N P dC

m ^ n is given in Table 1. In most limnological applications, density is used for stability considerations. Hence, density refers to potential density, i.e., the density of a certain water parcel under normal atmospheric conditions (1013 hPa). As a consequence of the (small) adia-batic compressibility of water, in situ density increases with pressure, i.e., water depth, by about 5 x 10-10 Pa. This means that at 200 m depth, (potential) density and in situ density differ by about 10-3.

Density

As direct density measurements in the field are not accurate enough, indirect methods based on easy to

Stability

Stability of a water column derives from the density increase in the vertical. Hence, it is a measure for the

Table 1 Contribution of dissolved or suspended substances to the density of water

Substance

├čn[(kg/kg)J

Ca(HCO3)2

0.807

Mg(HCO3)2

0.861

Na(HCO3)

0.727

K(HCO3)

0.669

Fe(HCO3)2

0.838

NH4(HCO3)

0.462

CO2

0.273

CH4

-1.250

Air

-0.090

Modified from Imboden, DM and WUest A (1995). Physics and Chemistry of Lakes, pp. 83-138. Berlin: Springer-Verlag.

Modified from Imboden, DM and WUest A (1995). Physics and Chemistry of Lakes, pp. 83-138. Berlin: Springer-Verlag.

potential energy required for vertical excursion and for overturning of water parcels. Stability considerations can be made for an interface in the water column or for the entire stratified water body as a whole. If an energy source is known, the ratio of required and supplied energy yields a nondimensional number.

Differential Quantities The stability of a density stratification is quantified by

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