3-1. Temporal variation in the underwater soundscape
The temporal variation in the underwater soundscape is shown by the spectral probability density and spectrogram in Figure 3. At the median spectrum level represented by the magenta solid line in Figure 3A, spectral peaks near 330 and 3,200 Hz were conspicuous, and they seemed to be attributed to the calls of leopard seals. The sound pressure level of the most prominent peak at 328 Hz was 25 dB higher than the median spectrum level below 1 kHz, except in the bandwidth between 200 and 500 Hz, and this sound source was the dominant contributor to the increased ambient sound pressure level in this region. Relatively weak spectral peaks near 1,200 and 1,700 Hz were observed at the mean intensity spectrum level, represented by the magenta dashed line, but they were not sustained enough to be reflected at the median spectrum level. The feature represented by these four spectral peaks is also shown on the spectrogram in Figure 3B. These four spectral peaks exhibited almost synchronized occurrence patterns, with differences in sound pressure levels. They showed more prominent persistence on December 11 than on December 10 and presented relatively lower power from 10:00 to 18:00 on December 10. The temporal variations in the sound pressure levels are discussed in Section 3-3 using correlation with call counts of leopard seal vocalizations.