“I'm angry at the people who decided that phosphate was growing algae. I'm not sure that I believe that.” –Sue Wright, Texas
Sue Wright, quoted above, was upset because in 2010, sixteen American states banned the sale of dishwashing detergent containing high levels of phosphorous, an aquatic pollutant that sometimes causes eutrophication (algal blooms). Unfortunately, phosphorous is a rather effective component of detergent, so phosphorous-free dishwashing detergents did not immediately perform quite as well as their predecessors. This led some consumers (like our pal Sue) to complain to detergent manufacturers, state governments, consumer protection agencies, and the media.
What I like most about Sue’s complaint is that her anger was directed toward “the people who decided that phosphate was growing algae” rather than the policymakers who drafted and enacted the legislation. Her implied logic is exquisite – a factual claim has resulted in legislation that negatively affects some aspect of my life, therefore I don’t believe this factual claim and furthermore am angry at those who made it!
So, who specifically should Sue have directed her anger toward? Which jackass scientist “decided that phosphate was growing algae”?
The answer, unsurprisingly, is that many independent studies (involving various research groups) have demonstrated that phosphorous pollution, under some conditions, will stimulate algal growth and lead to eutrophication (see Schindler 2006 for a review). Here, I will focus on just one of these studies, perhaps the most influential.
My real motivation for discussing this particular paper is the recent announcement that the Canadian Government is discontinuing its operation of the Experimental Lakes Area (ELA), a collection of 58 pristine lakes that for over 40 years have been set aside for long-term ecosystem monitoring and ecosystem-scale experiments (more on the ELA later).
Green sludge
In the 1960s and 70s, many North American rivers and lakes, especially the Great Lakes, were experiencing rapid declines in water quality (see here and here). Industrial and municipal effluents were stimulating the growth of algae and other aquatic plants (termed ‘eutrophication’) leading to unsightly mats of green sludge, oxygen depletion, massive die-offs of fish and other aquatic life, and problems with the taste and odour of municipal drinking water.
The August 1969 issue of Time Magazine describes the then deteriorating state of Lake Erie:
"Each day, Detroit, Cleveland and 120 other municipalities fill Erie with 1.5 billion gallons of inadequately treated wastes, including nitrates and phosphates. These chemicals act as fertilizer for growths of algae that suck oxygen from the lower depths and rise to the surface as odoriferous green scum. Commercial and game fish … have nearly vanished ... Weeds proliferate, turning water frontage into swamp. In short, Lake Erie is in danger of dying by suffocation."
The public, industry, and all levels of government agreed that something had to be done to curb the declining state of North American waterways. However, there was disagreement over the most effective course of regulatory action because at the time, scientists and policymakers were still debating which nutrients were responsible for eutrophication. Was algal growth primarily limited by carbon, nitrogen, or phosphorous?
Schindler 1974
Experiments are the best way to establish causation, but are not always feasible. For example, the best way to test the anthropogenic climate change hypothesis would be to release copious quantities of greenhouse gas into the atmospheres of a random sample of earth-like planets, leave another randomly-chosen bunch of planets untouched, and then compare change in climate across the two groups of planets. Clearly this is not feasible, and clearly we can’t experimentally pollute a bunch of lakes just for the sake of science. Right? Wrong. Well, wrong to the second assertion at least.
The aforementioned Experimental Lakes Area is (was) a wonderful place where scientists could manipulate whole lakes to test hypotheses on the scale of entire ecosystems. In the late 1960s and early 70s, David Schindler – a Canadian limnologist who at the time was director of the ELA – oversaw a number of whole-lake experiments designed to determine which nutrient (out of carbon, nitrogen, and phosphorous) was primarily responsible for eutrophication.
In an initial experiment, Schindler et al. added copious amounts of nitrogen and phosphorous to Lake 227 which naturally had an extremely low concentration of dissolved carbon. If algal growth was primarily limited by carbon (and not nitrogen or phosphorous), then the N + P treatment should not stimulate the growth of algae. However, this was not the case. Within weeks of the treatment, Schindler et al. observed that Lake 227 “was transformed into a teeming, green soup” with algal concentrations up to two orders of magnitude higher than nearby untreated lakes. Clearly, low levels of carbon had not been limiting the growth of algae.
In a second experiment, Schindler et al. divided another lake, Lake 226, into two equal halves using a large vinyl curtain that was sealed into the sediment and surrounding bedrock. The team added an equivalent amount of carbon and nitrogen to both halves of the lake, but added phosphorous to only one side. This manipulation resulted in what James Elser at Arizona State University has called “the single most powerful image in the history of limnology”.
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Figure 1. Lake 226 following fertilization with carbon, nitrogen, and phosphorous (below divider) versus carbon and nitrogen only (above divider).
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Just a few months after the nutrient additions began, the side of the lake receiving C + N + P was completely covered by a bloom of blue-green algae whereas algae levels on the C + N side were essentially unchanged from when the nutrient additions began. It was abundantly clear that phosphorous had been limiting the growth of algae in Lake 226.
In a final experiment, Schindler et al. manipulated a third lake, Lake 304, to test whether, and how quickly, a lake could recover from phosphorous-induced eutrophication. The team measured the concentration of algae in Lake 304 at approximately monthly intervals over the course of five years, between 1969 and 1973. For three of those years, 1971–1973, the lake received additions of carbon and nitrogen, and for two years, 1971–1972, also received phosphorous. The experiment therefore mimicked what might happen if governments took steps to limit the amount of phosphorous entering a polluted water body. The general finding was that summertime algal concentrations increased dramatically in 1971 and 1972 when the lake was being fertilized with C + N + P, but returned to near baseline levels in 1973 after phosphorous fertilization was discontinued.
