DEEP-SEA MACROBENTHIC COMMUNITIES AT CONTRASTING SITES OFF PORTUGAL, PRELIMINARY-RESULTS .2. SPATIAL-DISPERSION

Small-scale, biogenic processes are thought to be important in creating a mosaic of microhabitats that allow co-existence of the large numbers of rare species found in deep-sea sediments. However, the large-scale effects of hydrodynamic disturbance seem likely to be important in high-energy settings on the deep-sea bed. This is investigated by examining spatial dispersion patterns in the macrobenthic populations at two deep-sea sites off Portugal. The widely differing conditions of sediment disturbance at the two sites are related to differences in macrobenthic species diversity and dominance. Closely grouped, transponder-mapped, replicate box core samples, each divided into 25 ''vegematic'' subcores permitted analysis of dispersion patterns at two spatial scales at each site. One site was located on the Tagus Abyssal Plain (TAP), and the other in the nearby Setubal Canyon (SC) where ripple bedforms indicate Vigorous bed flow, probably with a tidal periodicity. Greater aggregation at the scale of tens of metres is indicated from the observed numbers of SC species per box core being wide underestimates of the value predicted by rarefaction of the pooled total compared to those from TAP. Variance/mean ratios for individual species of bivalves and tanaids indicated more aggregation in spatial distribution between box cores from SC than from TAP. Dispersion chi-square analysis also indicated more non-randomness at SC between box cores than for the TAP cores. However, low faunal abundances in the TAP samples render the results of analyses for this site somewhat inconclusive and therefore the trend indicating less aggregation at TAP compared to SC must be regarded as weak for the groups examined. Analyses of the distribution of tanaids, and bivalves at a 100 x 100 mm subcore scale showed no convincing significant departure from random expectation, but low abundances in these groups reduces' the sensitivity of the tests applied. Between-core aggregation detected at SC may reflect habitat Variation resulting from the varying depth within the area of sampling, while within-core patchiness could be influenced by the ripple bed forms on the sediment.