Silent oceans: ocean acidification impoverishes natural soundscapes by altering sound production of the world's noisiest marine invertebrate

Abstract

Soundscapes are multidimensional spaces that carry meaningful information for many species about the location and quality of nearby and distant resources. Because soundscapes are the sum of the acoustic signals produced by individual organisms and their interactions, they can be used as a proxy for the condition of whole ecosystems and their occupants. Ocean acidification resulting from anthropogenic CO2 emissions is known to have profound effects on marine life. However, despite the increasingly recognized ecological importance of soundscapes, there is no empirical test of whether ocean acidification can affect biological sound production. Using field recordings obtained from three geographically separated natural CO2 vents, we show that forecasted end-of-century ocean acidification conditions can profoundly reduce the biological sound level and frequency of snapping shrimp snaps. Snapping shrimp were among the noisiest marine organisms and the suppression of their sound production at vents was responsible for the vast majority of the soundscape alteration observed. To assess mechanisms that could account for these observations, we tested whether long-term exposure (two to three months) to elevated CO2 induced a similar reduction in the snapping behaviour (loudness and frequency) of snapping shrimp. The results indicated that the soniferous behaviour of these animals was substantially reduced in both frequency (snaps per minute) and sound level of snaps produced. As coastal marine soundscapes are dominated by biological sounds produced by snapping shrimp, the observed suppression of this component of soundscapes could have important and possibly pervasive ecological consequences for organisms that use soundscapes as a source of information. This trend towards silence could be of particular importance for those species whose larval stages use sound for orientation towards settlement habitats.

Keywords: climate change; noise; ocean acidification; snapping shrimp; sound; soundscape.

Figure 1.

Acoustic spectra for a control versus an elevated CO₂ marine soundscape. (a) Dusk and evening chorus of snapping shrimp (peak frequency ∼ 4 kHz) and sea urchins (peak frequency ∼ 1.2 kHz) at White Island. Acoustic power was averaged among multiple days of sampling in multiple sites (N = 2 per treatment) and is reported along a colour scale. (b) Median acoustic power spectra for recordings obtained at dusk averaged among multiple days of sampling at White Island (New Zealand) (N = 2 per treatment), Vulcano (Italy) (N = 3 per treatment) and Ischia (Italy) (N = 2 per treatment). (Online version in colour.)

Figure 2.

Effect of ocean acidification on biological soundscapes and snapping shrimp noise. (a) Full bandwidth SPL and (b) mean number of snaps per minute ± s.e. in field recordings at Vulcano and Ischia (Italy) and White Island (New Zealand) at dusk (sunset to 1 h past sunset) over 5 min recordings. Different letters indicate statistically significant (p < 0.05) differences. (c) Scatterplot showing the relationship between frequency of snaps and full bandwidth root mean squared SPL. Data points represent single recordings replicated across both time and space.

Figure 3.

(a) Mean (±s.e.) number of snaps produced overnight in laboratory experiments by individual snapping shrimp (N = 10 per treatment). (b) Mean (±s.e.) peak SPL of defensive snaps produced upon stimulation (N = 7 per treatment) and of snaps produced overnight in undisturbed conditions (N = 10 per treatment). Different letters indicate statistically significant (p < 0.05) differences.