Underwater plant sounds: Diel acoustic patterns of four submerged aquatic macrophytes

Passive acoustic monitoring is an emerging survey method in freshwater environments. Unidentified ticking sounds appeared to be a major contributor to freshwater soundscape. These unidentified ticking sounds in the soundscape may be related to aquatic plant primary production in the freshwater environment as such ticking sounds have previously been described in the aquatic plant Elodea canadensis. This plant emits sounds linked to gas bubble trails in response to exposition to light. Therefore, the aim of this study was to investigate the sound features of different aquatic plant species, and their daily acoustic cycles under natural sunlight. To achieve this, the sounds of four species (Callitriche sp., Elodea nuttallii, Myriophyllum spicatum and Potamogeton lucens) were recorded in field mesocosms over 13 summer days. Ticking sounds were emitted by each species during the day. Our results showed that these ticking sounds had similar acoustic features in the four species. The number of ticking sounds was best identified by the Acoustic Complexity Index over the 0-2 kHz frequency band (ACI_0-2kHz). Similar daily curves with sound peaks between 17:00 and 19:00 were observed in all species, which was later than the maximum sunlight intensity. Further research is required to understand the mechanisms behind the sound production, whether these sounds are detectable in more complex soundscapes typical of freshwater environments, and whether these sounds have an impact of on the rest of the underwater soundscape.