Main sources of ocean ambient sound at low frequencies include anthropogenic activities such as shipping and seismic explorations (<200 Hz), and physical events such as winds and waves (up to 1 kHz). The contributions to ambient sound from anthropogenic sources have been increasing, although the patterns differ across different spatial scales. We investigated low-frequency (<1 kHz) ambient sound across the North Pacific and Atlantic Oceans as well as the Gulf of Mexico and the Arctic. One site in the eastern North Pacific has been monitored intermittently from 2004, but most other locations had data recorded only during 2010’s.
The highest recorded levels of low-frequency ocean ambient sound were in the Gulf of Mexico and the lowest were in the Arctic. However, there was a large level of variability within each area, as well as between the ocean basins. In the Gulf of Mexico, for example, there was 10 dB of variability in ambient sound at 40 Hz (shipping noise band) across sites, but only about 4 dB at 100 Hz. Most of the difference at 40 Hz was likely resulting from the location of the recorders relative to the main shipping lanes and seismic exploration areas. Sites in Southern California also had large variability in sound at 40 Hz and 100 Hz (up to 20 and 15 dB, respectively) with the variability linked to the local bathymetry and exposure of recorders to different local basins or the open ocean. Generally, southward facing sites in Pacific and Atlantic Oceans had lower sound levels at low frequencies, likely corresponding to lower levels of shipping traffic across those areas. Long-term time series from a site in the eastern North Pacific showed decrease in low frequency ambient sound in 2009-10 relative to 2004, presumably the result of decrease in worldwide shipping during the global recession. However, the trends since then have been variable and harder to interpret in the light of shipping trends.
At most monitored locations there was a strong seasonal contribution to ambient sound from marine mammal calls, most commonly blue and fin whales, but also humpback whales. Understanding of ambient sound trends is important for developing baselines on the levels of noise marine animals may be exposed and used to in their environment. Additionally, it allows placing any increases in noise and experiments on noise impacts into appropriate context of ocean ambient sound.
