RA-228-DERIVED NUTRIENT BUDGETS IN THE UPPER EQUATORIAL PACIFIC AND THE ROLE OF NEW SILICATE IN LIMITING PRODUCTIVITY

Ra-228 activities in the upper ocean (surface to similar to 850 m) of the equatorial Pacific between 9 degrees N and 12 degrees S along simi lar to 140 degrees W were measured at five stations during the JGOFS E qPac 1992 Survey I cruise, when El Nino conditions prevailed in the ar ea. The vertical profile of Ra-228 at each station consists of measure ments made on 2 to 3 m(3) of water collected using submersible filtrat ion systems in situ from 10-11 depths. Ra-228 activities in the surfac e mixed layer range from similar to 5 dpm m(3) at northern stations to similar to 1.5 dpm m(-3) near the equator. They decrease markedly bet ween 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 equa tor and downwelling between similar to 3 degrees N and 10 degrees N. O n the basis of the Ra-228 and nitrate distributions, estimations of up ward vertical fluxes of nitrate at various depth horizons at each of t he stations have been made. Maximum fluxes of similar to 2.0 to 3.5 mm ol 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 N-15 tracer experim ents, this new production estimate is significantly higher than the re ported particulate organic carbon fluxes derived from the Th isotope a nd floating trap measurements. This implies that a significant fractio n of the export production may have occurred in the form of dissolved organic carbon. The 228 Ra-derived new production of 0.8 x 10(15) g C year(-1) for the equatorial Pacific region east of the dateline is app roximately one-half of the value obtained by Chavez and Barber (1987) for a non-El Nino period. The recycled 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'' silicat e, the silicate component that is decoupled from nitrate cycling and e xported to the deep sea.