Aggregation
Aggregates begin as the colloidal fraction, which typically contains particles sized between 1 nm and several micrometers. The colloidal fraction of the ocean contains a large amount of organic matter unavailable to grazers. This fraction has a much higher total mass than either phytoplankton or bacteria but is not readily available due to size characteristics of the particles in relation to potential consumers. The colloidal fraction must aggregate in order to be more bioavailable. Aggregation theory outlines main mechanisms by which marine aggregates may form and are as follows:
- Brownian motion
- Brownian motion, also known as the Brownian Pump in this case, describes the interaction between individual particles in the colloid fraction. Particles in Brownian motion interact randomly due to the impact of solute molecules. These interactions lead to collision and aggregation of these small particles. The small aggregates then collide and aggregate with other aggregates and particles until the aggregate in question is several micrometers in diameter.
- Shear
- Once particles have aggregated to several micrometers in diameter, they begin to accumulate bacteria, since there is sufficient site space for feeding and reproduction. At this size it is large enough to undergo sinking. It also has the components necessary to fit the "aggregate spinning wheel hypothesis." Evidence for this has been found by Alldredge and Cohen (1987) who found evidence of both respiration and photosynthesis within aggregates, suggesting the presence of both autotrophic and heterotrophic organisms.
- Differential Settling
- This form of aggregation involves particles sinking at different rates and their collision to form aggregates.
- Diffusive Capture
- Diffusive capture describes a particle that is advected into the diffusion-limited boundary layer of another particle and is eventually captured/collides with that particle.
- Surface Coagulation
- Aggregates may also form from colloids trapped on the surface of rising bubbles. For example, Kepkay et al. found that bubble coagulation leads to an increase in bacterial respiration since more food is available to them. In fact, a storm at sea could increase bacterial respiration by up to 36 fold for two to four hours.
- Filtration
- Particles and small organisms floating through the water column can become trapped within aggregates. Marine snow aggregates are porous, however, and some particles are able to pass through them.
- Bacterial Motility
- It is unclear how relative this mode of aggregation is to marine snow, but there have been limited observations of bacteria moving fast and far enough to capture colloidal particles.
Aggregation theory represents a two state system. At low cell concentrations aggregation is relatively unimportant and somewhat more unlikely. However, at higher cell concentrations it becomes increasingly important. A model has been proposed to characterize the formation of marine aggregates and the loss due to sinking:
- C1 is the concentration of the cells
- r is the radius of each cell
- G is the shearing rate
- α is the stickiness coefficient
- g is the growth rate.
Thus, aggregation of marine particles is more prevalent when cell and particle concentration is higher (e.g. algal blooms)
Read more about this topic: Marine Snow