Explanation for the decline of bacteria introduced into lake water.

The sizes of the populations of individual bacterial species diminished following their addition to water from lakes with different trophic levels at temperatures of 5, 10, 15, and 30°C. Some species persisted after their initial reduction in cell numbers, but others were undetectable after 3 to 15 days. The decline of these introduced bacteria was not a result of their inoculation at higher densities than are found in nature. The death of most of the test species was not the result of starvation, abiotic factors, bdellovibrios, or bacteriophages. Despite the presence of lytic bacteria, the lake water did not have lytic activity against the test species. Protozoan predation was a significant factor in the fall in bacterial population sizes because protozoa increased in numbers as the bacterial density fell, the suppression of protozoa led to the elimination or delay of the decline of the bacteria, and the addition of protozoa to lake water in which indigenous protozoa were suppressed produced the same pattern of bacterial elimination as in untreated lake water.