A lime-green River Lagan caught the eye of one commuter cycling the towpath to Belfast.
Caroline Bloomfield was cycling from Lisburn when a mile-long stretch of the river stopped her in her path.
"I was about three miles along the towpath when I noticed the water was bright lime-green. I had no idea why, but I took a photograph because it was so striking," she said.
The Northern Ireland Environment Agency is investigating.
Caroline tweeted her photograph on Monday and people replied that it was algal bloom and not such good news for the life of the river.
"People came back on Twitter and said it was run-off of fertiliser from the fields that causes the algal bloom," she said.
"They said it is not good for wildlife because it takes the oxygen out of the water."
She said that the picture is of a "side tributary, not on the main river".
A spokesperson for the Department of Agriculture said that the Northern Ireland Environment Agency (NIEA) had only very recently been made aware of the report of a possible algal bloom in the Lagan canal.
"From the evidence available, NIEA do not believe the picture supplied is of an algal bloom, but that this is an accumulation of water weed.
"In order to confirm the situation, a water quality inspector has been deployed to the area to investigate.
"Coincidently, NIEA collected three routine water quality samples in the Lagan Canal yesterday, but saw no evidence of the presence of algae in the waterway at that time."
The department said that an algal bloom can occur in warmer weather conditions when there can be a rapid increase in the growth of microscopic algae.
"These can be visually striking in colour and are known as an 'algal bloom'. The discolouration of the water results from the high density of pigmented cells.
"Algal blooms are often green or blue-green in colour, but may also be other colours such as dark brown or red depending on the species of algae.
"Freshwater 'algal blooms' can be the result of an excess of nutrients, particularly phosphorus in the water. Some blue-green algal blooms are potentially toxic."
It said the NIEA normally takes samples in order to identify whether reported blooms are a potentially toxic algal species and, if necessary, inform the local council who can decide if access to the waterway should be restricted.