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

We investigated the relative roles of Fe and silicic acid availabilities in regulating Si uptake rates across the Polar Frontal Zone in the Pacific Se ctor of the Southern Ocean (59-68 degreesS, 170 degreesW) during the US JGO FS Antarctic Environment Southern Ocean Process Study (AESOPS). Meridional gradients in silicic acid concentration ([Si(OH)(4)]) of about 0.25-0.56 mu M km(-1) were observed in this area during austral spring and summer, 1997- 1998, with [Si(OH)(4)] ranging from < 1 to 15 <mu>M On the north side of th e gradient to 40-60 muM on the south side. In two pairs of shipboard bottle -enrichment experiments conducted north and south of the Si gradient in spr ing and summer, we measured the effects of Fe, Zn and Si additions on Si-32 (OH)(4) and (NO3-)-N-15 uptake rates, biogenic silica concentrations and Si (OH)(4):NO3- uptake ratios. Fe addition had little or no effect on Si uptak e rates in enrichments conducted in the low-Si waters north of the Si gradi ent. 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 b oth spring and summer. Fe addition decreased Si(OH)(4):NO3- uptake ratios b y 2-5 times, largely due to stimulation of NO3- uptake rates. Zn addition h ad no effect on Si(OH)(4) and NO3- uptake rates. Short-term (24 h) Si addit ions had varying effects on Si uptake rates, depending on season and locati on. In spring, additions of 40 muM Si to water from bottle enrichments, con ducted north of the Si gradient tin situ [Si(OH)(4)] similar to 15 muM) did not increase Si uptake rates initially, but did increase uptake rates afte r 8 days. In the summer enrichment north of the Si gradient tin situ [Si(OH )(4)] similar to 5 muM), 50 muM Si additions doubled in situ. Si uptake rat es in the initial water collected for the enrichment, and increased Si upta ke rates as much as 16-fold during the experiment. South of the Si gradient , where in situ [Si(OH)(4)] was > 40 muM in both spring and summer, Si addi tion had no effect on in situ Si uptake rates in the initial enrichment wat er nor on any Si uptake rates measured during the experiment. Our results i ndicate that both Fe and Si availabilities regulate Si uptake rates and sil ica production in the Southern Ocean along 170 degreesW. Fe limitation appe ars to restrict Si uptake rates south of the Si gradient and plays a role i n preventing Si depletion south of the ACC, where ambient [Si(OH)(4)] never fell below 40 muM during 1997-1998; Our experiments in the Seasonal Ice Zo ne at 62 degrees suggest that Si uptake in this area switched from being Fe -limited in the spring, when in situ [Si(OH)(4)] was > 40 muM, to Si-limite d in the summer, when [Si(OH)(4)] was < 5 <mu>M. Thus, while Fe limitation could be reducing Si uptake rates in this area, it does not prevent:eventua l Si drawdown. Our experiments also indicate that Si and Fe co-limitation m ay 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 interact ion between Fe and SI limitation in these waters and the high Si(OH)(4):NO3 - uptake ratios observed at in situ dissolved Fe concentrations can have a large impact on Si and N biogeochemistry in the Southern Ocean. (C) 2000 El sevier Science Ltd. All rights reserved.