# Concentration ranges of nitrogen? For a lake at Guangdong province, phosphorus concentration is measured at 0.01 mg/L. At which of the following concentration ranges of nitrogen, phosphorus is the...

Concentration ranges of nitrogen?

For a lake at Guangdong province, phosphorus concentration is measured at 0.01 mg/L. At which of the following concentration ranges of nitrogen, phosphorus is the limiting nutrient for this lake, compared with nitrogen?
(a) 0.01~0.02 mg/L
(b) 0.04~0.06 mg/L
(c) 0.1~0.2 mg/L
(d) 1~2 mg/L

(can have one or more than one answer!)

Thanks!

gsenviro | College Teacher | (Level 1) Educator Emeritus

Posted on

The lakes exhibit nutrient limited growth depending on the TN:TP ratio, where TN and TP are total nitrogen and total phosphorus.

Depending on various scientific literature, the range can be anything between > 17 to >50 (molar basis) for phosphorus limited growth.

Using these ranges and the molar weights of N and P as 14 and 31, resp.

we can use TN/TP > 50 (to be on the safer side), TN = TP x 50

Given, TP = 0.01 mg/l / 31 = 0.00032 mmoles/lt

and TN = TP x 50 = 0.00032 x 50 mmoles/lt = 0.016 mmoles/lt x 14 = 0.226 mg/lt.

So TN > 0.226 mg/lt.

We can also use the other accepted limit (TN/TP> 17) and get, TN > 0.077 mg/lt.

so either option (c) or (d) is correct.