Deep-Sea Research II, Vol. 42, No. 2-3, pp. 465-477, 1995 zyxwvutsrqp 0%7-0645(95)ooo3&5 Copyright fQ 1’995 Elsevia science Ltd Printed in Great Britain. AU rights reserved 0967-0645/95 s9.50+ 0.00 zyxwvutsrqpon The die1 cycle in the integrated particle load in the equatorial Pacific: A comparison with primary production IAN D. WALSH,* SUNG PYO CHUNG,* MARY JO RICHARDSON* and WILFORD D. GARDNER* (Received 5 July 1994; in revised form 13 February 1995; accepted 2 March 1995) Abstract-As part of the U.S. JGOFS EqPac process study beam c profiles were obtained during two time-series occupations of the equator at 14o”W (‘IT008 and lTO12). CTD/transmissometer profiles were routinely performed three times a day, roughly at dawn, noon, and just prior to sunset. Additionally, ‘die1experiment’ days of intensive profiling (every 3 h) were conducted twice during lTUO8 and three times during TTO12. The beam attenuation profiles clearly show a die1 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 die1 change in the particle load. Apart from changes in scattering and effective cross section, the die1 change in the integrated particle load (IPL) represents the cycling of mass into and out of the small particle pool. The daytime increase in the integrated particle load (AIPL, 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 TlBO8 cruise was 26 mmol C m -’ day-’ (n = 7, SD = 7) and 41 mmol m -’ day-’ (n = 15, SD = 13) for TTo12. The integration of the 0.1% light level was 29 mmol mm2day-i during ‘lTOO8,and 41 mmol mm2day-’ for lTO12. 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 sifu, primarily grazing and aggregation. INTRODUCTION Optical variability in the upper ocean derives from changes in the concentration and optical properties of suspended particles. Phytoplankton are a key, if not dominant, component of the small particle concentration (e.g. Kiefer and Austin, 1974; Kirk, 1983; Siegel et al., 1989; Olson et al., 1990; Dickey, 1991; Stramska and Dickey, 1992). Transmissometers have been an effective tool in measuring optical variability, from which we can infer spatial and temporal variability of particle concentrations (e.g. Gardner et al., 1990, 1993; Dickey ef al., 1993). Die1 variations of the beam attenuation coefficient (c) at 660 nm have been used to estimate primary production (Siegel et al., 1989; Cullen et al., 1992; Walsh et al., 1992). However, laboratory studies have found that physiological changes can contribute to changes in the beam attenuation coefficient (Ackleson et al., “ Department of Oceanography, Texas A & M University, College Station, TX 77843, U.S.A. 465