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Habitat, population and leaf characteristics of Zostera marina L. on Roberts Bank, British Columbia Moody, Robert


The sand and mud flats of the Fraser River foreshore support extensive meadows of the seagrass Zostera marina L. (eelgrass). Industrial, residential and recreational developments threaten these valuable foreshore areas. A study was undertaken into the habitat requirements and population and morphological characteristics of eelgrass on southern Roberts Bank, British Columbia to provide information which would help minimize the potentially deleterious effects of such developments on the eelgrass resource. Water temperatures and salinities and wave motion on southern Roberts Bank all approach the world-wide optima for eelgrass. The upper distributional limit of eelgrass was lower than those of other Pacific Coast eelgrass populations. The sandy nature of the substrate influences "desiccation" which, in turn, controls the intertidal limit of eelgrass growth. Light availability determines the lower distributional limit of eelgrass in other areas. These two factors, the sandy substrate and reduced light availability in the turbid estuarine waters of the Fraser River foreshore, appear to be responsible for the narrow depth range of eelgrass on southern Roberts Bank. A stratified random sampling technique was used to determine seasonal changes in eelgrass standing crop, turion density and leaf dimensions at five elevations, located at 0.5 m depth intervals, from the upper to the lower limits of eelgrass growth. A pronounced decline in both turion density and leaf standing crop occurred in late summer. Throughout the study period, leaf standing crops and turion densities were greatest at the three intermediate study elevations. Reduced leaf standing crops were found near the upper and lower edges of the eelgrass bed; no significant difference in standing crops was found for these two elevations. Turion densities were also lower near the upper and lower depth limits of eelgrass and a significant difference in turion densities was found between these two elevations, with the lowest turion density recorded near the lower limit of eelgrass. Near the upper edge of the eelgrass bed, turion weights and mean leaf lengths were one-half those of the lower elevations. A synthesis of the available information indicates that depth-related factors strongly influence certain morphological and population characteristics of eelgrass on southern Roberts Bank.

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