p_stl

Phosphorus settling rate outside the mid-year nutrient settling period

A negative settling rate indicates that the reservoir sediments are a source of P. A positive settling rate indicates that the reservoir sediments are a sink for P.

For natural lakes, measured phosphorus settling velocities most frequently fall in the range of 5 to 20 m/year although values less than 1 m/year to over 200 m/year have been reported (Chapra, 1997). Panuska and Robertson (1999) noted that the range in apparent settling velocity values for man-made reservoirs tends to be significantly greater than for natural lakes. Higgins and Kim (1981) reported phosphorus apparent settling velocity values from –90 to 269 m/year for 18 reservoirs in Tennessee with a median value of 42.2 m/year. For 27 Midwestern reservoirs, Walker and Kiihner (1978) reported phosphorus apparent settling velocities ranging from –1 to 125 m/year with an average value of 12.7 m/year.

The table below lists recommended apparent settling velocity values for phosphorus (Panuska and Robertson, 1999):

Description of water body nutrient dynamics	Settling velocity [m/year]
Shallow water bodies with high net internal phosphorus flux	<= 0
Water bodies with moderate net internal phosphorus flux	1 - 5
Water bodies with minimal net internal phosphorus flux	5 - 16
Water bodies with high net internal phosphorus removal	> 16