One hundred years ago, exploration of the Gulf of Maine involved dabbing lard on the end of a lead weight to collect a bit of bottom sediment. Today, we have much more sophisticated tools to analyze the geological characteristics of the sea floor.
According to Walter Barnhardt, Director of the Woods Hole Coastal and Marine Science Center, a division of the United States Geological Survey (USGS), “as a technology-dependent process, sea floor mapping is always changing.”
Barnhardt notes that with recent advances in electronics, computing power and satellite navigation, he and his colleagues have access to state-of-the-art instruments designed to detect depth, sediment type, and bottom topography in vivid detail.
“We have the world’s best marine electronics,” said Barnhardt, “Our technicians are as important as the PhDs!”
But sea floor mapping is not just an exercise in trying out the latest in technology; it provides an in-depth understanding of the interactions of human activities and coastal and marine processes that is quite literally laying the groundwork for marine spatial planning and ecosystem-based management.
Navigational charts, designed primarily to identify hazards to surface vessel traffic, provide only the barest hint of what lies under the sea. Today’s sea floor maps, by contrast, accurately represent the geological forms and processes of the sea bed.
Sea floor mapping is accomplished today in a three-fold process that starts with the use of sound-emitting instruments carried onboard or towed behind research vessels and designed to collect information from an inhospitable environment not easily assessed by direct observation.
Barnhardt describes how one approach, multibeam bathymetry, is used to analyze the sea floor: “The instrument emits a series of pings on either side of the vessel that creates a swath of data; the vessel goes back and forth over the bottom overlapping swaths, just like a lawnmower. How wide is the swath? It depends on the water depth. In shallow water, the swath is pretty narrow. Imagine holding a flashlight pointed at the floor; as you move the flashlight higher in the air, the swath gets wider. The narrower the swath the higher the resolution of the information produced.”
Because the width of the swath is a function of the depth of the water, identifying small features, such as shipwrecks, is most easily accomplished in relatively shallow water. In waters more than a mile deep, multibeam bathymetry can pick out submarine landslides and canyons along the edge of the continental shelf.
The second step occurs in the computer lab as the huge amounts of raw data produced by the instruments—a terabyte (1,000 gigabytes) of data can be generated in a month of surveying—is processed to associate signals in the data with known geological phenomena. Underwater photography and video and the collection of sediment samples are used to ground-truth data processing.
The third and final step involves analysis and interpretation of the processed data by marine geologists, a practice that, Barnhardt said, “requires intelligence, education and imagination.”
Satellite-based global positioning systems (GPS) allow for detection of subtle changes in the sea floor over time. As the position of the research vessel shifts with the wind, tides, and currents, its GPS ensures that the data collected by onboard instruments is georeferenced within a fraction of an inch. This allows for data from subsequent surveys to be compared at a very fine scale. Such analyses are useful in detecting the rate of erosion of a navigational channel, for example. Monitoring the dynamics of the sea floor also allows scientists to establish models that can be used to predict the rate of retreat of a shoreline or the fate of contaminated sediments.
Sea floor maps, done right and made widely available, “are useful for many purposes,” said Barnhardt; “A large scale characterization of the seabed can be used for identifying gravel resources, performing hazards analysis, and siting infrastructure.”
USGS partners with other agencies to develop maps for specific purposes; recent projects included working with the Bureau of Ocean Energy Management, Regulation and Enforcement (BOEMRE) to identify areas appropriate for development of wind power and with the Commonwealth of Massachusetts to support ocean planning within state waters.
Dan Sampson, GIS/Data Manager in the Massachusetts Office of Coastal Zone Management, described this state-federal partnership as “an opportunity to leverage our funds to do more; we are able to use the data collected to draft policy.”
The Massachusetts Ocean Management Plan, the first of its kind in the US, was based in part on sea floor maps generated by the partnership.
“We also use the maps to review proposals for projects such as pipelines and gravel mining,” Sampson said. “We can get involved in a project upfront, before a final site is selected. We can guide project proponents to appropriate sites, less likely to raise conflicts with other uses.”
Is there a role for sea floor mapping in fisheries management? As the most persistent feature of fish habitat, the sea floor is the most easily mapped—and most easily incorporated in regulations designed to protect areas important to fish. However, although the ocean bottom is an important component of fish habitat, it is not the only component or even, in ecological terms, necessarily the most important one.
Barnhardt noted that maps produced by the USGS describe the physical properties of the sea floor and are of limited use in managing fisheries, given our current understanding of the link between the sea floor and the overlying ecosystem.
“While there are geologic controls on fish habitat, the jump that is difficult to make is understanding the correlation of a species of fish to the bottom.”
Sampson, involved in mapping habitat in state waters, concurs: “We are at the beginning stages of understanding the linkages between the bottom and the animals that live in the water column; the relationship is hideously dynamic and complicated!”
Currently, fisheries management in the US is based on the development of fisheries management plans for individual species or groups of similar species. Models of fisheries production describe fish stocks in terms of gains, due to recruitment, and losses, due to harvesting and natural mortality. Not enough is known about the relationship of such gains and losses to characteristics of the sea floor to incorporate habitat maps in the stock assessment process.
The National Marine Fisheries Service (NMFS) envisions a shift to ecosystem-based management, which will be focused on the suite of fisheries prosecuted within a given ecologically defined area. Building on the stock assessments that form the basis of the current approach to management, ecosystem-based management will incorporate ecological interactions among commercially harvested species but also the relationship between these species and the ecosystem properties of the area. Sea floor mapping will play an important role in determining and defining the areas to which ecosystem-based management will be applied.
Implementation of the National Ocean Policy (NOP), established by executive order by President Obama in 2010, calls for use of coastal and marine spatial planning as the means for achieving ecosystem-based management for all activities, including fisheries, in the US Exclusive Economic Zone. It is a sure thing that this comprehensive approach to managing the nation’s ocean resources will start at the bottom—with state-of-the art-sea floor mapping.
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