Pond scum is nasty, but also more insidious than we realized. Cyanobacteria are able to change the way nitrogen and phosphorus cycle in lake water, allowing them to maintain themselves after an initial fertilizer pulse.
Also known as blue-green algae or pond scum, cyanobacteria form dramatic blooms under the right conditions, turning clear water murky. Being toxic to most other life forms, they can turn a beautiful lake or slow moving river into an ecological death zone. The consequences for humans achieved widespread attention when residents of Toledo, Ohio, were reduced to drinking, and even washing in, bottled water. While the take-over of one-fifth of one of their Great Lakes was astonishing, in drier parts of the world such events can be destructively common.
Such blooms have been occurring for billions of years, but have recently become more frequent as a result of nitrogen and phosphorus run-offs from excessive application in agriculture. However, blooms can last long after the initial drivers have been consumed.
“We usually think of cyanobacteria as responders to human manipulations of watersheds that increase nutrient loading, but our findings show they can also be drivers of nitrogen and phosphorus cycling in lakes,” says Professor Kathryn Cottingham of Dartmouth College, lead author of the paper in Ecosphere. "This is important because cyanobacteria are on the increase in response to global change—both warming temperatures and land use — and may be driving nutrient cycling in more lakes in the future, especially the clear-water, low-nutrient lakes that are so important for drinking water, fisheries and recreation."
The secret to cyanobacteria's capacity to maintain dominance over a body of water is through the release of nitrogen and phosphorus that other phytoplankton can't access. Prior to Cottingham's work, it was known that some cyanobacteria species can use nitrogen gas dissolved in water, just as symbiotic bacteria fix nitrogen from the air, converting it to a form plants can use.
It is only now being recognized that some cyanobacteria can release phosphorus that has settled in lake bed sediment, or is trapped in the muddy water just above. Once the cyanobacteria that accessed these nutrients reaches middle and surface waters, the nutrients become available to others that would not have been able to mine the original phosphorus themselves.
Credit: Cottingham et al. Cyanobacteria transport phosphorus upward in spring, on daily migrations or to near sedimentary waters.
The consequence is that an initial burst of nutrients, such as what often happens when fertilizer runs off from farms, can trigger a cyanobacteria outbreak that changes the ecology of the water source into a new state. “Cyanobacterial blooms warrant attention as potential drivers of the transition from a low-nutrient, clear-water regime to a high-nutrient, turbid-water regime,” the authors write. “A prediction of particular concern given that such blooms are reported to be increasing in many regions of the world due in part to global climate change.”
Shifting any ecosystem back can be challenging. As a general rule, it is better to maintain the resilience of an ecosystem prior to a bloom, where cyanobacteria numbers are low enough that other species remain dominant.