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Monitoring the expanding distribution of non-indigenous dwarf eelgrass Zostera japonica in a Pacific Northwest USA estuary using high-resolution digital aerialphotomaps
Citation:
Young, D. R., P. J. CLINTON, D. T. SPECHT, T. H. DEWITT, AND H. LEE. Monitoring the expanding distribution of non-indigenous dwarf eelgrass Zostera japonica in a Pacific Northwest USA estuary using high-resolution digital aerialphotomaps. Presented at Spatial Science, East Perth, AUSTRALIA, January 02 - 06, 2008.
Impact/Purpose:
The proliferation of non-indigenous species is a world-wide issue. Environmental managers need improved methods of detecting and monitoring the distribution of such invaders over large areas.
Description:
The proliferation of non-indigenous species is a world-wide issue. Environmental managers need improved methods of detecting and monitoring the distribution of such invaders over large areas. In recent decades, numerous estuaries of the Pacific Northwest USA have experienced the expansion of the non-indigenous dwarf eelgrass Zostera japonica. This marine angiosperm appears first to gain a foothold in the upper intertidal zone, where it can persist at relatively low densities for many years. In Yaquina estuary on the central Oregon coast, Z. japonica first was observed in 1974, and a July 1997 aerial photomap of drained tideland and corresponding ground survey showed that the distribution of the dwarf eelgrass was limited to the uppermost portion of the intertidal zone mainly in the lower estuary. This paper reports the subsequent development of a procedure to process digital aerial orthophotography obtained by scanning 1:20,000 false color, near-infrared film at 12 microns to obtain classifications of native eelgrass, benthic green macroalgae, and non-indigenous dwarf eelgrass using a hybrid digital image classification procedure involving interactive vegetation index masking, unsupervised isoclassification and manual raster editing. The ground pixel size obtained is 0.25 m x 0.25 m, and evaluation of a 10 pixel square area yields a minimum mapping unit of 2.5 m x 2.5 m. The image classification is evaluated using results from a detailed ground survey of randomly selected stations of the same area, positioned using a sub-meter GPS, to obtain a classical accuracy assessment. Transport via hovercraft greatly facilitates locating the randomly assigned ground stations within each class stratum. A second aerial photosurvey conducted in July 2007 and subsequent digital image classification showed a two order-of-magnitude increase in the distribution of this non-indigenous species. Thus, application of this remote sensing procedure has documented the rapid expansion of the distribution of Z. japonica throughout Yaquina estuary in recent years, and demonstrated the utility of this monitoring technique to environmental managers concerned with evaluating the spread of non-indigenous plants in estuarine intertidal ecosystems.