Primary production of marine snow during and after an upwelling event1

Marine snow (flocculent macroscopic particles) collected during an upwelling event near Santa Barbara, California, was recently formed and inhabited by phytoplankton physiologically similar to those free‐living in the water. At least 20% of all primary production, Chl a , and phytoplankton occurred on this marine snow, suggesting that a significant fraction of new organic carbon entering the upwelling food chain is available to large‐particle feeders or passes directly through the detrital food chain on particles. In contrast, marine snow collected 2 weeks after the upwelling event was inhabited by a physiologically distinct community of aging, senescent phytoplankton; <1% of total primary production and Chl a occurred on this marine snow. High dark fixation rates, high RUBPCase activity on photosynthetically depressed snow, and high ammonium concentrations within this snow suggest that chemoautotrophic bacteria, possibly nitrifying bacteria, may be particularly active on these macroscopic particles.