Role of silicon as a limiting nutrient to Antarctic diatoms: evidence from kinetic studies in the Ross Sea ice-edge zone

During the austral summer of 1990 an intense, diatom-dominated, ice-edge phytoplankton bloom in the southwestern Ross Sea resulted in depletion of silicic acid, nitrate and phosphate to concentrations much lower than is typical for Antarctic surface waters. Silicic acid was depleted to < 6 μM within the core of the meltwater field, where biogenic particulate silica concentrations exceeded 20 μmol l−1. Three Si uptake kinetic experiments were conducted on natural phytoplankton assemblages from the nutrient-depleted zone; 30Si-labeled Si(OH)4 was used to measure the uptake rate at concentrations ranging from 1 to 20 μM above ambient. Dependence of the specific uptake rate (V) on silicic acid concentration conformed well to a Michaelis-Menten saturation model; maximum uptake rates (Vmax) ranged from 0.0022 to 0.0028 h−1 which corresponds to maximum growth rates of 0.08 to 0.10 doublings d−1. Half-saturation constants (km) ranged from 1.1 to 4.6 μM, a range similar to values found in other areas of the ocean and considerably lower than those previously reported for several Antarctic diatom species in culture studies. Results indicate detectable, but weak, substrate limitation of silicic acid uptake rate by the naturally occurring diatom assemblage in the western Ross Sea. Significant Si limitation in other subsystems of the Southern Ocean would be possible only if their resident diatom assemblages had much lower affinity for silicic acid than we observed.