A seasonal progression of Si limitation in the Pacific sector of the Southern Ocean

We conducted kinetic experiments using the radioisotope 32Si to assess Si limitation in the Pacific sector of the Southern Ocean between 53° and 71°S from early December 1997 through mid March 1998. A diatom bloom developed near the edge of the pack ice in late November or early December and then propagated southward, depleting silicic acid in surface waters from >45 to <5 μM, and occasionally to <1 μM in an area that extended from ∼61° to 65°S. We performed kinetic experiments of two kinds: 26 multiple-concentration experiments in which the specific rate of silicic acid uptake (V) was measured as a function of the extracellular silicic acid concentration ([Si(OH)4]), and 61 two-concentration experiments in which V was measured only at the ambient concentration ([Si(OH)4]a) and one concentration 30–70 μM above ambient. te of Si uptake was almost always limited by [Si(OH)4]a in the Si-depleted waters within the bloom and to its north. In those waters V increased with [Si(OH)4] in accordance with the Michaelis-Menten saturation function, and [Si(OH)4]a often limited V to <40% of its potential maximum (Vmax). Si-limiting conditions followed the bloom southward during spring and summer, and extended ∼500 km south of the Antarctic Polar Front (APF) by late January. een of the 87 kinetic experiments showed no Si limitation. All but three of those non-limiting sites were in Si-rich waters south of the APF. However, there was occasional, relatively weak limitation of Si uptake even south of the APF, at [Si(OH)4]a >40 μM. This unexpected result was caused by very low affinity for Si uptake (high KS in the Michaelis–Menten function). Seventy percent of all measured KS values (49 of 70) were between 0.7 and 10 μM, a range only about twice that reported from elsewhere, but the remaining 21 KS values ranged from 10.7 to 61 μM. Eleven of these were >20 μM, which is extraordinarily high in comparison with most values from other oceans. There was no clear tendency for KS to change with latitude or with [Si(OH)4]a, but there was an apparent seasonal progression in which both the highest KS values and the variability in KS increased with time. The initial slopes of the kinetic curves, (Vmax/KS, a measure of the ability to exploit low concentrations) for the Southern Ocean diatom assemblages were very low compared to other regions of the ocean. In the Southern Ocean data 52 of 73 initial slopes (71%) are <2 (mM h)−1 while only one of 37 initial slopes (3%) from other systems is that low. The lowest slopes were observed south of the APF, which may be a consequence of the high silicic acid concentrations that prevail in that region. The ability to compete for silicic acid at low concentrations appears not to impart a significant competitive advantage in the Si-rich system south of the APF.