Colloidal organic carbon and phytoplankton speciation during a coastal bloom

Abstract

During the 1995 spring bloom in Bedford Basin, Nova Scotia, dissolved organic carbon (DOC) in the photic zone was separated into low-molecular-weight and colloidal size fractions by cross-flow ultrafiltration. DOC, colloidal organic carbon (COC) and low-molecular-weight organic carbon (LOC) were then analyzed by high-temperature catalytic oxidation. COC (associated with the production of phytoplankton exudates) did not follow the concentration of bulk chlorophyll a (chi a) or the total number of phytoplankton cells. Instead, surface-active COC (that adhered to the ultrafiltration system) was produced early during the bloom when the diatom Skeletonema costatum was at a maximum. Later on, as the bloom began to decline, less surface-active COC (that remained largely free of the ultrafiltration surfaces) was produced and was associated more with variations in cell number of Chaetoceros socialis, the predominant diatom for most of the bloom. These results suggest that chl a or the total number of phytoplankton cells may not be reliable indices of the production of COC. On the contrary, the results suggest that relatively high COC concentrations were associated with specific diatom species. In addition, the maximum amount of COC was associated during the early stages of the bloom with a diatom (S.costatum) that was a small fraction (<5.6%) of total phytoplankton cell number. This finding, that relatively large amounts of colloids were associated with exopolymer production during the onset and development of a bloom, does not agree with reports suggesting that the production of exopolymers by diatoms is primarily an end-of-bloom occurrence.