Chronic substrate limitation of silicic acid uptake rates in the western Sargasso Sea

Abstract

The kinetics of Si uptake at the JGOFS Bermuda Atlantic Time-series Study (BATS) Site in the western Sargasso Sea (31°50′N, 64°10′W) were examined on nine cruises between November 1991 and November 1993. Si uptake rates were severely substrate-limited on every cruise during all seasons. Surprisingly, the observed uptake kinetics were among the most inefficient examined to date despite persistently low (<0.9 μM) ambient silicic acid concentrations ([Si(OH) 4 ]) throughout the upper 100 m. Uptake kinetics did not usually conform to the Michaelis-Menten function. Rather, rates increased approximately linearly with increasing substrate concentration to ca 5.5 μM, the highest concentration tested, on seven of the nine cruises. Half saturation constants on the remaining two cruises when some degree of hyperbolic response to increasing [Si(OH) 4] was observed were 1.6 ± 0.55 and 2.6 ± 1.5 μM (s.e.). The slopes of the linear kinetic curves ranged from 1.5 × 10 −3 to 8.3 × 10 −3 h −1 μM −1 with no clear seasonal trend. Ambient [Si(OH) 4] restricted in situ uptake rates to < 12–16% of those measured at the highest concentration employed, ca 5.5 μM. These results indicate that substrate limitation of Si uptake is both chronic and severe in this region of the Sargasso Sea. The observed severity of uptake rate limitation strongly suggests Si limitation of diatom growth rates. Comparison of estimated in situ Si uptake rates with silica dissolution rates within sediment traps deployed at the base of the euphotic zone suggest that diatom assemblages with the observed uptake kinetics would require [Si(OH) 4] greater than ca 0.6 μM to support Si uptake rates sufficient to balance losses due to dissolution. This may explain why Si(OH) 4 is never depleted to the nanomolar concentrations observed for NO −3 and HPO 4 2− at this site. However, essentially all Si(OH) 4 in excess of the threshold concentration of ca 0.6 μM is utilized by diatoms, suggesting that Si limits the yield of siliceous biomass. Limitation of the yield of diatoms by Si is consistent with the estimated mole ratio of N:Si:P supplied annually to the base of the surface layer from depth. That ratio, 20:5.2:1, demonstrates a clear deficit of Si(OH) 4 in the upward flux of those nutrients compared to Redfield proportions.