Proposed reductions in phosphorous runoff from agricultural lands might not be enough to counter Lake Erie’s increasing susceptibility to toxic algae blooms, according to University of Michigan researchers.
"Our results suggest that current phosphorus loading targets will be insufficient for reducing the intensity of cyanobacteria blooms to desired levels, so long as the lake remains in a heightened state of bloom susceptibility," said lead author Daniel Obenour of the university’s Water Center.
That “heightened state” led to nearly half a million Ohio and Michigan residents being deprived of drinking water for several days in early August because of a cyanobacteria bloom containing the toxin microsystin.
The problem seems to be that the blooms are becoming more sensitive to phosphorus, according to the scientists.
But what has caused this and the corresponding increase in size of cyanobacteria blooms since the mid 1990s? That’s not so easy to explain.
Computer modeling revealed that a special form of phosphorus, DRP (dissolved reactive phosphorus) is more readily absorbed by algae, but it did not explain increased bloom susceptibility. Also, late-summer surface water temperatures did not increase enough to exacerbate the problem.
Exotic quagga and zebra mussels, however, could be a factor. The filter-feeding shellfish gorge on many species of phytoplankton, but avoid those that produce toxins. In other words, the latter now have less competition for nutrients, including phosphorus.
"We tested to see if the increase in the DRP fraction could be the cause, and it did not pass the test. It also does not look like water temperature is driving the increased susceptibility,” said Don Scavia, co-author and aquatic ecologist. “We're thinking it may have been the increase in mussels.
"As long as the lake remains in this heightened state of susceptibility, this problem is likely to persist,” he added. “That means we need to better understand what is driving the increased susceptibility and whether it can be controlled, or if deeper phosphorus reductions are needed.”