Predicting the Distribution of Marine Benthic Biotopes
491
1990). The maximum operating depth is limited by the power output of the echo
sounder used. The power output of high frequency echo sounders is generally, and was
in this case, too low to give sufficiently good second echo (E2) returns in water deeper
than approximately 45 m, depending on the nature of the seabed. The RoxAnn™
system requires both the 1st and 2nd echo returns to function properly, and this system
was therefore operating at the limit of its capability in the deep areas at the boundary of
the Papa Stour cSAC.
For Lochs Duich, Alsh and Long, acoustic data were required from shallow
waters down to over 100 m depth. A RoxAnn™ system with high frequency sounder
would not give ground discrimination in such deep waters. A RoxAnn™ with low
frequency sounder could discriminate in deeper waters, but would not provide
sufficient ground discrimination in shallow water. The system employed was a QTC™
acoustic ground discrimination system, also with a 200 kHz echo sounder, which
analyses the shape of only the primary echo return (Collins et al. 1996), and can
therefore operate successfully in both shallow waters and down to over 100 m depth.
The QTC™ system outputs depth and three parameters Q1, Q2 and Q3, to describe the
shape of the echo.
AGDS are designed to map in real time the physical characteristics of the
seabed, and do not take account of depth in their classifications of different seabed
types. This poses problems in the use of such systems for mapping of biological
communities, as the same biological communities may occur on very different seabed
types, and water depth is a fundamental factor in determining the type of biological
community present. It is therefore necessary to post-process the acoustic data and
calibrate them with ground validation data, for successful mapping of biological
communities.
Ground Validation
The acoustic data were ground validated by high quality drop-down video for epifaunal
information and/or grab sampling areas of seabed for infaunal information. Sites were
located where preliminary examination of the acoustic data indicated areas with
acoustically different properties; and in areas with apparently the same acoustic
properties, but with a wide depth range. In the latter case, validation sites were located
in both deep and shallow water within these areas. In sedimentary areas, grab samples
for infaunal and particle size analysis were taken in addition to the video sample.
Video recordings obtained were analysed to describe the physical and
biological characteristics of the seabed to compile an inventory of biotopes present
within the survey. Sites were assigned to biotope types according to the Marine Nature
Conservation Review (MNCR) biotope manual, Version 96.7 where possible (Connor
et al., 1996), or provisional new biotopes were described if a record did not fit into any
of the existing biotope types. In kelp forests and parks where the substratum type was
not visible, and where the variety and detail of understorey flora and fauna was not
clearly distinguishable from the video, some difficulty was encountered in allocating
sites to biotope types. This was because the biotope classification (Version 96.7) did
491
1990). The maximum operating depth is limited by the power output of the echo
sounder used. The power output of high frequency echo sounders is generally, and was
in this case, too low to give sufficiently good second echo (E2) returns in water deeper
than approximately 45 m, depending on the nature of the seabed. The RoxAnn™
system requires both the 1st and 2nd echo returns to function properly, and this system
was therefore operating at the limit of its capability in the deep areas at the boundary of
the Papa Stour cSAC.
For Lochs Duich, Alsh and Long, acoustic data were required from shallow
waters down to over 100 m depth. A RoxAnn™ system with high frequency sounder
would not give ground discrimination in such deep waters. A RoxAnn™ with low
frequency sounder could discriminate in deeper waters, but would not provide
sufficient ground discrimination in shallow water. The system employed was a QTC™
acoustic ground discrimination system, also with a 200 kHz echo sounder, which
analyses the shape of only the primary echo return (Collins et al. 1996), and can
therefore operate successfully in both shallow waters and down to over 100 m depth.
The QTC™ system outputs depth and three parameters Q1, Q2 and Q3, to describe the
shape of the echo.
AGDS are designed to map in real time the physical characteristics of the
seabed, and do not take account of depth in their classifications of different seabed
types. This poses problems in the use of such systems for mapping of biological
communities, as the same biological communities may occur on very different seabed
types, and water depth is a fundamental factor in determining the type of biological
community present. It is therefore necessary to post-process the acoustic data and
calibrate them with ground validation data, for successful mapping of biological
communities.
Ground Validation
The acoustic data were ground validated by high quality drop-down video for epifaunal
information and/or grab sampling areas of seabed for infaunal information. Sites were
located where preliminary examination of the acoustic data indicated areas with
acoustically different properties; and in areas with apparently the same acoustic
properties, but with a wide depth range. In the latter case, validation sites were located
in both deep and shallow water within these areas. In sedimentary areas, grab samples
for infaunal and particle size analysis were taken in addition to the video sample.
Video recordings obtained were analysed to describe the physical and
biological characteristics of the seabed to compile an inventory of biotopes present
within the survey. Sites were assigned to biotope types according to the Marine Nature
Conservation Review (MNCR) biotope manual, Version 96.7 where possible (Connor
et al., 1996), or provisional new biotopes were described if a record did not fit into any
of the existing biotope types. In kelp forests and parks where the substratum type was
not visible, and where the variety and detail of understorey flora and fauna was not
clearly distinguishable from the video, some difficulty was encountered in allocating
sites to biotope types. This was because the biotope classification (Version 96.7) did