A model of the formation of marine algal flocs by physical coagulation processes

Abstract

Aggregates of organic matter (“marine snow”) are highly visible phenomena oceanic waters which can control material fluxes to the deep sea. These aggregates take many forms and undoubtedly have many causes. One form frequently described is that composed of algae, usually diatoms, occurring after an algal bloom. A model combining kinetic coagulation theory and simple algal growth kinetics describes the dynamics of such an algal bloom. Results show this to be a two-state system in which coagulation processes are either unimportant when algal concentrations are low or dominant when they are high, with a rapid transition between the two states. Critical algal concentration for this transition is inversely related to fluid shear, algal size and stickiness, and is similar to values observed after a bloom. The role of coagulation in controlling particle dynamics for more complicated aquatic systems will depend on other biological particle production and consumption processes, such as zooplankton feeding and defecation. These results emphasize the importance of measuring algal sizes and abundances when studying floc formation.