Study: Neonicotinoid use disrupts Japanese lake ecosystem

In Japan’s Shimane Prefecture, the rice is planted along the Lake Shinji watershed at the very beginning of May – in fact, two of the “planting week” days are public holidays as the month makes its transition from April. So when Japanese scientist Dr. Masumi Yamamuro wanted to look at the impacts of the neonicotinoid pesticides used in rice paddies and running off into the water, she and her team knew May was the time to do it.

What the Japanese researchers found after collecting and processing decades of data is that this class of pesticides, so threatening on land to bees and pollinators, is implicated in lower fishing yields but beyond that, the shift is because the neonicotinoids aren’t harming just once species but are disrupting the aquatic food web.

Here’s what Yamamuro, an environmental studies professor in the University of Tokyo’s Graduate School of Frontier Sciences, and researchers from partner Japanese institutions did. They checked for four of the neonicotinoids they studied, which are most often used in Japanese agriculture, by sampling the Lake Shinji water in April, May and June, and conducting rigorous chemical and other tests.

Meanwhile, they collected historic data from local authorities responsible for monitoring the presence of zooplankton in the lake, beginning from 1981 and ending in 2017. They also collected the Lake Shinji Fisheries Cooperative Association data for two species – eel and smelt – dating from 1981 through 2014. They verified the regional use of the neonicotinoids, which began in nearby rice paddies in 1993.

There was a reason Yamamuro honed in on the two lake species. That’s because it was clear the juvenile eel populations thrived after the neonicotinoids were introduced, but the smelt population collapsed in the years that followed. According to the researchers’ paper, published in the journal Science, the annual smelt harvest went from 240 tons to just 22 tons on Lake Shinji.

Yet what makes them think it’s the neonicotinoids? It’s because they think their use is linked to vanishing zooplankton concentrations, which plummeted after neonicotinoids were first used, and the smelt depended on the food source in the broader ecosystem in a way that the eels – which saw their populations thrive – just didn’t. What that suggests is the neonicotinoid use is disrupting the aquatic food web and multiple species in it.

“To understand the long-term ecosystem impacts of contaminants, one must study entire ecosystems for a long time,” said marine scientist Olaf Jensen of Rutgers University, who wrote a commentary on the work in the same journal issue. “Yamamuro, et al, have done just this, demonstrating that neonicotinoid pesticides can affect entire food webs.”

While that may be the case at Lake Shinji, more research is needed into the use of neonicotinoids and the potential for harm on land, sea, and the delicate ecosystem balances between the two. Yamamuro and her colleagues believe the same disruption is likely to occur beyond Japan’s shores, because neonicotinoids are currently the most widely used class of insecticides globally.