You know that routine when the electricity goes out, but you find yourself flipping the lightswitch every time you walk into a room anyway? That’s how I imagine the people of Toledo felt last week when they went to fill up the tea kettle- take a shower- wash an apple- top off the dog dish- that sharp realization of, oh, no, I can’t do that. The toxic cyanobacteria Microcystis had rendered their tap water unpotable, and just like that, 400,000 people were living with water, water everywhere, and not a drop to drink. News articles covered the human interest angle thoroughly, but were often disappointingly vague about the science behind the water ban. That’s nearly as worrisome as the event itself, because without policy that accounts for the science, last week in Toledo could be next week elsewhere in the country.
Although cyanobacteria temporarily deprived Toledo of water, they are responsible for another necessity of life: oxygen. About 3.5 billion years ago, cyanobacteria were the first organisms to generate oxygen, paving the way for aerobic respiration. Fast forward to the present, and it practically appears that we’re thanking those cyanobacteria for the favor by providing them with nutrients aplenty, courtesy of agricultural and wastewater runoff. The traditional paradigm holds that cyanobacteria growth is limited by P availability, but that has been challenged by Dr. Hans Paerl, a professor here at UNC-IMS. Dr. Paerl’s work has shown that even freshwater cyanobacteria are at least co-limited by N, meaning that they require both N and P to thrive. Many cyanobacteria have special features known as heterocysts that facilitate N fixation- the conversion of atmospheric N2 gas to a form of N that can be used for growth. It was formerly believed that cyanobacteria obtained their N requirements from fixation, fostering a lax attitude towards N regulations. Why control N content in runoff when cyanobacteria had a virtually inexhaustible supply of N2 in the air? Dr. Paerl’s work, however, suggests that cyanobacteria can in fact capitalize on anthropogenic N, making regulation imperative.
Dr. Paerl has studied lakes in which cyanobacteria that cannot fix N2 outnumber those that can, even when low N concentrations in the surrounding water would appear to give N2-fixers an edge. (For a ridiculous analogy, imagine that you and a bunch of termites are locked in a room for days with piles of wooden boards and only two hamburgers. You manage to survive on the hamburgers, but the termites are starving to death even though they could eat the wood.) Studies by a number of scientists have also suggested that toxic cyanobacteria obtain less than 50% of their N requirements through fixation, implying that the remainder is met through sources such as fertilizer runoff. There are still a number of questions about cyanobacteria N use, and UNC grad students are on the front lines of the inquiry. Alex Hounshell, a first-year PhD student in the Paerl lab, is studying the role of organic N in cyanobacteria blooms. She explained that, because organic N includes a vast array of large molecules like amino acids, it is difficult to analyze but necessary to understanding the overall role of N in eutrophic systems.
Cyanobacteria are not inherently problematic, but blooms of cyanobacteria or algae can have troubling consequences for aquatic environments. Algae scums prevent light from entering the water, killing plants. When those plants decompose, oxygen is used up and critters like fish die, sometimes in great numbers. However, cyanobacteria blooms like the one that plagued Toledo are extra scary because they skip the series of unfortunate events and go straight for the punchline: the cyanobacteria themselves contain toxins that are deadly or at least deleterious to humans and animals. Toledo’s cyanobacteria Microcystis produces toxins known as microcystins that can cause fatal liver hemorrhaging in acute doses and liver cancer under chronic exposure. No one has concluded exactly why Microcystis needs the toxin, but we’re likely to see more of it in the near future. Toxic cyanobacteria blooms (AKA cyanoHABs, short for Harmful Algal Blooms) are more likely to develop in warm, stagnant waters, and they can withstand saltwater intrusion. It’s almost like they evolved with global warming in mind.
Recent papers on cyanoHABs sound eerily prescient when reviewed after the Toledo incident. They recall those movie scenes in which the protagonist is given some priceless heirloom and told to be extremely careful with it; you know the thing is going to get broken in about twenty minutes. Lake Erie was repeatedly cited as an example of a chronically eutrophied lake, but it’s noted that this wasn’t always the case. Until the 2000s, Lake Erie’s algae blooms had been kept in check by robust P regulations, which have since loosened. N limitations are much less stringent, however, and since the Microcystis of last week’s headlines are non-N2 fixers, they obtain N from the nutrient-rich runoff that continues to flow into Lake Erie. This scenario is compounded by the fact that cyanobacteria can still access decades-old N and P as it’s recycled in the lake. Their nutrient requirements are typically well-satisfied, and that means virtually unchecked growth.
The resulting cyanoHABs and Toledo water restrictions are a sorry example of what may be an inherent part of human nature, to wax philosophical for a moment: we like to analyze and attempt to solve existing problems, but have substantially less motivation for preventing those problems in the first place. Global warming springs to mind as a ready example, but the trend is evident in finance, legislation, and even personal relationships. So yes, there may be more funding for better TMDLs or new constructed wetlands after the Toledo incident, but why does it take a drinking water ban to rally that kind of action? This fix-it-when-it-breaks attitude is detrimental to environmental and public health, and, more pressingly, it’s bumming out grad students. Alex commented that, although she expects the Toledo cyanoHAB will make it easier to apply for funding, she’s rather cynical about the American attitude towards the environment. She’s seen too many instances of society taking resources for granted, and guesses that concern about Lake Erie will soon be supplanted by another headline-grabber.
But on the off chance that it’s not, what can communities do to prevent eutrophication in the future? Some solutions have prohibitive drawbacks. Dredging physically removes nutrient-laden soils but usually results in habitat loss. Bubblers that mix the water column only work in small bodies of water. Most watersheds do not have enough “spare” water to flush a system. The best way to avoid eutrophication is to prevent nutrients from entering the waterway in the first place, through strict regulations regarding runoff and through Best Management Practices like bioswales, constructed wetlands, and tree boxes. Fish or plants can also be added to aquatic environments, allowed to grow, and harvested to remove nutrients that are then tied up in biomass. Excess P concentrations can be reduced by adding clays to the water, where they will bind phosphate and prevent its release from the sediment.
For any of these changes to happen, though, communities need to decide to prioritize the well-being of their watersheds. And if it doesn’t take an entire American city turning off its taps to spur that kind of action, it’s hard to say what will.
For more info:
Otten TG, Xu H, Qin B, Zhu G, Paerl HW. 2012. Spatiotemporal Patterns and Ecophysiology of Toxigenic Microcystis Blooms in Lake Taihu, China: Implications for Water Quality Management. Environmental Science and Technology 46: 3480-3488.
Paerl HW, Xu H, McCarthy MJ, Zhu G, Qin B, Li Y, Gardner WS. 2011. Controlling harmful cyanobacterial blooms in a hyper-eutrophic lake (Lake Taihu, China): The need for a dual nutrient (N & P) management strategy. Water Research 45: 1973-1983.
Paerl HW, Hall NS, Calandrino ES. 2011. Controlling harmful cyanobacterial blooms in a world experiencing anthropogenic and climatic-induced change. Science of the Total Environment 409: 1739-1745.
Paerl HW & Otten TG. 2013. Harmful Cyanobacterial Blooms: Causes, Consequences, and Controls. Microbial Ecology 65(4): 995-1010.
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