Rivers are sometimes polluted with untreated sewage. Figure 6 shows some changes that occurred when untreated sewage entered a river. Which type of organism had the most rapid increase in numbers when sewage entered the river?

The graph shows the population changes of three types of organisms after sewage is introduced. Sewage is a source of food for decomposer bacteria. The line representing bacteria shows a very sharp, immediate increase right at the point where sewage enters. The lines for green algae and protozoa also rise, but their increase is less steep and occurs further downstream. Therefore, bacteria had the most rapid increase.