Photosynthetic oxygen bubble stream sounds from aquatic macrophytes, and their consequences for acoustic biodiversity inventories and acoustic communication in shallow freshwater settings

PLOS ONE | https://doi.org/10.1371/journal.pone.0317424 April 23, 2025 9 / 13 PLOS ONE Photosynthetic oxygen bubble stream sounds from aquatic macrophytes Each syllable was, on average, 279.45 ms in duration, with 8 or more well-defined harmon- ics (peak frequencies listed in Table 1). The lower fundamental harmonics (below 12 kHz) were the most intense and had the greatest duration. Audibly, this sound featured irregularly spaced, high-pitched, “phwui” sounds, presumably caused by the build-up and release of pressure. Exemplar 5 Recorded on July 19, 2021, in shallow (<20 cm) water, associated with a bubble stream visible from above. This was a prolonged, continuous sound, lasting over 4000 ms, with peak fre- quencies ranging from 1.6 to 4.7 kHz (Fig 5). Frequency exhibited an initial sharp increase, gradually decreasing to consistent frequencies after two seconds. Audibly, it consists of a single “bweezzz” that decreases in amplitude over the duration of the recording. These exemplar recordings were uploaded to the Xeno-Canto platform (https://xeno- canto.org, XC893189-XC893193), and the Freshwater Sounds Archive (https://fishsounds.net/ freshwater.js), along with the aquarium-based sago pondweed recording (XC920087), by Katie Campbell. Discussion The shallow water soundscape of Gull Lake, Alberta, was dominated by biological sounds produced by photosynthetic oxygen bubble streams from sago pondweed, along with occa- sional stridulations from water boatmen. Since the POBS were associated with single streams

of bubbles (not “pearling” on the plant surface), we believe that wounds were the source of the bubbles. These were likely caused by aquatic insects, water birds, or wave action. Prolonged oscillations were typical of the POBS, as expected from bubbles escaping through a small, elastic opening, via the buildup and release of pressure. The five sound exemplars presented here should not be interpreted as representing five discrete categories of Fig 5. Exemplar 5 spectrogram and waveform. A macrophyte bubble stream sound recorded July 19, 2021, at Gull Lake, Alberta. See Fig 1 for axis descriptions. https://doi.org/10.1371/journal.pone.0317424.g007