenic particles, contained within biogenic particles and labile lithogenic particulate iron) (de Baar et al, 1999;Löscher et al, 1997;Bowie and Sedwick, 2004). In the literature, the set of TDFe observations is much smaller than that of DFe.…”

A global compilation of dissolved iron measurements: focus on distributions and processes in the Southern Ocean

“…In the past decades, a large number of dFe measurements have been collected that provide seasonal variability data in the Southern Ocean (e.g., Martin et al, 1990;de Baar et al, 1995de Baar et al, , 1999Löscher et al, 1997;Lannuzel et al, 2011;Fitzwater et al, 2000;Sedwick et al, 1997Sedwick et al, , 1999Sedwick et al, , 2000Sedwick et al, , 2008Sedwick et al, , 2011Sohrin et al, 2000;Measures and Vink, 2001;Bowie et al, 2004Coale et al, 2005;Chever et al, 2010. However, despite these efforts, there are some months in different ocean regions and basins in which no dFe measurements have yet been made.…”