Iron and silicic acid concentrations regulate Si uptake north and south of the Polar Frontal Zone in the Pacific Sector of the Southern Ocean

• Published in: Deep-sea research. Part II, Topical studies in oceanography Vol. 47; no. 15; pp. 3315 - 3338
• Main Authors: M. Franck, Valerie, Brzezinski, Mark A, Coale, Kenneth H, Nelson, David M
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 2000
• Subjects: Marine; PS, Ross Sea
• ISSN: 0967-0645; 1879-0100
• DOI: 10.1016/S0967-0645(00)00070-9

We investigated the relative roles of Fe and silicic acid availabilities in regulating Si uptake rates across the Polar Frontal Zone in the Pacific Sector of the Southern Ocean (59–68°S, 170°W) during the US JGOFS Antarctic Environment Southern Ocean Process Study (AESOPS). Meridional gradients in silicic acid concentration ([Si(OH) 4]) of about 0.25–0.56 μM km −1 were observed in this area during austral spring and summer, 1997–1998, with [Si(OH) 4] ranging from <1 to 15 μM on the north side of the gradient to 40–60 μM on the south side. In two pairs of shipboard bottle-enrichment experiments conducted north and south of the Si gradient in spring and summer, we measured the effects of Fe, Zn and Si additions on 32 Si(OH) 4 and 15 NO 3− uptake rates, biogenic silica concentrations and Si(OH) 4 : NO 3 − uptake ratios. Fe addition had little or no effect on Si uptake rates in enrichments conducted in the low-Si waters north of the Si gradient. However, Fe addition increased Si uptake rates 3–5 times over controls in enrichments conducted in the high-Si waters south of the gradient, in both spring and summer. Fe addition decreased Si(OH) 4 : NO 3 − uptake ratios by 2–5 times, largely due to stimulation of NO 3 − uptake rates. Zn addition had no effect on Si(OH) 4 and NO 3 − uptake rates. Short-term (24 h) Si additions had varying effects on Si uptake rates, depending on season and location. In spring, additions of 40 μM Si to water from bottle enrichments, conducted north of the Si gradient (in situ [Si(OH) 4] ∼15 μM) did not increase Si uptake rates initially, but did increase uptake rates after 8 days. In the summer enrichment north of the Si gradient (in situ [Si(OH) 4] ∼5 μM), 50 μM Si additions doubled in situ Si uptake rates in the initial water collected for the enrichment, and increased Si uptake rates as much as 16-fold during the experiment. South of the Si gradient, where in situ [Si(OH) 4] was >40 μM in both spring and summer, Si addition had no effect on in situ Si uptake rates in the initial enrichment water nor on any Si uptake rates measured during the experiment. Our results indicate that both Fe and Si availabilities regulate Si uptake rates and silica production in the Southern Ocean along 170°W. Fe limitation appears to restrict Si uptake rates south of the Si gradient and plays a role in preventing Si depletion south of the ACC, where ambient [Si(OH) 4] never fell below 40 μM during 1997–1998. Our experiments in the Seasonal Ice Zone at 62° suggest that Si uptake in this area switched from being Fe-limited in the spring, when in situ [Si(OH) 4] was >40 μM, to Si-limited in the summer, when [Si(OH) 4] was <5 μM. Thus, while Fe limitation could be reducing Si uptake rates in this area, it does not prevent eventual Si drawdown. Our experiments also indicate that Si and Fe co-limitation may occur north of the Si gradient, such that Si uptake rates will not reach maximal levels until both Si and Fe limitations are relieved. The interaction between Fe and Si limitation in these waters and the high Si(OH) 4 : NO 3 − uptake ratios observed at in situ dissolved Fe concentrations can have a large impact on Si and N biogeochemistry in the Southern Ocean.