THE OCEANIC MIXED-LAYER PUMP

Diel variations in beam attenuation profiles in the upper water column were observed during Joint Global Ocean Flux Study (JGOFS) programs i n the North Atlantic (North Atlantic Bloom Experiment-NABE) and Equato rial Pacific (EqPac). Beam attenuation due to particles in surface wat ers increased during the day by as much as 70% and decreased at night in both studies. In the equatorial Pacific the magnitude of the variat ions was larger during cool non-El Nino conditions (October, 1992) tha n during El Nino conditions (March/April 1992). However, the percent i ncrease from the mean beam attenuation value for each time period was similar. The daytime increases result primarily from primary productio n, with possible contributions from changing optical effects of living cells. Nocturnal decreases in beam attenuation, reflecting particle l oss, could be caused by grazing, remineralization, and the production of large particles (aggregates, fecal pellets) with subsequent settlin g. The nocturnal decreases also were associated with increases in the surface mixed layer depth and increases in nutrient concentrations. Pa rt of these diet changes could result from mixing upward particle-depl eted, nutrient-enriched water from below the daytime mixed layer. Sinc e many biological and chemical species have strong gradients in the up per 100 m, regular oscillations in the depth of mixing can be an impor tant forcing function for vertical exchange in surface waters. When it occurs, this ''mixed-layer pump'' is important in sustaining new prim ary production and in removing particles from surface waters, and shou ld be included in models of oceanic surface mixing.