What’s all that racket! Soundscapes, phenology, and biodiversity in estuaries

Discussion In this study, we used a six-year passive acoustic dataset to understand the annual and inter- annual variability of an estuarine soundscape. Our findings show a strong relationship between temporal changes in acoustic activities of estuarine organisms and environmental fac- tors. We showed that the transition between winter and spring is a dynamic time-period with an increase in biological sound during the spring, which mirrors the increase in phytoplank- ton, zooplankton, invertebrates, and fish abundance that drive changes in primary, secondary, and tertiary productivity within estuaries [20, 21]. In years with warmer spring temperatures, this seasonal transition occurred earlier than in years with cooler spring temperatures. This means that temperature plays an important factor in initiating certain behaviors (e.g. spawn- ing), and earlier occurrences of these behaviors reflect an organismal response to climate vari- ability [8]. Soundscapes and limitations In addition to sounds of biological origin (e.g. snapping shrimp and fish sounds), factors such as physical and anthropogenic sounds, water depth, and bottom topography may affect received SPLs. Water flow, rain, wind, or wave action, unlike sounds of biological origin, occur randomly [22–25]. These sounds are dominant in the low (200–2000 Hz) and high (15–20 kHz) frequency bandwidths [23]. However, during our analysis, we identified and removed acoustic files that contained sounds associated with intense water flow, rain, wind/wave action,

and anthropogenic origin. In addition, this tidal estuary is subject to less wind and wave action as compared to open ocean environments. Thus, we are quite confident that the SPLs and pat- terns presented are of biological nature. Studies have shown that water depth and active space can affect sound propagation [24, 25]. In our studies, water depth and river width increased from the source towards the mouth and could potentially affect received SPL measurements. Soundscape phenology In our study, we showed that sound production in snapping shrimp and sound producing fish species could serve as potential indicators of climate driven changes in spring phenology. In fact, snapping shrimp respond very quickly to changes in temperature with increased snapping activity with warmer temperatures and decreased activity with cooler temperatures [26]. Warmer temperatures also have the potential to impact spawning phenology of certain fish species that have temperature-dependent gonadal development [27]. Our studies have shown that positive temperature anomalies increase sound production in fish, while negative temper- ature anomalies decrease calling [28]. In many fish species, spawning seasons are temperature dependent since biologically important processes (e.g. maturation of gonads) require specific temperature ranges. Similar to other studies, we detected a significant increase in snapping shrimp and fish acoustic activity in the spring and summer and a significant decrease in the fall and winter [e.g. 20, 21, 26, 29, 30]. These temporal variations in biological sound levels

indicate that there is a strong connection between sound production and the seasonal changes in estuarine diversity and productivity. In terrestrial ecosystems, the influence of climate change on phenology is well documented and significant; however, in marine environments, this aspect is understudied due to the inability to sample at the necessary time scales [4, 5, 8, 31, 32]. It is important to note that mon- itoring soundscapes can assist in climate and phenology studies. By tracking vocalizations of amphibians and birds, there is now clear evidence that climate influences the phenology of breeding and migratory patterns [33, 34]. For example, in a terrestrial system, Buxton et al. (2016) detected shifts in songbird phenology in varied thrush (Ixoreus naevius), Pacific wren PLOS ONE Estuarine soundscapes and biodiversity PLOS ONE | https://doi.org/10.1371/journal.pone.0236874 September 3, 2020 7 / 18