THE DIEL CYCLE IN THE INTEGRATED PARTICLE LOAD IN THE EQUATORIAL PACIFIC - A COMPARISON WITH PRIMARY PRODUCTION

As part of the U.S. JGOFS EqPac process study beam c profiles were obt ained during two time-series occupations of the equator at 140 degrees W (TT008 and TT012). CTD/transmissometer profiles were routinely perf ormed three times a day, roughly at dawn, noon, and just prior to suns et. Additionally, 'diel experiment' days of intensive profiling (every 3 h) were conducted twice during TT008 and three times during TT012. The beam attenuation profiles clearly show a diel cycle, with morning lows and evening highs. Transforming the beam c data into suspended pa rticle concentration, and then integrating the particle load to the 1% and 0.1% light depths for each day yields the diel change in the part icle load. Apart from changes in scattering and effective cross sectio n, the diel change in the integrated particle load (IPL) represents th e cycling of mass into and out of the small particle pool. The daytime increase in the integrated particle load (Delta IPL, defined as the I PL from the evening profile minus the IPL from the morning profile) wa s converted to carbon units by assuming a 0.4 particulate organic carb on (POC) to particulate matter concentration (PMC) ratio. Our estimate of the net daily POC increase to the 1% light level averaged over the TT008 cruise was 26 mmol C m(-2) day(-1) (n = 7, SD = 7), and 41 mmol m(-2) day(-1) (n = 15, SD = 13) for TT012. The integration of the 0.1 % light level was 29 mmol m(-2) day(-1) during TT008, and 41 mmol m(-2 ) day(-1) for TT012. As the optical method in situ includes the affect s of growth, respiration, mixing, settling, grazing and aggregation, o ur data are not directly comparable to C-14 uptake-based primary produ ction measurements. Rather, the difference between the optical estimat es of the change in the particle pool and primary production estimates can be ascribed to removal processes in situ, primarily grazing and a ggregation.