AN EXAMINATION OF OBSERVER BIAS AS A SOURCE OF ERROR IN SURVEYS OF SEAGRASS SHOOTS

Mensurative experiments investigated the effects of different observer s on estimates of the density of shoots of two species of seagrass: Po sidonia australis Hook and Zostera capricorni Aschers. Balanced progra mmes of sampling were used to examine variation in counts of seagrass shoots attributable to different observers, sizes of quadrats, depths and locations within large beds of each species of seagrass. A separat e experiment examined differences between novice observers and a more experienced observer, when an 'optimal' size of sampling unit was used . Estimated densities of Zostera shoots varied inconsistently among ob servers, quadrats, depths and locations. Differences between observers were not affected by the size of quadrat used to count Posidonia shoo ts, but varied between locations in the seagrass bed. Experience had o nly a minor impact on biases. Only two of 12 novices produced counts t hat were different from the experienced observer. These results emphas ize the importance of considering both accuracy and precision in the d esign of field studies of seagrasses.