Algal Turf Scrubbing: Creating Helpful, Not Harmful, Algal “Blooms”

Thanks to the “green revolution,” food has never been cheaper or more abundant. Ironically, high-performance agriculture threatens our future ability to feed ourselves. Agriculture requires so much nitrogen and phosphorus fertilizer that it damages our soils, and the excess fertilizer pollutes local waters and the oceans, causing harmful algal blooms and extensive dead zones, threatening already overfished aquatic food resources and destroying the ecological assets, such as coral reefs, that support them and the rest of ocean life.

Half of our agricultural land is either partially or totally compromised, and the degradation continues. Compounding the problem, inexpensive phosphorus is only available from a few easily exploited deposits that will be exhausted within decades.


These problems have been known for many years, but economic and political forces have bought time by ignoring the problem rather than attempting to prevent or remediate it. Consumers choose the cheapest products, but farmers need to make a profit. This situation conspires with permissive regulations that encourage destructive behaviors. Even if consumers change their preferences to more expensive sustainable products and regulations improve, the ongoing damage would only be slowed. The opportunity to prevent the problems caused by modern agriculture is gone. A long-term solution is needed to reverse the negative trends and to repair the damage already done.

In the late 1970s and early 1980s, Dr. Walter Adey, of the Smithsonian Institution, was studying coral reefs in the Caribbean. These reefs support a huge mass of aquatic organisms, based on highly productive algae growth as the primary source of carbohydrates supporting a complex food web. However, concentrations of nitrogen and phosphorus in the waters surrounding the reefs appeared too low to support such rapid algal growth. Dr. Adey realized that as long as continuous water input exists, and the right kind of algae are present (the kinds that attach and grow in one place, remaining immobile while the water flows past them), any amount of nitrogen and phosphorus above a bare minimum is enough to support rapid growth. The water’s motion keeps incoming nutrients mixed to feed the algae, while also keeping the algal filaments moving so sunlight can penetrate more deeply. Algae proliferate rapidly under these conditions, feeding a crowded ecosystem.

Adey’s discovery became a solution when the Smithsonian’s Natural History Museum built the then-largest indoor tropical aquarium and found that keeping unwanted algae and nutrient levels under control (always an issue with aquariums) was achievable by creating an external algae cultivation system mimicking Caribbean reefs. Dr. Adey’s discovery had led him to develop a technology known today as “algal turf scrubbing.” Simply providing an artificial stream (or “floway”) designed to facilitate attachment and rapid growth of algae, mimicking the Caribbean coral reefs, replicated a natural ecological cycle. The nutrients in the aquarium water, waste products from animals and decaying organic matter, feed the algae. The aquarium water enters the top of the floway, is “scrubbed” by the algal “turf,” and flows out the bottom, returning to the aquarium. Frequent harvest removes the turf and thus the nutrients that would otherwise contaminate the aquarium. This system operated as the aquarium’s sole water cleaning method for several years, with no species lost.

Algal turf scrubbing is not limited to aquarium use. Because algal growth is rapid even at low nutrient concentrations, it is one of the best treatment methods available for non-point agricultural pollution anywhere. In a sense, algal turf scrubbing is a stationary algal bloom, desirable and controllable rather than unwanted and toxic – algae harvested from a floway is algae that will not bloom in a lake, reservoir, or the ocean. Unlike the algae in harmful blooms, the algae in a floway are diverse, do not produce high levels of toxins, and do not rot in place depriving local waters of oxygen and killing fish and other aerobic species. Better yet, algal cells are everywhere in our environment, ready to attach and grow wherever the appropriate environment is provided, already adapted to the local environment. The ecological balance thus encouraged is natural and abundant. Dozens of tons of algae can be grown per year on a single acre, limited only by the husbandry of the farmer harvesting the biomass every few days so that the remaining cells can again proliferate to fill the provided niche.


If well-distributed throughout agricultural areas, algal turf scrubbing facilities can recapture the nutrients that run off of farms before they accumulate downstream and cause unwanted blooms. The algal biomass can be recycled locally as a slow release fertilizer, reducing the amount of mineral fertilizer that needs to be imported or manufactured and returning organic carbon to the soil, reversing centuries of damage. It can be used as a feed in agriculture and aquaculture, and as a source of biofuel.

While some land (including non-agricultural land) would have to be devoted to building algae farms, only a small percentage of any given river basin would be required to completely recycle the nutrients that escape into the water. The company HydroMentia has been building and operating large-scale algal turf scrubbing facilities for decades to treat agricultural runoff. About 20 million hectares (47 million acres) are needed worldwide to completely recycle agricultural nutrient pollution. This is no small task. One percent of the global economy would need to be diverted to algae farming. In exchange, algae farmers would restore ecological balance, basin by basin, reversing damage to their soils and reducing the use of mined and petroleum-based nutrients, rescuing fish stocks by reducing or eliminating algae blooms and dead zones. In the process, 10% of excess atmospheric carbon would be captured in the algal biomass.

Farmers have fed the world; now they have the means to cleanse the waters of the world as well. With algal turf scrubbing as a widespread agricultural practice, a new balance between producing abundant food and keeping our waters clean can be found, if we have the will and wisdom to make it happen.

These findings are described in the article entitled, Expanded algal cultivation can reverse key planetary boundary transgressions, recently published in the journal HeliyonThis work was conducted by D. Calahan and W. Adey from the Smithsonian Institution and E. Osenbaugh from Softforce, Inc.




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