Investigations of dissolved oxygen (DO) levels and related water quality in the Petchburi River, a regulated river system in western Thailand, were conducted at 3-month intervals from May 2010 to February 2012. The results indicated generally good water quality with DO levels >4 mg L−1. Variations in water quality, particularly the DO and ammonium nitrogen levels along the river, occurred due to discharge regulation and the natural characteristics of the river. The processes which contribute to DO levels include natural saturation (ca 57–78% in the upper river section and ca 44–76% in the lower river section) and aquatic plant DO production (ca 20–36% in the upper river section and ca 24–50% in the lower river section) which is also significant, while re-aeration would be less effective. The DO contribution from pondweed (family Potamogetonaceae) was of interest because of its successional capacity. Future control of river quality will require suitable management of aquatic plant species for conservation purposes.

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