THAILAND - Thanakrit Sornvanee, head of the Bangkok Metropolitan Administration (BMA) Water Quality Management Bureau's working team for water-quality analysis, said the ecosystem downgrade included water quality and trees.
Thanakrit said that feeding the fish was not usually allowed because park officials fed them and if people threw food into the water it could adversely affect the fish.
He said the protesters' daily activities had an impact on the ecosystem because many cooked and bathed there, hence more waste was being released into the lake.
He said the park's dissolved oxygen (DO) level peaked at 8.0 some days during the PDRC rally. If the DO was over 4, fish risked dying.
Recognising the risk, he said the BMA had adjusted its water-treatment strategy to ensure that the DO was not too high, including water from lake being pumped to a retention pool and treated before being released back into the lake.
On average, he said the BMA had managed to keep the DO at 4.
Thanakrit said his team normally tested the water quality once a month but now did it three times a week.
A park worker, who asked not to be named, said surface aerators had been added to the park to increase oxygen in the water but no chemicals were used because it might effect animal life.
"We don't worry about water quality because water will rehabilitate its own quality, but we are concerned about the trees in the park because we can't water trees in some area as protesters have set up their tents near them," he said.
The 360-rai (57.6-hectare) park, Bangkok's largest, offers rare open public space, trees and playgrounds for the city's residents and visitors, while people can rent boats for use on its large man-made lake.
Walking paths around the park cover 2.5 kilometres in total and are popular for evening strolls and jogging.
Officially, cycling is only permitted between 10am and 3pm. There is a smoking ban throughout the park, and dogs are not allowed.