Concentrations of nitrite and nitrate were measured using a
Hach DR/2500 spectrophotometer according to manufacturer’s
instructions (N-NO3 method 8039, N-NO2 method 10019). For
the measurement of total Kjeldahl nitrogen (TKN), the organic
nitrogen content was converted to ammonia for detection. 1 ml
Kjeldahl digestion solution (Clesceri et al., 1981) was added to
2.5 ml water sample in a 25 ml boiling tube. The tube was placed
in a block digestor at 185 ◦C for 3 h. Tubes were allowed to
cool. 200 l sample was mixed with 2.3 ml alkaline citrate (20%
w/v trisodium citrate, 1% w/v sodium hydroxide). For ammonia
(N-NH3) determination, 2.5 ml sample (water sample or
diluted digested TKN sample) was mixed with 0.1 ml phenol
solution (11.1% w/v phenol in ethanol), 0.1 ml sodium nitroprusside
(0.5% w/v) solution and 0.25 ml oxidizing solution (4:1 mix
of alkaline citrate and sodium hypochlorite (5% w/v)). Colour
development was allowed to proceed at room temperature and
in subdued light for 1 h. Absorbance was measured at 640 nm.