Boundary Layer

What is a boundary layer, and why is it important? It the layer of water that is microscopically close to the algae; the water molecules that actually touch the algae:

Attachment 7262

(from: http://www.grc.nasa.gov/WWW/K-12/airplane/boundlay.html )

This boundary layer area of the water has zero flow, because it has to have the same flow as the algae, which of course is zero. Since there is no flow (velocity) here, nutrient transport through it is slow. The faster the water flow, the smaller the boundary layer, and the faster the nutrients can get to/from the algae from the water. Here is one explanation:

http://www.advancedaquarist.com/2014/11/aafeature ...

"A diffusion gradient within the Momemtum Boundary Layer may occur when an organism extracts a substance from the water column. A deficiency occurs when the substance cannot diffuse quickly enough through the MBL. Hence, diffusion rates depend upon the substance’s concentration in the water column and the thickness of the MBL – a substance in high concentration (such as magnesium at ~1,300 mg/l) should diffuse in a satisfactorily manner through a thick boundary layer, while a substance in a low concentration (such as many micronutrients) require ‘good’ water motion which results in a minimal MBL."

Here is another explanation:

Seagrasses: Biology, Ecology, and Conservation, p 199, by AWD Larkum, Robert Joseph Orth, Carlos M. Duarte:

"As water flows through seagrass [or algae] beds, a boundary layer develops on the sediment surface, as well as on each seagrass [and algae] component exposed to the moving water. The faster the water moves, the thinner the diffusive boundary layer (DBL) becomes, and consequently, the faster the transfer of molecules from the water column to the sediment and/or seagrass [or algae]. It follows then that when currents [flow] are weak, the flux of molecules to the seagrass [or algae] surface may be limited by diffusion through the [boundary layer] (i.e., physical limitation). Under those conditions, many biological sites or enzymes in the seagrass [or algae] tissue are available to assimilate molecules when/if [!] they reach the plant's [or algal] surface."

A way to reduce the boundary layer is to introduce air, because the thickness of water in air is zero. The more air, the more the water is moved away from the algal cells and the thinner the boundary layer is. A waterfall scrubber has about a 5 to 10 mm thickness layer of fast moving water before it reaches air; an upflow scrubber has practially zero distance when an air bubble rubs the algae, because the inside of an air bubble is "dry":

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