L. Benedetticecchi et al., ESTIMATING THE ABUNDANCE OF BENTHIC INVERTEBRATES - A COMPARISON OF PROCEDURES AND VARIABILITY BETWEEN OBSERVERS, Marine ecology. Progress series, 138(1-3), 1996, pp. 93-101
Different procedures for underwater sampling of epifaunal organisms we
re compared for their robustness to bias due to observers and precisio
n using multifactorial sampling designs. Variability among 3 observers
was tested in relation to: (1) the method employed to estimate the pe
rcent cover of organisms (visual vs point-intercept technique); (2) th
e size of quadrats (50 x 50 vs 20 x 20 cm); (3) stress (sampling at th
e 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 o
f standard errors (over 3 replicates) obtained with each method and si
ze. Two cnidarians (Astroides calycularis and Leptosammia pruvoti) and
2 sponges (Petrosia ficiformis and Geodia cydonium) were considered i
n 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 whe
n used to estimate the percent cover of A. calycularis and L. pruvoti,
irrespective of method. In contrast, for P. ficiformis the small quad
rats were more precise if sampled with the visual method, while the re
verse occurred for the large quadrats. A trend toward a greater precis
ion of the large quadrats sampled with the visual method was evident f
or G. cydonium, although no significant effects were found. Pilot stud
ies and cost-benefit analyses were also used to determine the optimal
allocation of resources (time) for sampling epifaunal organisms at dif
ferent spatial scales in the cave, for each method and size of quadrat
s. The small units in conjunction with the visual method were most eff
icient. This procedure offered the best compromise between repeatabili
ty among observers, precision and maximization of replication for a fi
xed amount of resources. The large quadrats in conjunction with the vi
sual method were probably more adequate for G. cydonium, given the app
arent greater precision provided by this procedure for the large spong
e. Caution is recommended in employing different researchers working o
n the same sampling project in extreme environments, at least before d
ivergence among observers to changing environmental conditions is acco
unted for.