ABSTRACT: The silica cycle in the upper 200 m of the central North Pacific was examined to further assess the role of oligotrophic mid-ocean gyres in the global marine silica cycle and to evaluate the role of diatoms in the regional carbon and nitrogen cycles. Siliceous biomass in the upper 200<\f>m was very low (generally <50 nmol Si l^-1 of biogenic silica) with higher concentrations (100 to 280 nmol Si l^-1) observed occasionally in the deep chlorophyll maximum and in the nitracline. Doubling times for biogenic silica were generally between 2 and 5 d, suggesting fairly rapid diatom growth. Kinetic experiments showed widespread limitation of silica production rates by ambient silicic acid concentrations (0.9 to 3.0 µM). Inputs of iron inferred from high concentrations of lithogenic silica (up to 300 nmol Si l^-1) in surface waters did not stimulate silica production. Integrated silica production rates averaged 1.24 mmol Si m^-2 d^-1 (range 0.47 to 2.9 mmol Si m^-2 d^-1). This average is 2 to 3 times higher than those reported for other oligotrophic mid-ocean gyres, significantly increasing estimates of the fraction of global silica production occurring in these systems. A pronounced diatom bloom dominated by Mastogloia woodiana and Hemiaulushauckii was observed ca 200 km north of the Joint Global Ocean Flux Study (JGOFS) Hawaii Ocean Time-series (HOT) site in August 1995. A doming of isopycnals within the nutricline beneath the bloom suggested the presence of a cyclonic eddy that was enhancing local diatom production through eddy pumping. The exceptionally high biomass levels (up to 250 nmol Si l^-1) and high silica production rates (1.8 mmol Si m^-2 d^-1) within the bloom suggest that diatom blooms triggered by the mesoscale flow field may contribute significantly to regional silica production. Diatom primary production in the central North Pacific estimated from the overall average integrated silica production rate and diatom Si:C ratios is 9.5 mmol C m^-2 d^-1 (= 114 mg C m^-2 d^-1), which is 25% of the average primary productivity at the JGOFS HOT site. Silicic acid and nitrate supply rates to the euphotic zone inferred from the gradients of the 2 solutes in the nutricline are in about the same proportion as diatom cellular Si:N ratios. Thus, net consumption of the silicic acid supplied to the surface layer requires that diatoms account for a significant fraction of new production in the central North Pacific.

KEY WORDS: Diatoms · Elemental cycles · New production · Silica cycling · Silicon

Published in MEPS Vol. 167 (1998) on June 18
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998