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A water test tells you what is in a river on the day you sample it. The living things in the river tell you what it has been through for weeks. Ruth Patrick turned that idea into a working method, using diatoms, single-celled algae. This post covers her life, her 1948 survey, the device she invented, and how to read a sample count the way she did.
Who was Ruth Patrick?
The National Academy of Sciences biographical memoir by Thomas Bott and Bernard Sweeney records that Ruth Patrick was born in Topeka, Kansas, on 26 November 1907 and died on 23 September 2013, aged 105. Her father, a lawyer with a naturalist's interests, took her on walks to collect from woods and streams and let her look through his microscope. She received her own microscope at seven.
She studied botany at Coker College and took her MS (1931) and PhD (1934) at the University of Virginia, both on diatoms. In 1933 she began as a volunteer at the Academy of Natural Sciences of Philadelphia, became curator of its Leidy Microscopical Collection in 1937 while still unpaid, and joined the paid staff only in 1945. In 1947 she founded the Academy's Department of Limnology, the study of inland waters, renamed the Patrick Center for Environmental Research in 1983.
What did the 1948 Conestoga survey show?
In the summer and autumn of 1948, the memoir explains, Patrick led a study of streams in the Conestoga River basin in Lancaster County, Pennsylvania. Some stretches received farm runoff, some sewage, some industrial toxins, and some were fairly clean. Instead of looking for one indicator species, her team measured the sources of pollution, the water chemistry, and the variety of bacteria, algae, protozoa, rotifers, insects and other invertebrates, and fish.
The result: as pollution increased, biodiversity decreased. The memoir notes that Thomas Lovejoy later called this the "Patrick Principle". She set out the approach in a 1949 monograph on the Conestoga survey and in a 1950 paper with the same title, A proposed biological measure of stream conditions.
What is a diatometer?
To compare rivers fairly, Patrick needed every sample to grow under similar conditions. She designed the diatometer, a clear acrylic rack that holds glass microscope slides and floats near the surface of a stream. Diatoms settle on the slides and grow as they would on natural surfaces, and the slides can be lifted out and examined.
Using it on rivers across the United States, she found three patterns, as the memoir describes:
Natural community: many species, each with small to moderate numbers.
Organic pollution (such as sewage): fewer species, a few of them in very large numbers.
Toxic pollution: fewer species and fewer individuals of each.
Worked example: reading three sample slides
Here are made-up counts of 200 diatom cells from three sites, for practice only.
Site A: 25 species; the commonest species is 20 cells.
Site B: 6 species; the commonest species is 150 cells.
Site C: 5 species; only 40 cells could be counted on the whole slide, and the commonest species is 12 cells.
Work it through:
Share of the commonest species at A = 20 ÷ 200 = 10%. Many species, none dominant: the natural pattern.
Share at B = 150 ÷ 200 = 75%. Few species, one hugely abundant: the organic pollution pattern.
At C, both the species count and the number of cells are low: the toxic pattern.
Two mistakes to avoid. First, do not judge a site by total cells alone; B has plenty of cells and is still unhealthy. Second, compare sites only when samples were collected the same way, which is exactly the problem the diatometer solved.
Our guides to biodiversity and conservation and our environment in Class 10 cover the ecology around this idea. If you want to see how changing one factor shifts a system, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
How did her work reach beyond the lab?
The memoir records that Patrick estimated she studied between 800 and 900 streams and rivers around the world, and that she helped found the Stroud Water Research Center in 1967. She advised Presidents Johnson and Reagan, worked with Congress on anti-pollution legislation, took part in developing the Clean Water Act passed in 1972, and in 1975 became the first woman elected to the board of the DuPont Company. She was elected to the National Academy of Sciences in 1970, only the twelfth woman at the time.
The National Medal of Science citation (1996) honours her "for her algal research, particularly the ecology and paleoecology of diatoms, and for elucidating the importance of biodiversity of aquatic life in ascertaining the natural condition of rivers and the effects of pollution."
What can a student take from how Patrick worked?
She stood in rivers. The memoir recounts that when someone in India asked whether it was really she in a photo of her team standing in a river, she said it was, and that by getting into the river she learned things she could understand no other way. She also built teams across many specialities, and designed a tool so that her comparisons would be fair.
Try it now: invent counts for two ponds, one healthy and one polluted with sewage, and explain in two sentences how a diatom count would tell them apart.
