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 10 / 13 PLOS ONE Photosynthetic oxygen bubble stream sounds from aquatic macrophytes sounds. We suspect that the full range of sounds produced by POBS form a continuum, such that novel sounds will frequently be encountered. Our visits to other freshwater lakes and ponds in central Alberta revealed similarly diverse soundscapes wherever we placed the hydrophone among rooted macrophytes in clear water during sunny conditions. In all cases, we found that the soundscapes in these environments were dominated by POBS, with corixid stridulations as the only other biological components, although in some settings the vocalisations and splashings of nearby birds (e.g., American Coot (Fulica americana), and Canada Goose (Branta canadensis)) were audible through the hydrophone. It is well understood that freshwater sounds can originate from various sources, including fishes, amphibians, insects, macrophytes, and hydrological/geological processes [5,9–14]. In the freshwater aquatic literature, biological aquatic sounds are typically classified as either fast repetitive ticks (FRT) produced by air released from the swim bladders of fishes [39], or as stridulations produced by insects [12,39,40]. Since both POBS and FRTs involve bubbles escaping through an opening under pressure, we should expect POBS and FRTs to be similar. However, FRTs are shorter in duration, and occur more frequently at night due to nocturnal gas venting during the diel movements of fishes moving closer to the water surface [39]. It also

appears that FRTs are more characteristic of deeper water, unlike macrophytic bubble sounds [27]. Stridulatory communication has been well documented in the Corixidae [29–31]; however, corixid stridulations are both quantitatively and qualitatively easy to distinguish from POBS (see the Xeno-Canto platform for exemplars of corixid stridulations, https://xeno-canto.org). Many unknown freshwater aquatic sounds have often been attributed to insects [12], but we note that many POBS are similar to terrestrial insect sounds, and especially those of some cicadas. This similarity, however, may be coincidental, and may mislead researchers when characterising aquatic sounds. Since we did not hear stridulations other than three familiar corixid species, we suspect that other putatively sound-producing organisms at our study sites either did not produce sound at all, or that they only stridulate occasionally as an anti- predator defense, as do many terrestrial beetles. We therefore recommend caution when assuming that unidentified/unconfirmed aquatic sounds are produced by fishes or insects [12,16,40]. We believe that POBS are underreported in the current literature. We recognise the call from Rountree et al. [40] to further classify fish sounds in aquatic soundscapes, but expand it to encompass all aquatic sounds, including POBS. Only one paper to our knowledge has explicitly mentioned freshwater macrophytes as a major component of shallow, freshwater soundscapes [15]. We also suggest considering how differences in soundscapes likely correlate with lake zona-

tion (e.g., littoral, limnetic, profundal, benthic) and water parameters (e.g., salinity, pH, depth, trophic state, sediments). Because all of the POBS we heard were produced by sago pondweed, the diversity of potential sounds from this species is broad. This macrophyte is found globally in freshwater habitats <2.5 metres deep with currents <1 m/s [41]. Given its ubiquity, similar soundscapes are likely to exist in shallow freshwater lakes elsewhere. However, many other species of macrophytes are undoubtedly capable of producing a very similar range of sounds. Recognizing the ubiquity of POBS in shallow freshwater has implications for other studies as well. For acoustic bioinventory work, the diversity of plant-produced sounds has obvious consequences for the characterisation of sound sources in shallow freshwater. Understanding plant sounds will enhance the capacity for ecological conditions of ecosystems to be measured, primary production to be measured, pollution events to be characterised, and future initiatives as well. For behavioural studies of acoustic communication in shallow freshwater environ- ments, it is important to recognize that POBS produce the acoustic background against which