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The availability of dissolved trace metals such as iron (Fe), manganese (Mn), and zinc (Zn) may play an important role in regulating the growth and biomass of phytoplankton in waters of the antarctic continental shelves (Martin, Fitzwater, and Gordon 1990a). Little information is available, however, regarding the concentrations of trace metals in these waters during the phytoplankton growing season. Very low dissolved Fe concentrations of approximately 0.1 nanomole/liter (nM) have been reported for surface waters of the Ross Sea in January 1990 (Fitzwater et al. 1996), which, together with the results of bottle-incubation experiments (Martin et al. 1990a), suggest that Fe-deficiency may limit algal production in this region during the middle to late summer. During spring (November and December) 1994, we measured significantly higher dissolved Fe concentrations of 0.5-3.8 nM in these waters, and dissolved Mn concentrations of 0.08-0.83 nM (Sedwick, DiTullio, and Mackey 1995). These observations suggest that the dissolved Fe content of seawater in the Ross Sea may vary widely by season and that dissolved Fe availability decreases during the summer. The results we present here, from a survey of dissolved iron and manganese in this region during the early summer, are consistent with this hypothesis.
During an expedition aboard R/V Nathaniel B. Palmer in December 1995 and January 1996, water-column samples were collected at nine stations in the southern oceans and Ross Sea (see figure 1 and table) using trace-metal clean techniques as described in Sedwick et al. (1995). Samples were filtered through 0.4 micrometer-pore polycarbonate membranes, and dissolved Fe and Mn were determined at sea by flow-injection analysis following modifications of the methods of Measures, Yuan, and Resing (1995) and Resing and Mottl (1992). Station K1 was an ice-free, deep-ocean location well away from the antarctic continent, where low concentrations of both metals were expected in the upper water column. Stations K2-K9 were on the continental shelf, in water depths of less than 1 kilometer, and were occupied in conditions ranging from heavy pack ice to ice-free (see table). Dissolved major nutrients, nitrate plus nitrite, phosphate, and silicate were abundant at all trace-metal stations (concentrations >14 micromole/liter, 0.8 micromole/liter, and 45 micromole/liter, respectively; data not shown). Vertical concentration profiles of dissolved Fe and Mn for stations K1 and K3-K9 are presented in figure 2.
Our ability to collect open-ocean seawater and measure dissolved Fe and Mn without significant contamination is demonstrated by the smooth vertical concentration profiles and low concentrations (0.12-0.30 nM Fe, 0.21-0.28 nM Mn) obtained at station K1 (figure 2A), which are very similar to data reported by Martin et al. (1990b) for the southern Drake Passage. Except for station K3, water-column dissolved Fe concentrations were generally low (0.09-0.38 nM) in shelf waters of the Ross Sea, and tend to increase with depth, suggesting removal from the upper water column due to biological uptake or other scavenging processes (Landing and Bruland 1987). At station K3 (figure 2B), significantly higher dissolved Fe concentrations were measured at the surface (2.25 nM) and 20 meters depth (0.72 nM); we attribute these findings to the release of Fe from the melting sea ice (brash ice), which was present at this location. Martin et al. (1990b) have suggested melting sea ice as a possible source of Fe for surface seawater, and total Fe concentrations above 30 nM have been measured in snow collected from antarctic sea ice (Edwards and Sedwick 1996). The highest mixed-layer dissolved Mn concentrations of approximately 0.6 nM were also observed at station K3, consistent with dust inputs from melting sea ice. Dissolved Mn concentrations were low (<0.6 nM) in the upper water column at all other stations, increasing to higher concentrations (approximately 0.5-0.8 nM) below 100 meters depth, again suggesting biological/scavenging removal from the mixed layer.
Our data suggest that the concentrations of dissolved Fe and Mn are typically low (0.09-0.38 nM Fe, 0.03-0.58 nM Mn) in the upper hundred meters of the water column of the Ross Sea during the early summer, except where brash ice is present. These results are consistent with those of Martin et al. (1990a) and Fitzwater et al. (1996), who suggest that low concentrations of dissolved Fe may limit phytoplankton production in this region during the mid- to late-summer. The elevated concentrations of dissolved Fe and Mn observed in association with brash ice suggest that melting sea ice may be an important source of Fe and Mn into surface seawater around Antarctica and may explain the significantly higher dissolved Fe and Mn concentrations observed in these waters during the spring (Sedwick et al. 1995), when large amounts of annual sea ice are melting.
We thank chief scientist David Garrison, Antarctic Support Associates personnel, the captain and crew of R/V Nathaniel B. Palmer, and the other participants in the Collaborative Studies of Bloom Dynamics and Food Web Structure in the Ross Sea. We are grateful to Louis Gordon and Andrew Ross for the nutrient analyses, and to Ian Helmond and Terry Byrne for design and fabrication of the water samplers. Joe Resing and Chris Measures are thanked for assistance with the flow injection methods. This research was supported by National Science Foundation grant OPP 93-17431 and the Antarctic CRC.
Edwards, R., and P. Sedwick. 1996. Annual release of iron from seasonal sea ice in antarctic waters. Handbook and Abstracts, 1996 Australian Marine Sciences Association National Conference. [abstract]
Fitzwater, S.E., K.S. Johnson, R.M. Gordon, and K.H. Coale. 1996. Iron and zinc in the Ross Sea, 1990. Eos, Transactions of the American Geophysical Union, Supplement, AGU-ASLO Ocean Sciences Meeting Abstracts Volume, OS192. [abstract]
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Martin, J.H., S.E. Fitzwater, and R.M. Gordon. 1990a. Iron deficiency limits plankton growth in antarctic waters. Global Biogeochemical Cycles, 4, 5-12.
Martin, J.H., R.M. Gordon, and S.E. Fitzwater. 1990b. Iron in antarctic waters. Nature, 345, 156-158.
Measures, C.I., J. Yuan, and J.A. Resing. 1995. Determination of iron in seawater by flow injection analysis using in-line preconcentration and spectrophotometric detection. Marine Chemistry, 50, 1-10.
Resing, J.A., and M.J. Mottl. 1992. Determination of manganese in seawater using flow injection analysis with on-line preconcentration and spectrophotometric detection. Analytical Chemistry, 64, 2682-2687.
Sedwick, P., G. DiTullio, and D. Mackey. 1995. Iron and manganese in surface waters of the Ross Sea during the spring bloom 1994. Antarctic Journal of the U.S., 30(5), 199-201.