Seeing the Bottom Provides a New Perspective and Complements Multibeam Surveys

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Seeing the Bottom Provides a New Perspective and Complements Multibeam Surveys

By Brad Butman
September 1999

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shrimp surround anemone
Shrimp defense: Shrimp arrange themselves (nearly) symmetrically around a burrowing cerianthid anemone on Stellwagen Bank. The behavior may be a combination of protection from predators (cover your back) and availability of food. Food may be attracted by the anemone or may fall from suspension as the current becomes retarded while flowing around (or through) the anemone.

From July 18-21, Brad Butman and Page Valentine carried out a 3-day whirlwind ground-truth-sampling cruise for western Massachusetts Bay aboard the fishing vessel ISABEL S. A bottom grab sampler equipped with downward and forward-looking real-time color video and still camera was used to investigate 142 stations, take bottom video (~20 hours), still photos (~1,000), and grabs (~100). Under the watchful eyes of Ken Parolski and Dann Blackwood, the imaging systems functioned flawlessly. So did the weather.

The Massachusetts Bay segment of the ISABEL S. cruise was sandwiched within a longer (2-week) cruise carried out by Page to obtain similar observations in the Stellwagen Bank National Marine Sanctuary. Laura Hayes, Jeremy Malczyk, Nancy Soderberg, and David Walsh were also participants in the Mass Bay leg.

The objective of the cruise was to provide direct observations of the sea floor to help interpret the backscatter and shaded-relief images produced by multibeam surveys. The video observations changed the way we think about the sea floor in Mass Bay.

High-backscatter areas ranged from gravel pavement, cobbles, boulders, pipe clay, coarse sand with large bedforms, dumps of dredge and construction material, to clay fragments beneath a smooth surface, etc. Some of these interpretations could be made from the spatial patterns of the backscatter, but some were a surprise.

Brad monitors the seafloor
Inside the lab van, Brad monitors the sea floor as the video images are recorded. The forward-looking and downward-looking color video cameras provide complementary views of the sea floor. The position of the ship is displayed in real time over a GIS display of the multibeam images. As the sampler drifts over targets of interest, 35-mm still shots are taken.

Interpretations are confounded by anthropogenic activity. For example, cerianthid anemones are found in some places but are absent in other similar environments. Is this a result of bottom trawling, which would remove these animals, or from environmental conditions? Dumping is ubiquitous—some rubble is in distinct piles, but there is evidence for low-level dumps elsewhere.

One of the advantages of differential GPS and calm weather was that we were able to investigate some of the 10- to 100-m-scale features shown in the multibeam backscatter and shaded-relief images. Prior to the cruise, we hypothesized some of the meandering channels to be glacial outwash streams formed at lower stands of sea level. The video observations showed that they were floored with rounded cobbles, confirming the hypothesis.

Offshore of Scituate, high-backscatter features trending downslope turned out to be extremely coarse well-sorted sand (the sediment had the texture of grapenuts!) that was formed into large megaripples; the transition between these features and the adjacent muddy bottom occurred over a few meters. The processes that form and maintain these features are unclear. What appeared to be 'pinnacles' in the shaded-relief images turned out to be spectacular outcropping bedrock. All of the submerged 'drumlins' shallower than about 25 m had a distinctly different biological community—purple and yellow calcareous algae, kelp, etc.—compared to those deeper than 25 m.

In short, the backscatter data from multibeam show the spatial patterns, but interpretation benefits greatly from seeing the bottom—a picture is worth a thousand words (well maybe a few pixels)! The direct visual observations complement the multibeam data and demonstrate the value of using different tools that image at different spatial scales.

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September 1999

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in this issue: cover story:
Seeing the Bottom

Bathymetric Data: Waverunners

Sediment Database

Mamala Bay Web Site

Hurricane Camille

DOI Environmental Award

Hein Elected IMMS President

Robbins Attends FWC Ceremony

Visitors

September Publications List