Even the researchers were surprised by the result.
"I assumed there would be an increase, because we know that plastic is very long-lasting in nature. But actually what we could see here was that the plastic concentration has actually been constant, it is not accumulating," professor Torkel Gissel Nielsen from the Technical University of Denmark tells Radio Sweden.
He cites several possible explanations for this, it could be that the plastic particles have settled at the bottom of the sea, or that the water streams have brought the particles out into the North Sea instead - or that the plastic has been degraded by bacteria.
"There are a lot of potential gaps in our knowledge about how plastic is recycled in the environment that we need to consider in future studies," says Torkel Gissel Nielsen.
The study is based on analysis of fish and plankton samples collected in southern Baltic Sea over the past 30 years. This is the longest period that has been covered by a study into microplastics in the Baltics.