Pollution with plastic waste is not confined to the oceans but poses a growing threat to lakes as well.
They say the levels are similar to those found in samples taken from marine beach sediments.
They are concerned that these tiny plastic particles are accumulating in freshwater species and are "likely" to get into the food chain.
The research is published in the journal, Current Biology.
"It is, I think, a problem all over Europe and maybe all over the world” Prof Christian Laforsch University of Bayreuth
The problem of large amounts of plastic polluting the world's oceans has been well documented in recent years.
But research on the problems caused by plastic in lakes has been lacking.
Danger on the shore This new study looked at Lake Garda, a large, sub-alpine body of water. The researchers found significant concentrations of plastic in sediment samples. On the north shore they found around 1,000 larger particles per square metre and 450 micro-plastic particles in the same area.
"We were surprised," lead author Prof Christian Laforsch from the University of Bayreuth told BBC News.
"We have similar amounts of plastic particles in the sediment of the lake's ecosystem as we find in marine ecosystems."
Chemicals found in plastics can be poisonous, can damage endocrine systems or in some cases cause cancers.
They can also transport dangerous organic pollutants into clean environments like lakes.
Landfill to lake Previous research on fish and other marine creatures has shown that these species tend to accumulate tiny plastic fragments into their tissue. Prof Laforsch worries that this is happening in Lake Garda and elsewhere.
"What we show is that filter feeders and sediment feeders and organisms that feed on the surface layer of the lake, all swallow these plastic particles mistaking them for food.
"There might be impacts when it affects the hormone system, they could become sterile for example. It could also be, that when fish are feeding on these organisms they accumulate these particles also in their tissue."
The problem is being caused by human use of plastic materials say the researchers.
Apart from water sports, and tourist and fishing boats, the main sources of waste entering Lake Garda were discarded plastic products and debris which may originate from landfill sites.
The scientists are also concerned that the discovery of significant amounts of plastic in lake environments could have implications for human populations as the waters are often used for drinking and for agriculture.
And they argue that, as in the seas, plastic pollution is likely to be widespread in freshwater bodies.
"There is nothing particular about Lake Garda," said Prof Laforsch.
"We are testing in Bavaria and it looks pretty much the same. It is, I think, a problem all over Europe and maybe all over the world."
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