INTRODUCTION:

Benthic assemblages of typical Caribbean reef fauna cover with variable density three ridges that parallel the Broward County, Florida, coast at about 5-10m, 10-20m and 20-30m depth. Two of these ridges (the second and third) are drowned early Holocene coral reefs of 5 ky and 7 ky uncorrected radiocarbon age respectively. Figure 1 shows the position and profile of these reef ridges within Broward County. In response to changes in environmental factors (hydrodynamic exposure, ambient light, etc.) the characteristics of these benthic assemblages change with variable density. Acoustic remote sensing is a useful tool for mapping the spatial arrangement of these benthic assemblages and describing change in their arrangements over time. Benthic fauna with distinct growth forms can be detected and differentiated acoustically due to the unique acoustic roughness associated with each unique growth form. Figure 2 shows the principles of acoustic remote sensing and shows the levels of data integration necessary to achieve a complete, multi-level data set. Figure 3 shows the finished results of an acoustic habitat classification survey conducted in the area of coverage in Figure 1.

METHODS:

An acoustic bottom classification survey using QTCView 5 based on a 50 kHz transducer was conducted over all three of the reef ridges in Broward County, FL. Ground-truthing (Figure 4) was subsequently performed by SCUBA divers using a traditional 50 meter point-intercept transect method to obtain community and cover data from each of the reef ridges. These data were then compared and used to produce a final benthic habitat classification for the area of the survey (Figure 5).

RESULTS:

Results from the QTCView 5 survey (Figure 5) showed different acoustic classes on the first (nearshore) reef ridge and the second and third reef ridges, which both gave the same acoustic signature. Rubble beds inshore of the third reef-line showed a mixed acoustic signature. In situ transect data (Figure 6) showed that the differences in acoustic signature corresponded to different benthic assemblages: nearshore hardgrounds had low live cover (10%) and were dominated by algae and hydrozoa, the two deeper reef lines had the same acoustic signature and similar benthic assemblages (25-50% cover by tall sponges and gorgonians).

DISCUSSION:

Subtle differences in species composition existed, however since growth-form characteristics and thus acoustic surface roughness characteristics were the same in both areas, no acoustic split was achieved. The QTCView5 was also able to differentiate between stable sands covered by a thin red algae turf and more mobile sand without turf cover. It is therefore concluded that acoustic remote-sensing methods can be used to differentiate different benthic assemblages, as long as enough differences exist in the growth-form characteristics of the dominant species to provide for a unique acoustic roughness.

Acknowledgements:

Divers: Brian Ettinger, David Gilliam, Susan Thornton, Jaime Vernacchio;
Captain: Lance Robinson.

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