Ballasting effects of smectite on aggregate formation and export from a natural plankton community

Strong correlations between particulate organic carbon (POC) and ballast minerals have been observed in the deep ocean. This has led to the postulation that ballast minerals can enhance POC flux by increasing the density and sinking velocity of ballasted aggregates and/or that ballast minerals protect the aggregated organic matter from degradation. Here we experimentally tested the influence of the ballast mineral smectite on the formation, size, dry weight, size-specific sinking velocity, carbon-specific respiration rate, and total POC flux of marine snow aggregates formed in roller tanks from a natural plankton community isolated from the North Sea. This study shows that the inclusion of smectite offers no protection against degradation of organic matter in freshly produced or aged marine snow aggregates. The main effect of ballasting with smectite was an increase in the density of the aggregates and, therefore 2- to 3-fold higher size-specific sinking velocities. Mineral ballasting had no influence on the total volume of aggregates or the total aggregated amount of POC. Nevertheless, the effect of increased sinking velocities in the ballasted treatment resulted in 2.7 ± 1.6 times larger potential POC fluxes compared to the non-ballasted aggregates. This implies that the incorporation of ballast minerals into sinking organic aggregates can increase the efficiency of the biological pump.