Plankton at risk from seafloor mining surveys

Climate scientists are agreed that global warming will have significant long-term impacts on plankton, the creatures that underpin the health and productivity of global marine ecosystems and which play a critical role in the planetary carbon cycle, though they are less sure what exactly those impacts will be.

But more immediate effects from human reliance on fossil fuels are now clearer, thanks to research that shows acoustic survey techniques used to explore the seafloor for oil and gas deposits is associated with the widespread death of plankton.

The study by marine scientists from Curtin University, in Western Australia, and the University of Tasmania has been published in Nature Ecology & Evolution. It concludes that “potential large-scale modification of plankton community structure and abundance due to seismic survey operations has enormous ramifications for larval recruitment processes, all higher order predators and ocean health in general” and flags the “urgent need to prioritise development and testing of alternative surveying techniques”.

These results add to the growing body of evidence of the deleterious influence on marine life from  man-made underwater noise, such as the disruption of whale behaviour from naval sonar use.

Oceanic exploration for petroleum resources is done through acoustic imaging, by firing intense, low-frequency sound impulses down into the seabed. Those impulse signals are produced by arrays of “air guns” that simultaneously shoot air at high pressure (13.8 MPa, or 2,000 psi) into the water. Acoustic echoes captured by strings of hydrophones enable sub-sea images to be generated.

While details of the global extent of such survey activity are scarce, the authors provide some sense of scale with statistics from Australian waters: during 2014 and early 2015, an average of 15,848 km of petroleum-related marine seismic surveys were completed every three months.

To determine the impact of such activity, the research team conducted experiments off the south-east coast of Tasmania, measuring the effect of a single air gun on zooplankton, the small marine animals that typically graze on the plantlike phytoplankton found in abundance at depths to about 200 metres. Sonar surveys and net tows were used to measure both abundance and the ratio of dead to live zooplankton both before and after air gun use.

The results: the average abundance of zooplankton caught in nets fell by more than 60% in the hour immediately after the air gun was fired, compared to control areas, and two to three times more dead zooplankton were found at all range groups for all taxa. This mortality rate was “more than two orders of magnitude higher” than what has been assumed by previous modelling studies.

Although they did not directly study the zooplankton for cause of death, the researchers offer a hypothesis: many marine invertebrates, including zooplankton, use mechanoreceptors to detect vibrations. For most zooplankton, these mechanosensory systems may be extremely sensitive, responding to air-gun impulses signal by ‘shaking’ to the point where damage could accrue to sensory hairs or tissue.

“Impacted animals might not die immediately after air gun exposure, but rather may be disabled in their sensory capacity with an accompanying loss of fitness and so increased predation risk through time,” the authors suggest.