Ecosystem monitoring using high frequency active acoustics

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Ecosystem monitoring using high frequency active acoustics

Message  mdoble le Lun 29 Avr - 15:22

Ecosystem monitoring
using high frequency active acoustics



We aim to address the
considerable challenge of monitoring and quantifying the ecologically-key
mid-trophic levels (zooplankton) in the rapidly changing Arctic Ocean. We are
only just beginning to appreciate the gap in our understanding and expectations
of these ecosystems in the Arctic, as exemplified by recent findings (Arrigo et al., 2012) which measured under-ice
phytoplankton biomass hugely in excess of expectations and higher than typical
open-water concentrations.


While phytoplankton can be
relatively easily measured using fluorimetry, estimating the zooplankton
population is far more challenging. Traditional, labour-intensive methods such
as net sampling have many problems with avoidance by the larger organisms.
Traditional techniques are clearly not amenable to future application in
autonomous systems, which must be the future of all oceanographic monitoring - especially
in the Arctic.


Acoustic methods are “the only means that can efficiently observe
the large biomass of the mid-trophic levels at ecologically important temporal
and spatial scales
” (Wiebe et al.,
2002). Acoustic data are inherently ambiguous when measured in isolation,
however. The amplitude of the back-scattered energy depends on the interacting
effects of the number of animals, the species composition and their size
distribution and orientation. Inverse modelling techniques are able to untangle
these interacting factors, but only with sufficient independent knowledge of
the animals present.


This ‘independent knowledge’
(often referred to as ground truthing, though the glaring deficiencies of these
complementary techniques are the motivation for using acoustics in the first
place) is provided by optical techniques (underwater video profiler, or UVP)
and net sampling. Knowledge of the physical environment (CTD, velocity) is also
helpful. Scattering/absorption parameters, fluorescence & bioluminescence
sensors also aid interpretation. Integrated measurements are the key to
understanding.


A key factor in gaining
understanding of the acoustic signal is the use of multiple frequencies.
Different species have varying responses across the acoustic spectrum and the
ratios between backscatter at different frequencies can be used to identify the
dominant organisms in a swarm. Comprehensive pan-Arctic observations of the
acoustic signatures of key species are sought, in conjunction with a full suite
of supporting measurements (optical, physical, nets). The aim is to build a
library of acoustic backscatter characteristics, allowing us to move towards a
uniquely acoustic characterisation of the functional groups present, ultimately
incorporating this knowledge into an autonomous acoustic profiling system,
similar in operation to the highly successful Ice Tethered Profilers deployed
by LOCEAN and WHOI during DAMOCLES, and under development by the LOCEAN iAOOS
project.


Researchers involved: Martin
Doble (LOV), Gaby Gorsky (LOV)

mdoble

Messages : 6
Date d'inscription : 09/04/2013

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