Limitations of Experimental Design in Microorganism Growth Study

A research study was conducted to analyze the effects of varying concentrations of a specific pollutant on the growth rate of aquatic microorganisms. Researchers set up five separate tanks containing identical microorganisms and varying concentrations of the pollutant: 0 ppm, 5 ppm, 10 ppm, 15 ppm, and 20 ppm. The tanks were monitored over a period of two weeks, measuring the growth rate of the microorganisms using optical density metrics.

The control group consisted of microorganisms in the 0 ppm tank, while the experimental groups included the remaining tanks. Researchers recorded the optical density at day 0, day 7, and day 14. The hypothesis was that increasing concentrations of the pollutant would lead to decreased growth rates in the microorganisms.

Based on this experimental design, which statement best describes a potential limitation of the study?