CHRONIC SUBSTRATE LIMITATION OF SILICIC-ACID UPTAKE RATES IN THE WESTERN SARGASSO SEA

The kinetics of Si uptake at the JGOFS Bermuda Atlantic Time-series St udy (BATS) Site in the western Sargasso Sea (31 degrees 50'N, 64 degre es 10'W) were examined on nine cruises between November 1991 and Novem ber 1993. Si uptake rates were severely substrate-limited on every cru ise during all seasons. Surprisingly, the observed uptake kinetics wer e among the most inefficient examined to date despite persistently low (<0.9 mu M) ambient silicic acid concentrations ([Si(OH)(4)]) through out the upper 100 m. Uptake kinetics did not usually conform to the Mi chaelis-Menten function. Rather, rates increased approximately linearl y with increasing substrate concentration to ca 5.5 mu M, the highest concentration tested, on seven of the nine cruises. Half saturation co nstants on the remaining two cruises when some degree of hyperbolic re sponse to increasing [Si(OH)(4)] was observed were 1.6+/-0.55 and 2.6/-1.5 mu M (s.e.). The slopes of the linear kinetic curves ranged from 1.5 x 10(-3) to 8.3 x 10(-3) h(-1) mu M(-1) with no clear seasonal tr end. Ambient [Si(OH)(4)] restricted in situ uptake rates to <12-16% of those measured at the highest concentration employed, ca 5.5 mu M. Th ese results indicate that substrate limitation of Si uptake is both ch ronic and severe in this region of the Sargasso Sea. The observed seve rity of uptake rate limitation strongly suggests Si limitation of diat om 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 u ptake kinetics would require [Si(OH)(4)] greater than ca 0.6 mu M to s upport Si uptake rates sufficient to balance losses due to dissolution . This may explain why Si(OH)4 is never depleted to the nanomolar conc entrations observed for NO3- and HPO42- at this site. However, essenti ally all Si(OH)(4) in excess of the threshold concentration of ca 0.6 mu M is utilized by diatoms, suggesting that Si limits the yield of si liceous biomass. Limitation of the yield of diatoms by Si is consisten t with the estimated mole ratio of N:Si:P supplied annually to the bas e 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 com pared to Redfield proportions. Copyright (C) 1996 Elsevier Science Ltd