The population of freshwater plankton called Holopedium has doubled in Ontario lakes between the mid-1980s and the mid-2000s, reports a study published this week in Proceedings of the Royal Society B. The plankton are thriving in lakes that are low in calcium — and the calcium levels of many lakes have fallen in recent decades because of acid rain and logging.
Holopedium populations are also growing in places such as Nova Scotia and the West Coast, where they have already started clogging up water filtration pipes, says John Smol, a Queen’s University biologist who co-authored the new paper.
Smol, working with collaborators from across Ontario and at Cambridge University in the U.K., examined the sediments that have accumulated in recent decades at the bottom of low-calcium lakes. They looked for the shells of animals such as Holopedium and water fleas — both of which are crustaceans, distantly related to shrimp — in order to estimate how population levels have changed.
They found that in recent decades, Holopedium populations have been exploding, while populations of water fleas or Daphniids — an important food for many small fish and invertebrates — are declining.
“We’re probably going to continue to see this trend happening in future,” said Smol, who is concerned about the possibility.
“Fewer things can eat these Holopedium,” he told CBC’s Quirks & Quarks in an interview that airs Saturday. “They’re larger, they’re covered in this jelly — some organisms simply don’t have the mouth big enough to handle them.”
The creatures also contain lower levels of nutrients such as phosphorus than water fleas.
Smol is also concerned that the creatures could increasingly clog up water intake valves — about a fifth of drinking water in Ontario comes from low-calcium lakes, he said.
The researchers think the explosion in Holopedium populations and the decline of water fleas has to do with falling calcium levels in the lakes.
Water fleas need a lot of calcium to build their shells, but Holopedium do not.
The decline in calcium levels is caused in part by logging, which prevents calcium in trees from being returned to the soil and eventually the lakes.
'Exporting' calcium in lumber
““Basically, we’re exporting calcium from the wilderness and putting it in our houses,” Smol said.
Another factor is acid rain caused by industrial pollution in past decades. The acid rain caused calcium in the soil to wash into the lakes all at once instead of being released steadily over time. While acid rain has largely been eliminated with better pollution control, there’s no easy way to replenish the calcium that was already washed away. Reintroducing calcium into the soil relies on the weathering of rocks over thousands of years.
“We disrupted this process,” Smol said. “It’s a very slow process, so it’s very hard to speed it up.”
That means the amount of jelly in our lakes will probably keep increasing in the future.