Elevated environmental noise levels have the potential to interfere with the acoustic communication system of manatees by masking signals containing biologically important information. Communication theory recognizes several ways a sender can modify its acoustic signal to compensate for noise. These include increasing the source level of a signal, its repetition, its duration, or shifting frequency outside of the noise band. To test whether manatees can modify their vocalizations to compensate for noise, vocalization usage and structure were examined in terms of vocalization rate, duration, frequency, and source level. Results imply that manatees do increase their vocalization energy expenditure, or vocalization effort, as a function of behavior and calf presence. Increases in vocalization rate, duration, and source level were most pronounced when calves were present and during behaviors whereby animals tended to be more dispersed, which may suggest a cohesion function. In conditions of elevated noise levels, manatees increased call duration during feeding and milling behaviors when calves were present, suggesting that ambient noise levels do have a detectable effect on manatee communication and that manatees modify their vocalizations as a function of noise.