Potential diffusion of nutrients is the movement of molecules along a gradient on concentration difference between the average concentration of the interstitial water in the sediment and that of the whole water column.
To determine the surface rate of the potential diffusion, the average concentration (ciw) of the interstitial water down to the layer with maximum value and the average concentration of the whole water column (cw) has to be determined. The average water content (w) of the interstitial water in the sediment down to the layer with maximum value can be determined by drying it until constant weight at 105°C. By applying Fick's first law, the surface rate (vp) of the potential diffusion of a given nutrient can be estimated as related to a unit surface are of the sediment.
Equipment and chemicals to determine the concentration of the particular nutrients are necessary.
The surface rate of potential diffusion as related to a unit area of sediment:
vp(nmol cm-2 h-1) =
| where: | DC | = | the diffusion constants (cm2 . h-1) of the investigated nutrient |
| k | = | porosity estimated from  | |
| ciw | = | average concentration (nmol cm-3) of the investigated nutrient in the interstitial water (taken to the maximum water concentration) | |
| cw | = | average concentration of the investigated nutrient in the water column (nmol cm-3) | |
| hs | = | thickness of the top layer of the sediment (cm) |
Calculation of average concentrations:
| where: | n | = | number of sediment layers taken up to the maximum concentration of interstitial water |
| ciw | = | concentration of the investigated nutrient in the interstitial water of the i-th layer of the sediment (nmol . cm-3) | |
| hiw,i | = | thickness of the i-th layer of the sediment (cm) | |
| hiw | = | height of the sediment at the maximum concentration (cm) |
| where: | m | = | the number of water layers in the whole water column |
| cw,i | = | concentration of the investigated nutrient in the i-th layer of the water column | |
| hw,i | = | thickness of the i-th water layer (cm) | |
| hw | = | height of the whole water column (cm) |
The direction of diffusion in from the sediment to the lake water if vp value is positive, and from the water to the sediment if vp value is negative.
The surface rate of potential diffusion (vp) as related to an extended sediment region and to a longer period of time can be obtained from surface rate values (vp, ik) of potential diffusion characteristic of different regions and periods of time.
| where: | tik | = | duration of different periods of time |
| n | = | number of different periods of time | |
| Fik | = | area of different regions | |
| m | = | number of different regions | |
| K | = | constants depending on the units of measurements chosen. |
