The River Lagan, one of Northern Ireland's major waterways, has been attacked by a highly invasive aquatic plant. Floating pennywort has been discovered along its banks.
So far over five tonnes of the weed has been removed.
The plant is native to North America.
It was first brought into Ireland as a plant for tropical aquariums and ponds, but it has since escaped into the wild at a limited number of locations in Northern Ireland.
It is capable of growing at a rate of 20cm a day and once established it can quickly form thick floating mats across the water's surface.
Many different invasive species are putting native plantlife under increasing pressure and in some cases, are the single biggest threat to local plants.
John Early, from the government's Northern Ireland Environment Agency, said it was important that people keep a look out for floating pennywort.
He said river users should inform authorities of where they have seen it so it can be removed.
"Floating Pennywort is a highly invasive aquatic plant that could potentially establish in the slower moving margins and slower flowing ditches which flow into the River Lagan," he said.
The weed is also present in several other areas of Northern Ireland.
At two sites, including the Glastry Clay Pits in County Down, it has been removed after a major and expensive project.
But it is almost inevitable that it will return in strength if the areas aren't constantly monitored.
According to BBC Northern Ireland's environment correspondent Mike McKimm it costs tax payers in Northern Ireland millions of pounds every year to control and remove invasive species.
"In most cases the offending plant arrived at its location because of human interference. Often it is innocently dumped as part of gardening waste or aquatic material," he said.
People even tip unwanted fish and reptiles into local streams and ponds with devastating effects on the local habitat."
Ecologists in Northern Ireland are watching alien species encroaching on a number of fronts.
Slipper limpets, muntjac deer and Japanese ironweed all have the potential to devastate local habitats.
And the so-called Sudden Oak Death disease, a fungus attacking Japanese larch trees, has been found in at least five woodlands in Northern Ireland.
Ironically it almost certainly came in on non-native or alien plants imported for ornamental gardens.