228Ra-derived nutrient budgets in the upper equatorial Pacific and the role of “new” silicate in limiting productivity

228Ra activities in the upper ocean (surface to ∼850 m) of the equatorial Pacific between 9°N and 12°S along ∼140†W were measured at five stations during the JGOFS EqPac 1992 Survey I cruise, when El Niño conditions prevailed in the area. The vertical profile of 228Ra at each station consists of measurements made on 2 to 3 m3 of water collected using submersible filtration systems in situ from 10–11 depths. 228Ra activities in the surface mixed layer range from ∼5 dpm M−3 at northern stations to ∼1.5 dpm m−3 near the equator. They decrease markedly between about 100 and 300 m, to concentration levels of 0.1–0.4 dpm M−3. The distributions manifest the occurrence of upwelling near the equator and downwelling between ∼3°N and 10°N. On the basis of the 228Ra and nitrate distributions, estimations of upward vertical fluxes of nitrate at various depth horizons at each of the stations have been made. Maximum fluxes of ∼2.0 to 3.5 mmol N m−2 day−1, averaging 2.6 mmol N m−2 day−1, occur near the base of the euphotic zone, about 100 m below sea surface. The average nitrate flux translates to a potential new production of about 17 mmol C m−2 day−1. While close to the result of the 15N tracer experiments, this new production estimate is significantly higher than the reported particulate organic carbon fluxes derived from the Th isotope and floating trap measurements. This implies that a significant fraction of the export production may have occurred in the form of dissolved organic carbon. The 228Ra-derived new production of 0.8 × 1015 g C year−1 for the equatorial Pacific region east of the dateline is approximately one-half of the value obtained by Chavez and Barber (1987) for a non-El Niño period. cycled fluxes of silicate, nitrate and phosphate to the euphotic layer bear molar ratios Si:N:P = 0.8:1:0.06. It is proposed that in upwelling regions of the equatorial Pacific, surface productivity is limited by the availability of “new” silicate, the silicate component that is decoupled from nitrate cycling and exported to the deep sea.