Introduction to the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 14

Jones Bank

10 m

Seabed

1 km

Figure 1.6 See colour plates version . A series of short internal waves on the edge of
Jones Bank, Celtic Sea, as seen in the data from the 200 kHz transducer of an EK60
echosounder. Image courtesy of Clare Embling, University of Aberdeen, UK.

potential, and there are ongoing developments in the field of measuring some
metals, such as copper and manganese, using sensors attached to CTDs.
There are several instruments that measure the size spectrum of particles in
the ocean, such as the optical plankton counter (OPC) and the laser particle size
analysers (LISST). The difficulty with data from these instruments is that they
cannot identify the shape and nature of particles, a particular problem in shelf
environments when plankton, detritus and suspended sediments are all present.
Recent, exciting developments in the use of high-resolution digital imagery are
changing this. For instance, a holographic technique using a laser and digital
camera to image particle diffraction patterns and then de-convolving the pat-
terns to generate images of the particles has recently become available (Graham
and Nimmo-Smith, 2010 ).
For larger particles in the ocean, e.g. large zooplankton, fish larvae and fish, multi-
frequency acoustic instruments are used to assess large-scale horizontal and vertical
distributions. Considerable effort has been applied to the different acoustic signatures
of key zooplankton and fish species (e.g. Brierley, et al., 1998 ; Fablet, et al., 2009 ), so
that distributions of different species can be observed from vessels or from moorings.
We often find that many of these particles, particularly the zooplankton, concentrate
in gradients in the biogeochemical and physical structure of the water column, so that
these acoustic techniques can provide revealing pictures of the physical environment.
Figure 1.6 shows an example from some of our own work of acoustic backscatter
data from an EK60 scientific echosounder common in fisheries studies, providing a
startling image of internal waves over a bank in a shelf sea.