Submicrometer particles in northwest Pacific coastal environments: Abundance, size distribution, and biological origins

Abstract

Submicrometer particles (SMP) are suggested to be a critical component for organic matter transitions in seawater, but little is known about variations and controls of SMP in coastal systems. We examined vertical and horizontal distributions of SMP (0.4-1 μm in equivalent spherical diameter as measured by a resistive pulse particle counter) and biological variables (chlorophyll a concentration, abundance of bacteria, and heterotrophic nanoflagellates) in northwest Pacific coastal environments. The abundance and total volume of SMP in the upper 200 m varied in the range of 5 x 104-3 x 107 particles ml-1 and 4 x 103-3 x 106 μm3 ml-1, respectively. Over a large trophic gradient (Chl a, 0.02-4 μg liter-1), the total volume of SMP was strongly positively correlated with Chl a concentration (r = 0.90, P < 0.0001, n = 47) and with other microbial variables (r = 0.84-0.90), consistent with a hypothesis that SMP dynamics are closely related to microbial food-web processes. Notably, size distribution of SMP in upper waters often exhibited a distinctive peak at a size range of 0.6-0.7 μm, which was most pronounced in productive nearshore waters and became less evident with depth and with distance from the shore. A sonication experiment revealed that the 0.6-0.7-μm particles are primarily nonliving. We hypothesize that SMP, particularly the 0.6-0.7-μm component, are directly produced by biological processes. Our data suggest that SMP are a highly reactive and abundant component of detrital colloids and play important roles in material cycles within coastal systems.