Increases in both the number of boats and in boat traffic in recent decades have brought negative consequences for the environment, researchers at the University of Gothenburg reported.
"The toxic, anti-fouling hull paints on the boats release both heavy metals and toxic substances to prevent growth on the boats," earth science professor Kjell Nordberg said.
"Motor boats also emit large amounts of noxious hydrocarbons and acidifying substances into the water through their exhaust fumes, which have increased as boats have become greater in number and are fitted with more powerful engines that also consume more fuel."
Various sediment samples in one fjord showed not only high levels of polyaromatic hydrocarbons and heavy metals but also a lack of oxygen during late summer and autumn, he said.
"You could tell from the occasionally black sediments, laminations and bacterial mats that this has been going on since the 1990s," Nordberg said.
"There's not much alive here in the summer."