Even remote alpine lakes at risk from increased nutrient pollution
FRISCO — The immoderate use of fertilizers in the last half century is literally choking some lakes to death and raises the risk of human exposure to dangerous toxins, scientists said after studying the proliferation of blue-green algae.
Those organisms have spread much more rapidly than any other type of algae in North American and European lakes, according to McGill University scientists, who published their findings in the the journal Ecology Letters. In many cases, the rate of increase has sharply accelerated since the mid-20th century.
Their study represents the first continental-scale examination of historical changes in levels of cyanobacteria, the scientific term for the photosynthetic bacteria that form blue-green scum on the surface of ponds and lakes during hot summer months. Cyanobacteria blooms pose a serious threat to drinking-water sources, because certain species contain toxins harmful to the liver or nervous system.
“We found that cyanobacterial populations have expanded really strongly in many lakes since the advent of industrial fertilizers and rapid urban growth,” said McGill University biologist Zofia Taranu. “While we already knew that cyanobacteria prefer warm and nutrient-rich conditions, our study is also the first to show that the effect of nutrients, such as phosphorus and nitrogen, overwhelm those of global warming.”
Alpine lakes at risk
The researchers from France, Italy, Spain, the UK, Malaysia, and Canada expected to find an increase in cyanobacteria in agriculturally developed watersheds. But they were surprised to find cyanobacteria multiplying in many remote, alpine lakes, where warming temps and nutrient loading from atmospheric sources are bigger factors than direct runoff.
The rapid increase in cyanobacteria identified in the study points to the potential for a parallel increase in the concentration of harmful cyanotoxins, said Taranu, who is now a postdoctoral fellow at Université de Montréal. While potentially toxic species don’t synthesize toxins at all times, studies have shown that one of the best predictors of toxin concentrations in lakes is the total abundance of cyanobacteria.
Cyanobacteria can produce toxins that cause damage to the liver or nervous system. The most common symptoms of acute exposure to harmful algal blooms are skin rash or irritation, gastroenteritis and respiratory distress.
Chronic, low dose exposures over a lifetime may also result in liver tumors or endocrine disruption. Preliminary studies also suggest that a recently isolated cyanotoxin may become more concentrated across food chains and may be associated with the formation of progressive neurodegenerative diseases such as Alzheimer’s, Parkinson’s and ALS diseases.
Although this latter work is still controversial among scientists, “Our results underline the importance of further research in this area,” Taranu said.
“Our work shows that we need to work harder as a society to reduce nutrient discharges to surface waters,” said Irene Gregory-Eaves, an associate professor of biology at McGill and co-author of the study. “Because diffuse nutrient loading (as opposed to end-of-pipe effluent) is the main issue, we need to build collaborations to tackle this complex problem,” she said.
“For example, partnerships among freshwater scientists and farmers are starting to happen, and more of this needs to take place, so that we can strike a balance between maximizing crop yields and minimizing excess fertilizer application.”