Biogenic silica production and the diatom contribution to primary production and nitrate uptake in the eastern equatorial Pacific Ocean

Abstract

To quantify biogenic silica production rates and estimate diatom contribution to primary production and nitrate uptake we examined the distribution of silicic acid, biogenic silica and silica production along zonal and meridional transects between 110°W and 140°W in the upwelling zone of the eastern equatorial Pacific during December, 2004, and September, 2005. Silicic acid concentrations [Si(OH) 4] in the upper 75 m were consistently between 2.5 to 4.0 μM but were twice as high near the equator at 110°W, compared to 140°W, consistent with a strong eastward shoaling of the nutricline. Euphotic-zone integrated biogenic silica concentrations [bSiO 2] had a narrow range, averaging 9.8±1.4 mmol Si m −2 during 2004 and 10.4±4.0 mmol Si m −2 in 2005. Minima in [bSiO 2] occurred within the equatorial undercurrent on all transects; creating atypical profiles where [bSiO 2] decreased with depth from the surface and then increased below the equatorial undercurrent. Specific production rates of biogenic silica (V b) were highest (∼0.2-0.3 d −1) in the upper 50 m, decreased to <0.05 d −1 at the base of euphotic zone, and were typically near zero at 150 m. Unlike other open ocean systems, there was a strong diel cycle in silica production in the euphotic zone as daytime rates were consistently 2-3 times higher than night rates. V b was significantly correlated with ambient [Si(OH) 4], but only when rate data were separated by the fraction of photosynthetically active radiation penetrating the sea surface that was present at each sampling depth (%I 0). This interaction between [Si(OH) 4] and light on V b was highly systematic, with the extent of Si limitation of V b being a linear function of %I 0. Consistent with these effects, the average V b at stations within one degree of the equator doubled from 0.11 to 0.21 d −1 between 140°W and 110°W paralleling the doubling of [Si(OH) 4]. Vertically-integrated silica production rates (per day) tended to be higher within the zone of active upwelling (within±1° latitude of the equator), but high rates were also observed south of 2°S. Variability in silica production rates along zonal transects showed spatial coherence with changes in the meridional velocity flow field forced by tropical instability waves. Despite such perturbations the mean integrated rate of silica production was very similar between cruises, 1.6±0.6 mmol Si m −2 d −1 in 2004 and 1.3±0.7 mmol Si m −2 d −1 in 2005; consistent with the idea that phytoplankton rate processes are much less variable in the equatorial Pacific than in other systems. Comparison of silica production rates to rates of C and N use indicates that diatoms were responsible for ∼18% and ∼13-18% of total primary production and nitrate uptake, respectively. Our results suggest that diatoms, despite their minor contribution to the autotrophic community biomass (∼6%), have a disproportionally large impact on organic matter cycling in the eastern equatorial Pacific.