By Paul Macnab, Manager, The Gully Marine Protected Area, Canada
Until 1997, hydrographic coverage of The Gully, the largest submarine canyon in eastern North America, was minimal, with only 1% surveyed using single beam methods and widely-spaced vessel transects. Thankfully, that spotty treatment was expanded with multibeam surveys that supported the case for designation of The Gully Marine Protected Area in 2004. We now benefit from continuous hydrographic coverage for about 90% of the MPA. A number of multibeam surveys spanning MPA depths from 18 m to well over 3 km have revealed the size and shape of the canyon like never before. The Canadian Hydrographic Service and the Geological Survey of Canada undertook a series of shallow and mid-depth surveys, and deeper portions were mapped under special partnership arrangements with vessels of opportunity, including platforms mobilized by the oil and gas industry for nearby surveys.
Multibeam instantly exposed canyon features that were invisible in the first-generation hydrographic surveys. Geologists used additional data sources like sidescan sonar to interpret, classify, and map seabed characteristics, capturing the tremendous diversity of benthic habitats present in the MPA. Identified features include nine steep-sided feeder canyons and distinct ridge-valley formations along the main canyon walls plus bedrock outcrops. The thalweg or canyon floor clearly presents in the multibeam imagery as do notable bedforms at the head of the canyon including sand waves with some reaching heights over 11 m. Moraines, iceberg furrows, and other glacial relics are readily apparent on the canyon flanks. Seabed processes like erosion, sediment transport and deposition, slumping and gas seepage are also inferred from the survey data. Geological findings are presently being combined with optical survey results, biological records, grain size data, and oceanographic parameters to generate benthic habitat maps depicting texture, roughness, natural disturbance, scope for growth, and sensitivity. An example of a surficial geology map is at http://apps1.gdr.nrcan.gc.ca/mirage/full_result_e.php?id=224961.
Informing site management
There are many ways in which the improved seabed characterizations are informing site management. During the regulatory drafting process, the survey results enabled a more informed assessment of seabed risks as decisions were made about activities that would be allowed or restricted. Multibeam also produced an accurate basemap that in turn supported precise offshore legal descriptions including a fully protected core zone delineated along the 600-m isobath. A critical post-designation application has been in permitting. When MPA managers receive coordinates for proposed bottom-contacting activities, the locations are plotted in our GIS to enable direct comparison with seabed properties. This allows us to assess the likelihood and severity of impact before rendering an approval under the MPA regulations. Further, it provides an opportunity to suggest alternative locations where proponent objectives can still be met while avoiding potentially negative benthic impacts.
Having better seabed maps has also improved the planning process for research and monitoring activities. For example, when historical photographic and video sampling stations are compared with bottom type, we can gauge the extent of optical coverage and identify remaining habitat gaps for future sampling programs. We've also selected resilient low-slope environments for mooring deployments, and grab sample locations more likely to contain mud than sand, an important consideration when sampling for hydrocarbon contaminants trapped in surficial sediments. A final management application we'd mention relates to oceans literacy: Gully seabed maps and derived products like virtual fly-throughs and solid terrain models have been instrumental for stakeholder outreach and public education efforts.
For more information:
Paul Macnab, Department of Fisheries and Oceans, Halifax, Nova Scotia, Canada. E-mail: paul.macnab@mar.dfo-mpo.gc.ca