Sunday, May 9, 2010

Acoustic observation of living organisms reveals the upper limit of the oxygen minimum zone

Listening to the ocean for more comprehensive study of the seas

Oxygen minimum zones
(OMZs) are expanding in the World Ocean as a result of climate change and direct anthropogenic influence.
OMZ expansion greatly affects biogeochemical processes and marine life, especially by constraining the vertical habitat of most marine organisms.
Currently, monitoring the variability of the upper limit of the OMZs relies on time intensive sampling protocols, causing poor spatial resolution.

The French Institut de recherche pour le dĂ©veloppement (IRD) and the Instituto del Mar del PerĂș (IMARPE) have developed an innovative acoustic method to observe the evolution of oxygen minimum zones (OMZs) in the world's oceans.
This new technique makes it possible to measure these oxygen-free (anoxic) zones, which are home to many marine organisms, in greater detail.

The researchers measured the vertical distribution of marine organisms, such as plankton, crustaceans and fish, in the water using routine underwater acoustic observation techniques along the Peruvian coast.
The measurements, made with the help of echo sounders, allowed them to determine the oxycline, which delimits the top of the OMZ, with high precision.
Combining the data obtained in this way with regular hydrological measurements, they managed to compile high-resolution maps that are 50,000 to 100,000 times more precise than common hydrological profiles.

As well as shedding new light on the state and spread of OMZs, the scientists were also able to calculate precisely the size of the habitat available to the Peruvian anchovy, for instance, providing information that might be of interest for fisheries management.
(IRD also uses this information to estimate the habitable volume for the world's most exploited fish, the Peruvian anchovy)

'This method ... allows performing integrated studies since acoustic data provides information on most ecosystem components [...], to which we can add ancillary information (satellite data, vessel monitoring system, top predator tagging...),"
Such integrated approaches are crucial to implement the ecosystem approach to fisheries.

"Our methodology can also be applied to other ecosystems, e.g. oceanic dead zones, and opens new perspectives for comprehensive multiscale studies on the impact of physical forcing on organisms."

OMZs are a naturally occurring phenomenon, found at depths of about 100 to 1,000 metres. They are home to numerous organisms that are specially adapted to the low-oxygen environment, including not only anaerobic bacteria but also larger organisms such as vampire squid. Oxygen-loving organisms, however, cannot survive in these zones.

OMZs currently cover about 10% of the planet's surface.
But they have been spreading for the past 50 years due to global warming and humankind's impact on the seas through rising levels of chemical nutrients (eutrophication), for example.

This opportunistic method could be implemented on any vessel geared with multi-frequency echosounders to perform comprehensive high-resolution monitoring of the upper limit of the OMZ.
IRD's approach is a novel way of studying the impact of physical processes on marine life and extracting valid information about the pelagic habitat and its spatial structure, a crucial aspect of Ecosystem-based Fisheries Management in the current context of climate change.
The findings open up new perspectives for the study of the oceans as well as the management of fisheries resources.

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