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 obtained during two time-series occupations of the equator at 140°W (TT008 and TT012). CTD/transmissometer profiles were routinely performed three times a day, roughly at dawn, noon, and just prior to sunset. Additionally, ‘diel experiment’ days of intensive profiling (every 3 h) were conducted twice during TTW8 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 particle concentration, and then integrating the particle load to the 1% and 0.1% light depths for each day yields the diel change in the particle load. Apart from changes in scattering and effective cross section, the diel change in the integrated particle load (IPL) represents the cycling of mass into and out of the small particle pool. ytime increase in the integrated particle load (ΔIPL, defined as the IPL from the evening profile minus the IPL from the morning profile) was converted to carbon units by assuming a 0.4 particulate organic carbon (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 T-012. 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 effects of growth, respiration, mixing, settling, grazing and aggregation, our data are not directly comparable to 14C uptake-based primary production measurements. Rather, the difference between the optical estimates of the change in the particle pool and primary production estimates can be ascribed to removal processes in situ, primarily grazing and aggregation.