Several researchers followed the uptake of microbial protein through the use of 15N enriched microbial biomass (Avnimelech,2007; Burford et al., 2004; Epp et al., 2002; Preston et al., 1996). From the technical point of view, this method is well established. To obtain 15N enriched microbial biomass, a portion of 15N ammonium salt is added to the water. Burford et al. (2003) found that natural biota in shrimp pond water becomes enriched with 15N tracer within 1 to 2 h. Avnimelech (1999, 2007) used starch addition to the water in order to enhance the 15N ammonium immobilization in the microbial biomass and found negligible TAN residues 24 h after the addition of ammonium salt solution.