Sample collection and handling
Two sets of water samples were collected in 500 ml Teflon bottles, at a depth of 5 cm, from April to November, 2008 in four paddy field sites based on their age of construction These sites include 1975 (34 years old), 1977 (32 years old), 1988 (22 years old) and 2004 (five years old). A set of the samples was used for chemical analysis and the other for biological analysis. For easy identification, three drops of Lugol’s solution was added to the water samples used for biological analysis. In each collection, the two sets of samples were transported in a cooler packed with ice blocks (to stop further biological activities) to the laboratory for analysis.