High-resolution observations of aggregate flux during a sub-polar North Atlantic spring bloom

An aggregate flux event was observed by ship and by four underwater gliders during the 2008 sub-polar North Atlantic spring bloom experiment (NAB08). At the height of the diatom bloom, aggregates were observed as spikes in measurements of both particulate backscattering coefficient ( b bp ) and chlorophyll a fluorescence. Optical sensors on the ship and gliders were cross-calibrated through a series of simultaneous profiles, and b bp was converted to particulate organic carbon. The aggregates sank as a discrete pulse, with an average sinking rate of ∼75 m d −1 ; 65% of aggregate backscattering and 90% of chlorophyll fluorescence content was lost between 100 m and 900 m. Mean aggregate organic carbon flux at 100 m in mid-May was estimated at 514 mg C m −2 d −1 , consistent with independent flux estimates. The use of optical spikes observed from gliders provides unprecedented coupled vertical and temporal resolution measurements of an aggregate flux event.