Reducing the phosphorus content in Lake Erie and other lakes could have unintended effects in terms of toxicity, according to a new study.
The strategy of cutting the levels of phosphorus in lakes is an effective one for checking hazardous algal blooms. However, findings newly published by Technische Universität Berlin (TU Berlin) researchers revealed that this approach may worsen toxicity in water bodies.
The new research was published in the journal Science. Researchers from the University of Michigan (U-M) were also involved in it.
“The big advance here was to integrate our understanding of the microbiology of the blooms into predictive models,” said study co-author Gregory Dick, an environmental microbiologist from U-M. “The results suggest that biologically informed models are able to reproduce emergent properties of blooms that are not predicted by traditional models.”
The study hints at a need to revise the approach that is currently favored for tackling dangerous algal blooms.
Algal blooms in areas such as Lake Erie house cyanobacteria. Also known as blue-green algae, this group of photosynthetic microbes can produce strong toxins capable of harming aquatic animals. They can also mar the quality of drinking water.
Around half a million residents of the Toledo area were denied tap water for about three days in 2014 as a result of contamination caused by cyanobacteria. Microcystis, one of the microbes in the group, had released high levels of the powerful liver toxin microcystin (MC) into Lake Erie.
Cyanobacteria use microcystin for protection. The toxin attaches to sites on critical enzymes for life processes in the microbes, thus shielding them from harsh hydrogen peroxide.
Blue-green algae depend greatly on phosphorus for sustenance. This explains why reducing phosphorus levels has been a preferred strategy for combating cyanobacteria globally.
Oxygen levels in lakes reduce as cyanobacteria pack up. More worryingly, the reduction in their number enables some other bacteria in the group to produce more of the toxic microcystin for protection.
Certain strains of cyanobacteria produce loads of microcystin while some others make very little if any.
“This diversity among bacterial strains is precisely what is responsible for the phenomenon that a reduction in phosphorus can lead to an increase in MC production,” explained Ferdi Hellweger, lead author and Water Quality Engineering chair at the Institute of Environmental Technology at TU Berlin.
Phosphorus reduction insufficient
There is a limited supply of phosphorus in nature to sustain cyanobacteria. Algal blooms usually result from agricultural runoffs that carry the nutrient into water bodies, such as Lake Erie.
The governments of the U.S. and Canada have, therefore, set a target of cutting phosphorus runoff into Lake Erie by 40 percent. Before now, efforts have been made to reduce phosphates in fertilizers. Tertiary wastewater treatment was also used to reduce phosphorus.
But, while decreasing phosphorus levels may reduce the cyanobacteria population, it could lead to greater toxicity. This is because the levels of nitrogen, another vital nutrient, rise. Higher amounts aid the remaining bacteria to produce the protective microcystin.
Researchers used an agent-based model – for the first time – to simulate the behavior of blue-green algae in Lake Erie. They developed the model following a meta-analysis of 103 studies dating back as far as 1958 and involving 708 experiments.
According to the team, sunlight helps to activate the gene for microcystin production. Blue-green algae that were at the water surface most times were more likely to produce the most amounts of the toxin.
Like phosphorus, nitrogen is also limited in supply. The deaths of some blue-green algae mean less competition for nutrients for the remaining ones. It especially implies more nutrients for those that produce microcystin for their defense.
Researchers say this suggests a need to rethink how water bodies are currently managed. It is important to reduce not just phosphorus but also nitrogen to more effectively combat toxicity, they said.
“This study supports the idea that phosphorus reduction will successfully reduce the overall abundance of cyanobacteria, consistent with the goals of current policies,” Dick said. “However, it also suggests that reducing phosphorus will cause an increase in abundance of the subset of cyanobacteria that are able to produce toxin, resulting in more toxin overall.”
U-M researchers in this study took field measurements at Lake Erie’s drinking water intake for Toledo. They analyzed Microcystis cells in algal blooms using environmental genomic methods to calculate the proportion of those producing and not producing toxins.