Coagulation efficiency, organic-matter glues and the dynamics of particles during a phytoplankton bloom in a mesocosm study

The combination of phytoplankton aggregate formation and subsequent sedimentation has been proposed as a mechanism for termination of phytoplankton blooms. To test this hypothesis, the evolution of a phytoplankton bloom was studied in a mesocosm. During a seven-day period, the concentration and size of small particles (<51-μm equivalent spherical diameter) and their coagulation efficiency, or stickiness, which is the probability that when particles collide they will remain together, was measured daily. During the study, the concentration of particles increased eight-fold, their mean volume increased six-fold and their volume fraction increased 45 fold. Particle stickiness dropped from 0.8 at the beginning to <0.1 towards the end of the study and was positively correlated with particle growth rate. These results are opposite of what has been speculated to occur during phytoplankton blooms. The relationship between stickiness and putative organic-matter glues, such as transparent exopolymer particles (TEP) and carbohydrates, was examined. Particle stickiness was strongly correlated with the chlorophyll weight-specific TEP concentration and the chlorophyll weight-specific carbohydrate concentration (both total and surface-active fraction). These relationships also hold if TEP and carbohydrate concentrations are normalized by other measures of volume or mass of particles. These results, on the relationship between particle stickiness and the chlorophyll weight-specific concentration of putative organic-matter glues (TEP, surface-active carbohydrates), can explain previous observations that the coagulation efficiency of particles declines with time during blooms. Employing the observed particle concentrations, sizes and coagulation efficiencies, the critical concentration at which growth of particles is balanced by losses due to coagulation and subsequent sedimentation was estimated. The estimates of the critical concentration towards the end of the study are in good agreement with the observed maximum concentration of particles. The dynamics of particles during this study appear consistent with predictions from coagulation theory.