Distribution and abundance of epibenthic megafauna at a long time-series station in the abyssal northeast Pacific

Abundance and dispersion of epibenthic megafauna were examined at an abyssal site (4100 m depth) in the northeast Pacific (Sta. M, 34°50ʹN, 123°00ʹW). Photogrammetric techniques and line-transect theory were used to analyze photographs from nine towed camera-sled transects made on six cruises between June 1990 and October 1991. The megafaunal assemblage was dominated numerically by the tube-dwelling polychaete Paradiopatra sp. and echinoderms, representing 26–48% and 24–41 % of the total, respectively. Megafaunal densities were remarkably similar among the nine transects, ranging from 26,594 to 39,913 individuals 10−4m−2. While the overall assemblages in the nine transects were ⩽80% similar, based on pairwise comparisons, the relative contribution of each faunal category varied significantly between transects; densities of individual species from pairs of synoptic transects were significantly different (p < 0.05) in 43 of 69 cases. Analysis of dispersion was limited by small sample sizes, but overall the distributions of these organisms either did not deviate from random or tended toward aggregation. The dispersion of individual taxa often was highly variable within a single transect and among transects, indicating a high level of spatial heterogeneity. Spatial heterogeneity in density and dispersion overwhelmed any temporal variation that might exist, at least on short timescales (<1.5 years). These results illustrate how variable the benthos can be, even within a relatively small area, and that large areas need to be surveyed to adequately describe megafaunal community structure.