Estimating the abundance of benthic invertebrates: A comparison of procedures and variability between observers

Different procedures for underwater sampling of epifaunal organisms were compared for their robustness to bias due to observers and precision using multifactorial sampling designs. Variability among 3 observers was tested in relation to: (1) the method employed to estimate the percent cover of organisms (visual vs point-intercept technique); (2) the size of quadrats (50 x 50 vs 20 x 20 cm); (3) stress (sampling at the beginning vs the end of the dive), and (4) random factors that were likely to change from dive to dive. Precision was expressed in terms of standard errors (over 3 replicates) obtained with each method and size. Two cnidarians (Astroides calycularis and Leptosammia pruvoti) and 2 sponges (Petrosia ficiformis and Geodia cydonium) were considered in this study because of their abundance at the study site (a submarine cave). Much of the variability was related to significant differences between observers that changed from dive to dive for estimates of the cover of A. calycularis, and from dive to dive and with method for L. pruvoti. The small quadrats were more precise than the large ones when used to estimate the percent cover of A. calycularis and L. pruvoti, irrespective of method. In contrast, for P. ficiformis the small quadrats were more precise if sampled with the visual method, while the reverse occurred for the large quadrats. A trend toward a greater precision of the large quadrats sampled with the visual method was evident for G. cydonium, although no significant effects were found. Pilot studies and cost-benefit analyses were also used to determine the optimal allocation of resources (time) for sampling epifaunal organisms at different spatial scales in the cave, for each method and size of quadrats. The small units in conjunction with the visual method were most efficient. This procedure offered the best compromise between repeatability among observers, precision and maximization of replication for a fixed amount of resources. The large quadrats in conjunction with the visual method were probably more adequate for G. cydonium, given the apparent greater precision provided by this procedure for the large sponge. Caution is recommended in employing different researchers working on the same sampling project in extreme environments, at least before divergence among observers to changing environmental conditions is accounted for.