Effect of core surface area and sediment depth on estimates of deep-sea nematode genus richness and community structure

A variety of core sizes are used for sampling deep-sea nematodes but little is known about the potential effects of core dimensions on estimates of diversity and community structure. We investigated the effects of core surface area (subcores vs. cores; 6.6 vs. 66.4 cm(2)) and depth (shallow vs. deep subcores; 0-1 vs. 0-5 cm) on estimates of nematode genus diversity and community structure at six sites on the continental slope of New Zealand. We found that cores yielded significantly higher genus richness [expected number of genera in a sample of 51 individuals; EG(51)] than the smaller subcores (by up to a third), but found no significant difference between shallow and deep subcores. Conversely, nematode community structure was influenced by core depth but not surface area, reflecting a consistent shift in nematode community structure between surface and subsurface sediment layers among study sites. Average dissimilarity between shallow and deep subcores (45.2 %) was only slightly greater than average dissimilarity between subcores and cores (41.3 %); thus, the lack of a significant difference between subcores and the larger cores was likely due to the random (i.e., unpredictable) nature of horizontal variability in nematode community structure. Estimates of nematode diversity and community structure derived from subcores and the cores from which they were taken were not significantly correlated, suggesting that: (1) shifts in these attributes are not consistent between sites, and (2) patterns in nematode diversity and community structure are influenced by the choice of core size. The present study shows that a difference of a few centimetres in the physical dimensions of a core can have a substantial influence on estimates of deep-sea nematode diversity and community structure. Studies on spatial and temporal patterns of nematode diversity and/or community structure should therefore be based on cores with the same or similar dimensions. Meaningful comparisons of nematode diversity and community structure between environments should ideally take into consideration any potential differences in horizontal and vertical patchiness at small (cm) scales, and ensure that core surface area and penetration depths are sufficient to allow representative samples to be obtained across the entire range of environmental conditions sampled.