INTENSIVE AGGREGATE FORMATION WITH LOW VERTICAL FLUX DURING AN UPWELLING-INDUCED DIATOM BLOOM

The surfaces of most pelagic diatoms are sticky at times and may there fore form rapidly settling aggregates by physical coagulation. Stickin ess and aggregate formation may be particularly adaptive in upwelling systems by allowing the retention of diatom populations in the vicinit y of the upwelling center. We therefore hypothesized that upwelling di atom blooms are terminated by aggregate formation and rapid sedimentat ion. We monitored the development of a maturing diatom (mainly Chaetoc eros spp.) bloom in the Benguela upwelling current during 7 d in Febru ary. Chlorophyll concentrations remained consistently high during the observation period (similar to 500 mg Chi m(-2)) and phytoplankton gre w at an average specific rate of 0.25 d(-1). The diatoms were extraord inarily sticky, with stickiness coefficients of up to 0.40, which is t he highest ever recorded for field populations. Combined with estimate s of turbulent shear in the ocean such stickiness coefficients predict very high specific coagulation rates (0.3 d(-1)). In situ video obser vation demonstrated the occurrence of abundant diatom aggregates with surface water concentrations between 1,000 and 3,000 ppm. Despite the very high concentration of aggregates, vertical fluxes of phytoplankto n were very low, with fractional losses <0.005 d(-1), and the aggregat es thus seemed to be near neutrally buoyant. Losses due to copepod gra zing were also low (similar to 0.025 d(-1)). Most of the aggregates we re colonized by the heterotrophic dinoflagellate Noctiluca scintillans that feed upon diatoms in the aggregates. The system appeared to be i n near steady state; specific diatom growth rate, coagulation rate, an d loss rate due to N, scintillans feeding were all of the same magnitu de (0.25-0.3 d(-1)) and the latter two varied in concert. Our observat ions provide only partial support for the population retention hypothe sis because aggregate buoyancy and N, scintillans grazing efficiently reduced the vertical flux of aggregates in this system.