Determining bathymetric distributions of the eelgrass Zostera marina L. in three turbid estuaries on the eastern North Pacific coast

摘要

Improved methods for determining bathymetric distributions of dominant intertidal plants throughout their estuarine ranges are needed. Eelgrass (Zostera marina) is a seagrass native to estuaries of the northeastern Pacific and many other marine regions of the northern hemisphere. The techniques described here employed large-format aerial photography using false color near-infrared film with digital image classification, and the production of digital bathymetric models of shallow estuaries, such as those occurring in turbid waters of the United States Pacific Northwest. Application of geographic information system procedures to Z. marina classifications and estuarine bathymetries yielded intertidal eelgrass bathymetric distributions based on a very large number of data points. Similar bathymetric patterns were obtained for the three estuaries surveyed, and approximately 90% of the classified Z. marina occurred within the depth range -1.0 m to +1.0 m (mean lower low water). Comparison of these depth distributions with ground surveys of Z. marina lower depth limits indicated that the area of undetected subtidal eelgrass constituted <10% of the areal distributions classified (with >= 86% overall accuracy) in each estuary. Zostera marina areal distribution in one estuary was distinctly different from those in the other two systems, which illustrated the potential usefulness of this technique in exploring causal factors for such differences in areal distributions of estuarine intertidal vegetation.