Low molecular weight and colloidal DOC production during a phytoplankton bloom

Dissolved organic carbon (DOC) in seawater collected during a spring bloom in Bedford Basin, Nova Scotia, Canada, was separated into low molecular weight and colloidal size fractions by cross-flow ultrafiltration. Total DOC and the organic carbon in the 2 size fractions were analyzed by high temperature catalytic oxidation. DOC accumulated in the mixed layer at the beginning of the bloom and contained a substantial amount of surface active material associated with an increase in river discharge. There was little variation in total DOC at the height of the bloom. Instead, the low molecular weight fraction increased and reached a maximum 3 to 6 d after the chlorophyll maximum. The colloidal fraction of DOC also increased but, at its maximum, accounted for only 16% of the total. This maximum in colloidal DOC was achieved 8 d after the low molecular weight maximum and almost 2 wk after the chlorophyll maximum. Colloidal material may have coagulated with phytoplankton aggregates at the height of the bloom and limited the accumulation of DOC in the mixed layer. During the decline of the bloom, the downward transport of this material associated with aggregates may have contributed to the eventual accumulation of DOC below the mixed layer. As a result, the downward transport of DOC should be determined from estimates of the low molecular weight and colloidal DOC produced by phytoplankton and include the colloidal carbon associated with aggregates.