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 in the North Atlantic (North Atlantic Bloom Experiment-NABE) and Equatorial Pacific (EqPac). Beam attenuation due to particles in surface waters increased during the day by as much as 70% and decreased at night in both studies. In the equatorial Pacific the magnitude of the variations was larger during cool non-El Nino conditions (October, 1992) than during El Nino conditions (March/April 1992). However, the percent increase from the mean beam attenuation value for each time period was similar. The daytime increases result primarily from primary production, with possible contributions from changing optical effects of living cells. Nocturnal decreases in beam attenuation, reflecting particle loss, could be caused by grazing, remineralization, and the production of large particles (aggregates, fecal pellets) with subsequent settling. The nocturnal decreases also were associated with increases in the surface mixed layer depth and increases in nutrient concentrations. Part of these diet changes could result from mixing upward particle-depleted, nutrient-enriched water from below the daytime mixed layer. Since many biological and chemical species have strong gradients in the upper 100 m, regular oscillations in the depth of mixing can be an important forcing function for vertical exchange in surface waters. When it occurs, this ''mixed-layer pump'' is important in sustaining new primary production and in removing particles from surface waters, and should be included in models of oceanic surface mixing.