The study lake, which has been deliberately fertilized since 1969, stratifies during the summer, creating ideal conditions for the preservation of annual records of the algal community at the bottom of the lake. Study co-author Leavitt took a core from the lake bottom during winter while the lake was frozen, then analyzed the core in much the same way a botanist might analyze tree rings. Instead of using microscopes to identify the algae, the scientists used high-performance liquid chromatography to determine what kinds of algae were present, and how abundant they were, in sediment layers for every year from 1943 to 1990. Humans contribute to fertilization of lakes in a number of ways. Agricultural run-off is a major source of nutrients, but the burning of fossil fuels also contributes to the problem, since nitrates released into the air fall to earth in rain, said Cottingham.
Urbanization exacerbates the problem as well.
When rain falls on cities, it typically flows directly from city streets and storm sewers into lakes and rivers, bypassing the natural filtering process that occurs when rain percolates through soil. While this study only analyzed lake data, Cottingham suspects that other ecosystems could be experiencing similar phenomena. Virtually all ecosystems on Earth are currently being fertilized due to human activities. Furthermore, the study raises the broader issue of whether humans can accurately evaluate, predict and control their impact on the environment.