Soundscape analysis can be an effective tool in assessing seagrass restoration early success

Abstract

Restoration of vulnerable marine habitats is becoming increasingly popular to cope with widespread habitat loss and the resulting decline in biodiversity and ecosystem services. Lately, restoration strategies have been employed to enhance the recovery of degraded meadows of the Mediterranean endemic seagrass Posidonia oceanica. Typically, habitat restoration success is evaluated by the persistence of foundation species after transplantation (e.g., plant survival and growth) on the short and long-term, although successful plant responses do not necessarily reflect the recovery of ecosystem biodiversity and functions. Recently, soundscape (the spatial, temporal and frequency attribute of ambient sound and types of sound sources characterizing it) has been related to different habitat conditions and community structures. Thus, a successful restoration action should lead to acoustic restoration and soundscape ecology could represent an important component of restoration monitoring, leading to assess successful habitat and community restoration. Here, we evaluated acoustic community and metrics in a P. oceanica restored meadow and tested whether the plant transplant effectiveness after one year was accompanied by a restored soundscape. With this goal, acoustic recordings from degraded, transplanted and reference meadows were collected in Sardinia (Italy) using passive acoustic monitoring devices. Soundscape at each meadow type was examined using both spectral analysis and classification of fish calls based on a catalogue of fish sounds from the Mediterranean Sea. Seven different fish sounds were recorded: most of them were present in the reference and transplanted meadows and were associated to Sciaena umbra and Scorpaena spp. Sound Pressure Level (SPL, in dB re: 1 μPa-rms) and Acoustic Complexity Index (ACI) were influenced by the meadow type. Particularly higher values were associated to the transplanted meadow. SPL and ACI calculated in the 200-2000 Hz frequency band were also related to high abundance of fish sounds (chorus). These results showed that meadow restoration may lead to the recovery of soundscape and the associated community, suggesting that short term acoustic monitoring can provide complementary information to evaluate seagrass restoration success.

Keywords: Posidonia oceanica; Acoustic restoration; Fish sound; Mediterranean sea; Passive acoustic monitoring; Restoration success.

Fig. 1

(a) Occurrence (in percentage) of chorus (/kwas/ and RPS) per meadow type. (b) Fish sound occurrence (in percentage) per meadow type: LFPS low frequency fast pulse train, APPPS accelerating pulse period pulse series, PS pulse series with irregular period, DS down sweep sound, DSS down sweep series, RPS repeated pulse series produced by Sciaena umbra; /kwas/: Ultra-fast pulse series produced by Scorpaena spp. (c) Occurrence (in percentage) of chorus (/kwas/ and RPS) per hour.

Fig. 2

Fish richness (number of species/125 m²), abundance (number of individuals/125 m²) and biomass (g/125 m²) found by scuba UVC.

Fig. 3

Relationships between acoustic indexes (SPL_{200–2000 Hz}, ACI_{200–2000 Hz}, SPL_{2000–11,000 Hz} and ACI_{2000–11,000 Hz}) and meadow type as predicted by the LMMs. The points and bars are mean ± CI.

Fig. 4

Relationships between acoustic indexes (SPL_{200–2000 Hz}, ACI_{200–2000 Hz}) and fish sound abundance class (chorus and no chorus) as predicted by the LMMs. The points and bars are mean ± CI.

Fig. 5

Map of the study area (left) and mooring system (right), the latter as described in La Manna et al., 2021.

Fig. 6

Meadow types: dead, reference and transplanted. Photo credits: Ivan Guala.