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During the cruise, more than 40 hours of georeferenced digital sea-floor video were collected along 52 transects in water depths of 15 to 370 m in Glacier Bay to
The video sled was equipped with forward- and downward-looking color video cameras, a pair of lasers to provide scale (by producing two red dots on the sea floor exactly 20 cm apart), and sensors to measure water depth, heading, pitch, roll, and height above bottom. The data and video were transmitted to computers and digital recorders aboard the Alaskan Gyre by a 2,000-ft-long cable, then combined with the ship's position as determined from differential global-positioning-system (DGPS) data. During video collection, the scientists recorded real-time observations of sea-floor characteristics (including primary and secondary substrate type, slope, roughness, and benthic biomass), as well as the presence of benthic organisms and demersal fish (fish normally found near the seabed).
Commonly observed substrates include bedrock, boulders, cobbles, rippled sands, large sand waves, and mud. The sea-floor video revealed extensive beds of living Modiolus (horse mussels) and scallops, features that had not been previously described. Among the other important organisms observed were Tanner, Alaskan King, and Dungeness crabs; halibut, flounder, sole, pollock, rockfish, and sculpins; numerous shrimp; and sessile invertebrates, such as gorgonians, sea pens, sea stars, and urchins. Using these observations, coastal and marine geologists from the Pacific Science Center are collaborating with ecologists from the Alaska Science Center to investigate the relations between geologic features of the sea floor and the biologic communities that inhabit them.
Commercial-fishing closures mandated by Congress in 1999 in parts of Glacier Bay National Park and Preserve (southeastern Alaska) created a network of five protected areas, which make up one of North America's largest temperate-marine reserves. Twelve active tidewater glaciers remain in the Glacier Bay fiord, where historical rates of glacier retreat are among the highest documented worldwide (more than 60 km of retreat in the past 200 years). In collaboration with the National Park Service, the USGS conducted sidescan- and multibeam-sonar surveys in 1998 and 2001 to map the acoustic reflectance and bathymetry of the sea floor in lower and central Glacier Bay. These data provide information about the geologic characteristics of the sea floor and offer the opportunity to link studies of sea-floor geology and benthic ecology.
Geologic features of the sea floor provide essential habitat for benthic communities that sustain the demersal (sea floor) and pelagic (surface and water column) fisheries of the world ocean. An understanding of sea-floor composition and sedimentology, benthic-habitat change, and trophic relations is essential to make informed resource-management decisions and to evaluate the design and long-term utility of marine reserves.
Global climate change may be accelerating glacier retreat and associated sedimentation in Glacier Bay, impacting essential habitats for commercially and ecologically valuable fish and crabs. Sediment samples, gravity cores, and digital bed-sediment images that were collected during the cruise will be used to examine rates and patterns of sedimentation, grain-size distribution, and organic-carbon content in the region.
A detailed geographic-information-system (GIS) database has been constructed, using the software program ArcGIS (with help from Pete Dartnell, USGS Western Coastal and Marine Geology Team [WCMG], Menlo Park, CA); the database contains geophysical, videographic, and sedimentologic data from WCMG cruises conducted in 1998, 2001, and 2004, as well as data contributed by ecologists at the Alaska Science Center. This GIS database is being used to produce a benthic-habitat map that characterizes the sea-floor geology of Glacier Bay and illustrates the distribution of biologic communities. The map will be used by scientists, managers, and the National Park Service to
Methods developed in the collection and analysis of data from Glacier Bay will be useful in studies of other areas where geologic change is occurring over biologically meaningful time scales. Collaboration between geologists and biologists enables the sharing of knowledge and resources to improve our understanding of habitat structure and function and to make predictions regarding future change in these marine systems.
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