In the last three decades, more freshwater lakes across the planet are seeing intense summer algal blooms with potentially harmful effects, according to the first long-term global survey of dozens of freshwater lakes.
Harmful algal blooms (HABs) occur when colonies of simple aquatic plants collectively known as algae grow out of control, producing toxic effects that can harm people, fish, shellfish, marine mammals, and birds, according to the National Oceanic and Atmospheric Administration (NOAA). HABs occur due to a variety of reasons, from natural instigators like Kilauea’s volcanic eruption to human activities such as agriculture, urban development, and climate change.
"What we call an algal bloom is really a bloom of phytoplankton, which are microscopic organisms that photosynthesize," study author Anna Michalak told IFLScience. "Phytoplankton forms the base of the aquatic food chain, and are therefore not bad per se. But, when the blooms get too intense or when types of phytoplankton that form toxins take over, the blooms become harmful to people and to ecosystems."
Last year, a persisting red tide in Florida resulted in the deaths of marine mammals and dozens of sea turtles while earlier this summer, family dogs across the US died from exposure to fatal freshwater algae. Algal blooms can have detrimental effects on drinking water, agriculture, fishing, recreation, and tourism to the tune of around $4 billion every year in the US.
Until now, it was unclear whether algal blooms were getting worse on a global scale.
To come to their conclusions, researchers analyzed 30 years of data from NASA and the US Geological Society Landsat 5 near-Earth satellite, which monitored the surface of the planet between 1984 and 2013 at 30-meter resolution. The researchers also partnered with Google Earth Engine to process more than 72 billion data points. Publishing their work in Nature, the researchers found that algal blooms are “getting more widespread and more intense” across the globe in 71 large lakes across 33 countries on six continents. During the summertime, which correlates with peak intensity, the researchers found algal blooms increased in more than two-thirds of lakes and decreased in just six.
What remains unclear is exactly why this is happening. There is no consistent pattern across lakes with worsening blooms when the researchers accounted for factors like fertilizer use, rainfall, or temperature. Of the lakes that improved over the 30-year period, only those that saw the least warming were able to improve, leading scientists to believe that climate change is likely making it more difficult for lakes to recover.
Earlier studies have argued that either temperature, or rainfall, or nutrient addition through fertilizer use and other processes were the most important factors, but the researchers say catalysts may be much more localized. Warmer waters accelerate the growth rate of phytoplankton, and can also preferentially benefit more harmful types of phytoplankton. Therefore, as lakes get warmer, it becomes more difficult to manage water quality.
"What we found is that it depends on the local conditions around a particular lake. We found examples where each of these factors was the dominant driver, but no silver bullet that could explain changes across all lakes," said Michalak. "The one very clear signal is that those lakes that were able to sustain improvements in bloom conditions had either warmed very little or had in fact cooled over the period that we examined. This strongly suggests that warming may already be hindering recovery in other lakes."
The researchers conclude that their work highlights the importance of identifying factors that make some lakes more susceptible to climate change. With that information, they say communities can develop water management strategies that incorporate conditions influenced by human activities and climate change.