The copepod Acartia sp. is more sensitive to a rapid pressure drop associated with seismic airguns than Calanus sp.

Abstract

Airguns used in seismic surveys release high-pressure air, generating sound waves that may have adverse effects on marine life. However, knowledge of how seismic exposure impacts zooplankton is limited. One key characteristic of seismic signals that could potentially cause damage is a rapid pressure drop. In this study, the rapid pressure drop (~2 bar) was re-created in the laboratory using a pressure tube. To determine the range at which this drop occurs, the sound field around a seismic airgun array was modeled. The effects of this pressure drop on mortality and swimming behavior were tested in 2 common copepods, Acartia sp. and Calanus sp., both immediately and 5 h after treatment. Pressure-exposed Acartia sp. showed higher mortality rates (0 h: 5.6%; 5 h: 10%) compared to the controls, while mortality in Calanus sp. only increased after 5 h (3.3%). The swimming speed of pressure-exposed Acartia sp. (0 h: 0.49 mm s-1; 5 h: 0.52 mm s-1) was lower than in the control treatment, whereas the swimming speed in pressure-exposed Calanus sp. (2.64 mm s-1) only differed immediately after treatment. This study demonstrates that a rapid pressure drop can negatively affect zooplankton mortality and behavior at close range. The results also show that Acartia sp. is more sensitive to this pressure drop than Calanus sp., suggesting potential species-specific impacts from seismic exposure. Identifying the sound characteristics that can be harmful to zooplankton allows for a more accurate assessment of the most affected species and the range at which impacts can occur.