TY - JOUR
T1 - Impact of iron on silicon utilization by diatoms in the Southern Ocean
T2 - A case study of Si/N cycle decoupling in a naturally iron-enriched area
AU - Mosseri, Julie
AU - Quéguiner, Bernard
AU - Armand, Leanne
AU - Cornet-Barthaux, Véronique
PY - 2008/3
Y1 - 2008/3
N2 - Biogenic silica stocks and fluxes were investigated in austral summer over the naturally iron-fertilized Kerguelen Plateau and in nearby high-nutrient, low-chlorophyll (HNLC) off-plateau surface waters. The Kerguelen Plateau hosted a large-diatom bloom, with high levels of biogenic silica (BSi) but relatively low silicic acid (Si(OH)4) uptake rates (1100±600 mmol m-2 and 8±4 mmol m-2 d-1, respectively). Diatoms of the naturally iron-enriched area presented high affinities for silicic acid, allowing them in combination with a beneficial nutrient vertical supply to grow in low silicic acid waters (<2 μM). Si(OH)4 acid uptake rates were also compared with carbon and nitrogen uptake rates. As expected for diatoms growing in favourable nutrient conditions, and from previous artificial iron-enrichment experiments, Si:C and Si:NO3 elemental uptake ratios of the natural diatom community of the plateau were close to 0.13 and 1, respectively. In contrast, diatom communities in the HNLC waters were composed of strongly silicified (high Si:C, Si:NO3 uptake ratios) diatoms with low affinities for Si(OH)4. Although the Si:NO3 uptake ratio in the surface waters of the plateau was close to 1, the apparent consumption of nitrate on a seasonal basis was much lower (∼5 μM) than the apparent consumption of silicic acid (∼15 μM). This was mainly due to diatoms growing actively on ammonium (i.e. 39-77% of the total nitrogen uptake) produced by an intense heterotrophic activity. Thus we find that while Fe fertilization does increase N uptake with respect to Si uptake, rapid recycling of N decouples nitrogen and carbon export from silica export so that the "silicate pump" remains more efficient than that of N (or P). For this reason an iron-fertilized Southern Ocean is unlikely to experience nitrate exhaustion or export silicic acid to the global ocean.
AB - Biogenic silica stocks and fluxes were investigated in austral summer over the naturally iron-fertilized Kerguelen Plateau and in nearby high-nutrient, low-chlorophyll (HNLC) off-plateau surface waters. The Kerguelen Plateau hosted a large-diatom bloom, with high levels of biogenic silica (BSi) but relatively low silicic acid (Si(OH)4) uptake rates (1100±600 mmol m-2 and 8±4 mmol m-2 d-1, respectively). Diatoms of the naturally iron-enriched area presented high affinities for silicic acid, allowing them in combination with a beneficial nutrient vertical supply to grow in low silicic acid waters (<2 μM). Si(OH)4 acid uptake rates were also compared with carbon and nitrogen uptake rates. As expected for diatoms growing in favourable nutrient conditions, and from previous artificial iron-enrichment experiments, Si:C and Si:NO3 elemental uptake ratios of the natural diatom community of the plateau were close to 0.13 and 1, respectively. In contrast, diatom communities in the HNLC waters were composed of strongly silicified (high Si:C, Si:NO3 uptake ratios) diatoms with low affinities for Si(OH)4. Although the Si:NO3 uptake ratio in the surface waters of the plateau was close to 1, the apparent consumption of nitrate on a seasonal basis was much lower (∼5 μM) than the apparent consumption of silicic acid (∼15 μM). This was mainly due to diatoms growing actively on ammonium (i.e. 39-77% of the total nitrogen uptake) produced by an intense heterotrophic activity. Thus we find that while Fe fertilization does increase N uptake with respect to Si uptake, rapid recycling of N decouples nitrogen and carbon export from silica export so that the "silicate pump" remains more efficient than that of N (or P). For this reason an iron-fertilized Southern Ocean is unlikely to experience nitrate exhaustion or export silicic acid to the global ocean.
KW - Biogenic silica
KW - Diatoms
KW - Indian sector
KW - Iron fertilization
KW - Kerguelen Islands
KW - Limiting factors
KW - Silicon
KW - Southern Ocean
UR - http://www.scopus.com/inward/record.url?scp=43049157970&partnerID=8YFLogxK
U2 - 10.1016/j.dsr2.2007.12.003
DO - 10.1016/j.dsr2.2007.12.003
M3 - Article
AN - SCOPUS:43049157970
SN - 0967-0645
VL - 55
SP - 801
EP - 819
JO - Deep-Sea Research Part II: Topical Studies in Oceanography
JF - Deep-Sea Research Part II: Topical Studies in Oceanography
IS - 5-7
ER -