Deep-Sea Research, Vol.37, No. 3, pp. 401-411, 1990. 0198-0149/90$3.00+ 0.00 Printedin Great Britain. ~) 1990 PergamonPressplc RAPID RESPONSE PAPER Distribution of macroaggregates and fine-grained particles across a continental margin and their potential role in fluxes WILFORD D. GARDNER* and IAN D. WALSH* (Received 24 March 1989; in revisedform 1 September 1989; accepted 11 September 1989) Abstract--The distribution and size of large marine aggregates (>0.5 mm) photographed in situ were compared with the small-particle distribution determined from beam attenuation (660 nm) in a section across the shelf and slope of the northern Gulf of Mexico. On the shelf changes in beam attenuation matched the trends in large particle concentration. Over the slope, beam attenuation profiles resembled the aggregates profiles in the upper 400 m with a small surface mixed layer maximum, a decrease through the upper thermocline to a minimum, and a small intermediate nepheloid layer (INL) in both large and small particles between 300 and 400 m. At the two deepest stations beam attenuation remained low from the bottom of the INL to the seafloor, while concentrations of large particles increased through the lower water column to concentrations greater than in the surface water. The distributions suggest that both large and small particles are resuspended from the upper slope and advected seaward, but on the mid slope this mode of transport occurs mostly as large aggregates. From calculations of settfing speeds based on estimates of particle density, it appears that these aggregates are important in both the horizontal and vertical flux of particles along continental margins. INTRODUCTION AND BACKGROUND FROMtheir production and introduction in surface waters to their incorporation into the sediments, particles in the ocean are cycled and redistributed by biological, chemical and physical processes. The redistribution of these nutrient-rich materials has important implications for understanding and quantitatively modeling biogeochemical processes in the oceans. In the open ocean, surface production is generally the most significant source of particles, but near the continental margins the seafloor can also be an important source of particles as a result of resuspension and lateral advection. The distribution of particles in the oceanic water column traditionally has been studied by making optical measurements of light scattering or beam attenuation (BiscAYv and Err~ra~IM, 1977; PAK et al., 1988), by collecting water samples for filtration or electronic particle volume analysis (BREWER et al., 1976; SH~-LVON et al., 1972), or by in situ filtration * Department of Oceanography, Texas A&M University, College Station, TX 77843, U.S.A. 401