Sound characterization and fine-scale spatial mapping of an estuarine soundscape in the southeastern USA

Estuaries are areas known for biological diversity, and their soundscapes reflect the acoustic signals used by organisms to communicate, defend territories, reproduce, and forage in an environment that has limited visibility. These biological sounds may be rhythmic in nature, spatially heterogeneous, and can provide information on habitat quality. The goal of our study was to investigate the temporal and spatial variability of sounds in Chechessee Creek (Stns CC1 and CC2) and an adjacent saltwater impoundment (Great Salt Pond, GSP) in South Carolina, USA, from April to November 2016. Fixed recording platforms revealed that sound pressure levels (SPLs) were significantly higher in CC1 and CC2 compared to GSP. We detected some biological sounds in GSP (snapping shrimp genera Alpheus and Synal pheus, silver perch Bairdiella chrysoura, oyster toadfish Opsanus tau, spotted seatrout Cynos cion nebu losus, Atlantic croaker Micropogonias undulatus, and American alligator Alligator mississippiensis); however, biological sound was much more prevalent in CC1 and CC2. In Chechessee Creek, snapping shrimp, oyster toadfish, and spotted sea trout sounds followed distinct temporal rhythms. Using these data, we conducted spatial passive acoustic surveys in Chechessee Creek. We discovered elevated high frequency SPLs (representing snapping shrimp acoustic activity) near an anti-erosion wall, as well as in creased low frequency SPLs (indicating spotted sea trout spawning aggregations) near the anti-erosion wall and at the mouth of Chechessee Creek. This study has de monstrated the utility of combining stationary and mobile recording platforms to detect acoustic hotspots of biological sounds. © The authors 2020. Open Access under Creative Commons by Attribution Licence. Use, distribution and reproduction are un restricted. Authors and original publication must be credited. Publisher: Inter-Research · www.int-res.com Mar Ecol Prog Ser 645: 1–23, 2020 invertebrates (Bohnenstiehl et al. 2016, Monczak et al. 2019), fish spawning patterns (e.g. LowerreBarbieri et al. 2008, Montie et al. 2015, 2016, 2017, Monczak et al. 2017), and the foraging and communication of apex predators (Herzing 1996, Janik 2000, McCowan & Reiss 2001, Vergne et al. 2009, Rosenblatt et al. 2013, Monczak et al. 2019). Current technology allows long-term monitoring of marine soundscapes, which can provide critical data on species presence and key behaviors over various time scales (i.e. seasonal, lunar, daily, and tidal rhythms). By listening to and quantifying the behaviors of soniferous organisms, soundscape ecology can provide an additional metric to gauge habitat quality and the health of marine ecosystems. Findings indicate that estuaries in the southeastern USA are rich in biological sound. These sounds can include the acoustic behavior of snapping shrimp (Alpheus and Synalpheus spp.) (Bohnenstiehl et al. 2016, Monczak et al. 2019); the courtship calls of ecologically important fish species such as silver perch Bairdiella chrysoura, oyster toadfish Opsanus tau, and Atlantic croaker Micropogonias undulatus, and economically important species such as black drum Pogonias cromis, spotted seatrout Cynoscion nebu losus, and red drum Sciaenops ocellatus (Luczko vich et al. 2008a, Montie et al. 2015, Monczak et al. 2017); and vocalizations of apex predators such as bottlenose dolphins Tursiops truncatus (e.g. Monczak et al. 2019) and American alligators Alligator mississippiensis. Studies that have described the soundscapes of estuaries in the USA include a comparison of oyster Crassostrea virginica reefs and soft-bottom habitats in Pamlico Sound, North Carolina (Lillis et al. 2014a); soundscape patterns and processes in a shallow estuarine reserve in Middle Marsh, North Carolina (Ricci et al. 2016); and temporal rhythms of the soundscape of a deep tidal river estuary, the May River, South Carolina (Monczak et al. 2019). Soundscape data can provide an understanding of what organisms experience acoustically as they move through an estuary. In fact, there is some evidence that sound gradients serve as settlement cues to organisms that pass through the soundscape. Field experiments showed that oyster larval recruitment was significantly higher on larval collectors exposed to oyster reef sounds compared to controls with no sounds (Lillis et al. 2015). Overall, habitats rich in biological sound may be more favorable for spawning, residence, and settlement by invertebrates and fish because they provide acoustic cues indicating that these habitats are rich in resources (Mann et al. 2007). Thus, fine-scale acoustic mapping of estuaries at appropriate times is necessary because many organisms produce sound only during specific seasons or during specific times of the day. Recent reports have shown that estuarine soundscapes exhibit distinct temporal rhythms that vary over tidal, daily, lunar, and seasonal time scales (Ricci et al. 2016, Monczak et al. 2017, 2019). To advance our knowledge and understanding of sound level gradients in an estuary, we experimented with a lowcost method to create spatial heat maps of biological sound within specific periods. We focused our studies on Chechessee Creek and an adjacent saltwater impoundment in South Carolina, USA. The specific objectives of this study were to (1) compare low frequency (i.e. more indicative of sounds originating from fish) and high frequency (i.e. more indicative of snapping shrimp snaps) sound pressure levels (SPLs) between Chechessee Creek (Stns CC1 and CC2) and a saltwater impoundment (Great Salt Pond, GSP); (2) characterize the types of biological sounds and temporal patterns in these habitats from acoustic data obtained from stationary recorders; and (3) perform fine-scale spatial mapping of low and high frequency SPLs in Chechessee Creek using a towable recorder. We argue that both stationary fixed recorders and mobile recording platforms are best used in tandem as a means to understand the spatial variation in the soundscape because some species follow specific patterns in sound production that are correlated with temporal and environmental variables. 2. MATERIALS AND METHODS