Some two billion zebra mussels now live in Lough Key. A few years ago they didn't exist in Ireland. But they are now found all over the Shannon system and nobody can tell precisely what effects this colonisation will have.
A research team from the School of Science at the Institute of Technology in Sligo is trying to establish a link between the zebra mussel population and nutrient levels in the water.
Frances Lucy is the lead researcher in the three-year study, which is funded by the Environmental Protection Agency. She has been involved in zebra mussel research since 1998.
She says one immediate problem is that there is "a dangerous concept out there" that zebra mussels are a solution to water pollution.
Phosphates in the water from agricultural fertilisers and household detergents fertilise plant growth in the water, including the phytoplankton, which zebra mussels feed on. The green pigment in phytoplankton has been taken in the past as an indication of the levels of water pollution.
With the arrival of zebra mussels, the water appears much clearer. Ms Lucy says that in Lough Key it is now possible to see to a depth of three metres compared to just 1 1/2 metres in 1998.
"But because they might mask pollution doesn't mean it has gone away," she says.
One aspect of the study will be to examine the impact of a new sewage treatment plant at Boyle which has a phosphate removal system. She says the way water quality is assessed will have to change.
In the past chlorophyll levels from the green pigment in phytoplankton were measured as an indication of nutrient pollution. But now chlorophyll levels may be dropping but the phosphates are still there.
She says when the plant at Boyle was being planned it was assumed it would be easy to assess the change in water quality but this was no longer true.
One possible effect is that while the zebra mussels are stripping the lake of phytoplankton, they are depositing nutrients at the bottom of the lake which will make more plants grow. They are also causing more light to get to the bottom of the lake which would also help these plants to grow. Water transparency could also change fish behaviour.
Anglers have also pointed out that by covering the bottom of lake they could also affect the hatches of certain types of insects, which provide vital food for trout and salmon.
Ms Lucy refers to the mussel as a "dynamic, invasive species" and says population densities reach 100,000 per square metre. She says there are "literally carpets of zebra mussels" on stony areas at the bottom of the lake.
The research team believes, although it is difficult to prove, that the population may have peaked because all the available space has been taken up. Generally the mussels are found in three areas, on stony areas of the lake bed, on reed beds, and on the much larger native swan mussel. A Marine Institute study has concluded that this native species has now practically been wiped out because of the colonisation of the zebra mussel.
They have an amazing capacity to reproduce, with a female producing about 30,000 eggs a year over two years before dying off. But the population may have reached a peak in Lough Key because the available space is used up and food supply is limited.
Ms Lucy stresses that they are only in the early stages of what they hope will extend beyond the three years of the current study. Earlier this year a small number of zebra mussels were found on Lough Gill, although early indications are they may have been caught in time.