The protection of earth's biodiversity requires a sophisticated understanding of how human activities can affect the relative abundances of species in natural ecological communities. Here, we report on an introductory biology laboratory activity in which students quantified biodiversity while investigating one of the most important controls on the biodiversity of an ecosystem: nutrient availability. Students established microcosms of six species of phytoplankton ("algae") in 50-mL beakers and exposed them to five different levels of inorganic nutrients. After two weeks, students used hemocytometers to count cells and compare the relative abundances of the algal species (i.e., their community composition) at different nutrient levels. The effect of nutrient level on biodiversity (measured by Simpson's reciprocal index) was significantly curvilinear, and best described as "U-shaped." Specifically, the algal community was most diverse at the lowest nutrient level, least diverse with a small amount of added nutrients, and intermediate in diversity at the highest nutrient levels. This convenient, quantitative investigation provided students an opportunity to consider how anthropogenic influxes of nutrients into ecosystems can lead to eutrophication, and how this phenomenon can have negative effects by decreasing the biodiversity of ecological communities. [For "Using Algal Microcosms in Introductory Biology Lab. II: The Influence of Biodiversity on Ecosystem Productivity," see EJ1285505.]