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Calanoid copepods dominate mesozooplankton communities in temperate and Nordic seas. The ability of copepods to remain and feed in productive surface waters depends on their ability to overcome downward flows. In this study, we assessed the swimming performance of subarctic Calanus spp. and tested how the copepods can retain their vertical position in a representative range of downward currents (between 0 and 5.4 cm s-1) simulated in a downwelling flume. Mean vertical and horizontal copepod swimming velocities and accelerations, movement periodicity and trajectory complexity were obtained by tracking individual trajectories in the field of view of 2 cameras. Copepod swimming velocity increased with increasing downward flow and matched downward flows up to 2 cm s-1. Beyond 2 cm s-1, animals were still able to significantly reduce their sinking rates, but their motions changed. Trajectories became simpler, swimming velocities changed on shorter time scales and instantaneous acceleration increased. These results are consistent with predictions of balancing depth retention against encounter rates with food and predators. Frequency distributions of vertical swimming speeds were mostly unimodal, with entire experimental populations responding in the same way. Coordination of movements and the ability to resist moderate downwelling flows can result in the accumulation of copepods in large surface swarms as observed in the field.
Calanoid copepods dominate mesozooplankton communities in temperate and Nordic seas. The ability of copepods to remain and feed in productive surface waters depends on their ability to overcome downward flows. In this study, we assessed the swimming performance of subarctic Calanus spp. and tested how the copepods can retain their vertical position in a representative range of downward currents (between 0 and 5.4 cm s-1) simulated in a downwelling flume. Mean vertical and horizontal copepod swimming velocities and accelerations, movement periodicity and trajectory complexity were obtained by tracking individual trajectories in the field of view of 2 cameras. Copepod swimming velocity increased with increasing downward flow and matched downward flows up to 2 cm s-1. Beyond 2 cm s-1, animals were still able to significantly reduce their sinking rates, but their motions changed. Trajectories became simpler, swimming velocities changed on shorter time scales and instantaneous acceleration increased. These results are consistent with predictions of balancing depth retention against encounter rates with food and predators. Frequency distributions of vertical swimming speeds were mostly unimodal, with entire experimental populations responding in the same way. Coordination of movements and the ability to resist moderate downwelling flows can result in the accumulation of copepods in large surface swarms as observed in the field.
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.
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