Iron and zinc effects on silicic acid and nitrate uptake kinetics in three high-nutrient, low-chlorophyll (HNLC) regions

Abstract

Effects of Fe and Zn availabilities on silicic acid (Si(OH) 4 ) and nitrate (NO 3 -) uptake kinetics in natural phytoplankton assemblages were investigated in 3 high-nutrient, low-chlorophyll (HNLC) regions: off central California, the Southern Ocean and the eastern tropical Pacific. Fe addition significantly increased the maximum potential uptake rate (V max ) of Si(OH) 4 (Si V max ) by 2 to 3 times relative to controls in 8 of 10 experiments. Fe addition significantly increased the maximum potential uptake rate of nitrate (NO 3 - V max ) by 2 to 3 times in all 6 experiments measuring the effect of Fe on NO 3 - V max . Four experiments quantified the effect of Zn addition on Si V max and NO 3 - V max . Zn had a significant effect on Si V max in only 1 experiment, increasing values 2-fold relative to controls. Zn addition had significant effects on NO 3 - V max in 3 experiments, increasing NO 3 - V max almost 2-fold in 1 instance and decreasing NO 3 - V max by 23 to 46 % in 2 instances. Combined Fe and Zn additions had effects on Si and NO 3 - V max that were similar to those of Fe addition alone. In 4 experiments measuring Fe and Zn effects on the half-saturation constant (K s ) for Si(OH) 4 uptake (Si K s ), Fe addition increased Si K s over 3-fold relative to controls in one experiment and Zn addition decreased it by 61 % in another. The effect of Fe and Zn addition on the half-saturation constant for NO 3 - uptake (NO 3 - K s ) could only be measured at 1 location. Here, Fe addition had no effect, and Zn addition alone and in combination with Fe decreased NO 3 - K s by 68 %. Si(OH) 4 :NO 3 - drawdown ratios calculated from changes in nutrient concentrations during the experiments were very similar to Si V max :NO 3 - V max ratios measured at the end of the experiments using tracers, and the effects of Fe and Zn addition on drawdown ratios paralleled the effects of Fe and Zn on V max ratios. In general, changes in Si(OH) 4 :NO 3 - drawdown and V max ratios were driven by changes in NO 3 - drawdown and NO 3 - V max . Our data show that Fe and Zn availabilities have a direct effect on Si(OH) 4 and NO 3 - uptake kinetics in natural phytoplankton assemblages in HNLC regions, and that changes in kinetic parameters may be driving changes in nutrient drawdown ratios, providing further proof that Fe limitation may accelerate the 'silica pump' and lead to seasonal Si(OH) 4 limitation. These results have significant implications for Si and N biogeochemistry and new production in low-Fe regions. Changes in nutrient uptake kinetics can also have important implications for phytoplankton species succession and ecological dynamics.