The Abundance and Distribution of Eelgrass (Zostera spp.) in Tillamook Bay Estuary, OR

Executive Summary

Seagrass beds have long been recognized as critically important components of estuarine ecosystems and are declining worldwide. In the Pacific Northwest, eelgrass ( Zostra spp.) beds may be adversely affected by human impacts from agriculture, forestry, oyster culturing and shellfish harvesting. As part of the continuing assessment of the estuarine environment, Tillamook Bay National Estuary Project undertook a study to measure eelgrass abundance and distribution throughout Tillamook Bay (30 km2 ) hoping to establish an initial baseline of eelgrass bed density and distribution and identify a means of monitoring the bay environment in terms of cover and substrate that was both accurate and cost effective.

A prototype airborne imaging system was used to collect multispectral data for Tillamook Bay at 1 m spatial resolution using 3 spectral bands (blue, red, and infrared). Over 300 individual images were taken during a period of maximum low tide in July 1995 in order to capture the majority of eelgrass beds exposed. Tillamook Bay characteristically dewaters over much of its area during these tidal events. Images were mosaiced then georeferenced prior to classification; however, due to the variability in sun angle and atmospheric conditions during the 4+ hour data capture time, each image had to be classified separately using an iterative unsupervised classification technique. Images were then recombined to make the final map. A number of site visits were used to enhance map thematic accuracy. Two densities of eelgrass beds plus a number of other vegetation classes and substrate types could be mapped for the entire bay using the results from the image classification.

Eelgrass beds were found to cover nearly 11% of the area of Tillamook Bay with the majority of the dense beds (composed of mostly Zostera marina L. ) in the northern 1/2 of the bay (Figure 1). The other species of eelgrass found in the bay, Zostera japonica , was found to grow almost exclusively in the southernmost 1/3 of the bay where sedimentation is most severe. This separation of the two species was observed during the site visitations, since Zostera japonica was indistinguishable from Zostera marina in spectral signature. While a reasonable assessment of the bay sediments could be made, highly discriminatory substrate mapping proved impossible due to the confounding effects of the varying degrees of wetness of the different substrate types.

Multispectral airborne imaging proved to be an effective way to map eelgrass in the type of ecological setting found in Tillamook Bay. With some improvement in mosaicing and edgematching of the imagery, this type of technology could be a very accurate, cost-effective alternative to mapping eelgrass dynamics. Merging these data with other ancillary data layers (e.g., oyster leases, tidal prism, water flow, and salinity) estuarine processes can now be examined through a series of spatio-temporal models.

For more information, please contact Jim Strittholt