Photosynthetic quotient (P.Q) values with depth o f rivers in Trengganu were found to be lowered by high levels o f indigenous ammonium nitrogen, ranging from 1 .2 -0 .8 , contrary to temperate conditions where high levels o f nitrate increase the P.Q values from 1.5 to 2.0. This is attributed to the fact that nitrate, in lieu o f ammonia, produces two extra molecules o f oxygen per atom o f nitrate nitrogen assimilated, culminating in higher P.Q values. Nevertheless, the present findings tally well with that o f marine phytoplankton (ie. P.Q is 1.2) when ammonia is the prime source o f nitrogen.