Control of silica production by iron and silicic acid during the Southern Ocean Iron Experiment (SOFeX)

Abstract

We examined the role of Si limitation in mediating the response of siliceous biomass and silica production to mesoscale Fe fertilization in the high-silicic acid high-nitrate waters of the Antarctic and in the low-silicic acid high-nitrate waters of the Subantarctic during austral summer. Iron fertilization stimulated biogenic silica production and silicic acid depletion in both regions. Si limitation significantly curtailed the response of silica production to Fe in the Subantarctic, but not in the Antarctic. Additions of silicic acid to Fe-enriched waters of the Subantarctic more than doubled specific silica production rates beyond the increase caused by Fe alone. This result, combined with the presence of Si limitation both inside and outside of the fertilized patch in the Subantarctic, indicates that silica production in the Subantarctic is regulated by both Fe and Si during austral summer. Fe increased the ability of Subantarctic diatom assemblages to take up low concentrations of Si over an order of magnitude by increasing maximum uptake rates and lowering half-saturation constants for silicic acid uptake. Our observations indicate that the dramatic gradient in Si availability across the Antarctic Circumpolar Current exerts a strong control on the contribution of diatoms to new production following Fe enrichment in the Southern Ocean during austral summer. With sufficient Fe to allow nitrate depletion, the abundant silicic acid in the Antarctic would allow diatoms to dominate nitrate use; however, low silicic acid concentrations in the Subantarctic would restrict the fraction of available nitrate that would be consumed by diatoms to ca. 5%.

Effects of iron on the elemental stoichiometry during EIFEX and in the diatoms <i>Fragilariopsis kerguelensis</i> and <i>Chaetoceros dichaeta</i>

“…Fe limitation directly decreases the uptake rates of SO diatoms for silicic acid (Brzezinski et al, 2005;De La Rocha et al, 2000;Franck et al, 2003;Franck et al, 2000). However, it has been hypothesized that increased Si uptake is caused by an increased duration of the cell wall synthesis phase.…”

Coccolithophores and calcite saturation state in the Baltic and Black Seas

“…Scarcity of silicate starts to become limiting to silica production and diatom growth at concentrations below about 3 µmol kg −1 (Brzezinski et al, 1998(Brzezinski et al, , 2005. Levels of silicate in summer were originally greater than this several decades ago but have subsequently declined, in both the Baltic Sea (Papush and Danielsson, 2006;Rahm et al, 1996) and in the Black Sea (Humborg et al, 1997;Konovalov and Murray, 2001), probably because of river damming (e.g.…”

The effects of dilution and mixed layer depth on deliberate ocean iron fertilization: 1-D simulations of the southern ocean iron experiment (SOFeX)

“…The coefficient used in the quadratic terms in aggregation mortality for small phytoplankton and diatoms was also lowered from 0.009 to 0.006 (decreasing aggregation losses). The half saturation constant (Ks) for silica uptake at the North Patch was estimated at 0.63 μM by Brzezinski et al (2005) during the SOFeX. We changed from our standard value of 1.0 μM to 0.63 μM only for North Patch (silicate concentrations were always high and non-limiting at the South Patch).…”

“…3). Brzezinski et al (2005) stated that the biogenic silica values obtained from the sampling regions were diluted by surrounding waters. This resulted in the lowering of biogenic silica at the South and North Patches.…”

“…During the SOFeX, biogenic silica and silica production rates were estimated by Brzezinski et al (2005). At the South Patch, the model lags the observed rise in silica production in the fertilized case by about a week (Fig.…”