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Underwater Videographic Survey of Submerged Aquatic Vegetation in Sequim Bay, WA
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A time sequence of airborne infrared imagery provides a unique view of phenomena associated with a turbulent tidal intrusion
into a stratified bay. During flood tide, cooler water from the Strait of Juan de Fuca is observed to penetrate Sequim Bay
(Washington, U.S.V.) as a turbulent jet. After separating from the shoreline, the jet collapses into the stratified middle
part of the bay, forming a mushroom-shaped head consisting of a semicircular plunge front and areas of recirculating flow.
As the plunge front advances into the estuary, a set of nonlinear internal waves emerges and propagates toward the relatively
stagnant southern part of the bay, where they are a potential source of vertical mixing. This range of phenomena is expected
based on laboratory studies, but has not been seen previously in a natural setting.
Although seagrasses have been identified as vital living marine resources, their distribution has not been rigorously quantified at many locations. This fact is often due to the high cost of sampling seagrass habitats, especially those with deep-water plants that cannot be sensed from aerial platforms. We present a cost-effective method of estimating basal area coverage of submersed vegetation that uses differential global positioning system data linked to underwater video images of the bottom. Our sampling design and statistical procedures are identical to estimating proportions using cluster sampling with unequal cluster sizes. This method has several advantages over other techniques: (1) confidence intervals around basal area coverage estimates permit hypothesis testing of changes over time; (2) sampling efficiency is better than simple random sampling with quadrats; (3) deep-water zones out of the range of aerial platforms can be sampled; (4) video images provide positive identification of plants which is not possible with acoustic techniques; and (5) the techniques provide a permanent archive of visual images that can be analyzed for other bottom attributes, such as other vegetation, macro-invertebrates, and gross sediment types. This method has some limitations: (1) it is not possible to sample extremely shallow or turbid waters and under some physical structures; (2) it is impractical to sample very large regions; (3) errors in differential global positioning system data must be accounted for; and (4) seagrass density must be measured subjectively. We illustrate our sampling methods and data analysis with an example from Puget Sound, Washington, USA.
Keys to the Seaweeds and Seagrasses of Southeast Alaska
- P W Gabrielson
- T B Widdowson
- S C Lindstrom
Gabrielson, P.W., T.B. Widdowson, and S.C. Lindstrom. 2006. Keys to the Seaweeds and
Seagrasses of Southeast Alaska, British Columbia, Washington and Oregon. Phycological
Contribution No. 7, University of British Columbia, Department of Botany. 209 pp.
A study of sampling and analysis methods: Submerged Vegetation Monitoring Project at year 4. Nearshore Habitat Program, Aquatic Resources Division, Washington State Department of Natural Resources
- P Dowty
Dowty, P. 2005. A study of sampling and analysis methods: Submerged Vegetation
Monitoring Project at year 4. Nearshore Habitat Program, Aquatic Resources Division,
Washington State Department of Natural Resources, 1111 Washington St SE, 1 st Floor,
PO Box 47027, Olympia, WA.