Acoustic propagation measurements were conducted in a Thalassia testudinum meadow in the Lower Laguna Madre, a shallow bay on the Texas Gulf of Mexico coast. A piezoelectric source transmitted frequency-modulated chirps (0.1 to 100 kHz) over a 24-h period during which oceanographic probes measured environmental parameters including dissolved oxygen and solar irradiance. Compared to a nearby less vegetated area, the received level was lower by as much as 30 dB during the early morning hours. At the peak of photosynthesis-driven bubble production in the late afternoon, an additional decrease in level of 11 dB was observed.