The Lancaster Canal, which winds its way through the city, is beginning to resemble a 'carpet of sludge', with some people fearing that the waterway is polluted.
The thick layer of mossy, brown scum first colonised the canal next to Shelley Road, Ashton in February, and is now oozing its way north towards Ingol.
But the Canal & River Trust (CRT), which is responsible for the upkeep of Lancaster Canal, said the 'sludge' is simply part of the waterway's natural life-cycle.
According to the Trust, the canal has been invaded by azolla, an invasive, non-native species of water fern.
The azolla can quickly spread if not treated, preventing local insects and amphibians from reaching the surface.
Dog walkers and boat dwellers have been told that the plant is not harmful to people or animals, but it can clog up the rudders and steering mechanisms of barges.
In a bid to reclaim the canal from the insidious pest, a colony of thousands of tiny beetles will be released into the affected waterway next week.
The tiny insects, known as weevils, are to be introduced to the canal where they will banquet on the thick carpet of azolla covering its surface.
But it might be a few weeks before Lancaster Canal is returned to its natural blue/green splendour in Preston.
This is because the CRT has been granted funding to treat the canal, but the weevils are currently 'out of stock'.
They are in high demand across the UK this spring and the Canal & River Trust is currently on a waiting list for the hungry critters.
A spokesman for the Canal & River Trust said: "I have spoken with our local ecologist about the azolla and she has advised that we plan to introduce weevils into the navigation.
"These are small beetles which feed on the weed and using weevils normally yields quick results.
"We were hoping to start this a couple of weeks ago. Sadly there seems to be a delay with the supply and we are still awaiting the delivery.
"As soon as we take receipt, they will be put into the canal.
"In the meantime, the local team have been installing some warning signage.
"I understand that some of this has been vandalised and removed. The teams will endeavour to replace this when resources allow."