Ratios of Si, C and N uptake by microplankton in the Southern Ocean

Nutrient uptake ratios and the Si:C:N composition ratios of particulate matter were measured within a diatom bloom located between the Antarctic Polar Front and the edge of the receding pack ice in the Pacific sector of the Southern Ocean along 170°W from 2 December 1997 to 3 January 1998. Rates of silicic acid, nitrate, ammonium and urea uptake were measured along with simultaneous estimates of net and gross carbon uptake derived from net oxygen production and 18O gross production rates. Concurrent measures of silica dissolution rates allowed net silicic acid uptake to be estimated. Comparison of integrated rates of gross silicic acid uptake, gross carbon uptake, and total nitrogen (nitrate+ammonium+urea) uptake revealed that gross uptake ratios within the bloom closely matched Redfield proportions. The mole ratio of gross Si uptake to total N uptake averaged 0.96±0.08, indistinguishable from the mean value of 0.90 characteristic of nutrient-replete diatoms in culture. The mean gross Si uptake: gross C mole uptake ratio, 0.20±0.02, was 50% higher than the typical diatom Si:C ratio of 0.13. Gross C:total N uptake mole ratios averaged 5.2±0.4, reasonably close to the Redfield value of 6.6. In contrast, average net uptake ratios were significantly higher than Redfield (C:NO3−=9.1±1.5, Si:C=0.52±0.12, Si:NO3−=3.7±0.4) and exceeded their corresponding gross uptake ratios by 180–380%. On average, net uptake was somewhat enriched in C, and greatly enriched in Si, over gross uptake; ratios of net uptake exceeded corresponding Redfield (C:N) or typical diatom (Si:C, Si:N) ratios by 38% for C:N, by a factor of 4.0 for Si:C and by a factor of 4.1 for Si:N. Thus, there was a transition from the plankton taking up nutrients in nearly Redfield proportions, to particles becoming slightly enriched in C over N and highly enriched in silica relative to organic matter through the combined effects of metabolic losses and food web processes. These results point to the operation of an efficient silicate pump that enriches particles in silica relative to organic matter prior to their export. We hypothesize that low [Fe] accentuated the decoupling between the cycling of silica and organic matter in the bloom. The average [Fe] in the mixed layer (0.13–0.29 nM) falls in a range that has been shown experimentally to dramatically increase Si(OH)4:NO3− uptake ratios by plankton assemblages in our study area. That result, together with the observation that gross uptake ratios of Si(OH)4 to total N were nearly in Redfield proportions, suggests that the main effect of low [Fe] was to increase reliance on regenerated nitrogen sources by inhibiting nitrate uptake. More efficient recycling of particulate nitrogen in surface waters compared to biogenic silica led to the selective depletion of silicic acid over nitrate, creating a positive feedback between [Fe] and the silicate pump, where low [Fe] accelerates the onset of Si limitation and diminishes the export of organic matter relative to silica.