Sediment structural properties mediating dominant feeding types patterns in soft-bottom macrobenthos of the Northern Adriatic Sea

In sediments sampled in Northern Adriatic Sea invertebrate benthic macrofauna was examined in comparison with both substratum bulk parameters (average grain size, sorting, total organic matter content) and attributes of grain size classes (specific organic matter content of dimensional fractions). The aims were: 1) to determine whether variations of community overall parameters (total spatial density, number of taxa) and in numerically dominant feeding types (deposit-feeders and suspension-feeders) patterns were related to changes in sediment general properties; 2) to evaluate whether the observed patterns of abundance for deposit- and filter-feeders had more valid correlates in specific properties of substratum dimensional fractions. Number of taxa and animal spatial density resulted conventionally related to substratum bulk parameters while feeding types patterns in comparison with sediment total organic matter content exhibited a reciprocal negative effect; only when both feeding types and sediment structure were resolved to their specific attributes in terms of prevalence of tubicolous taxa among deposit-feeders and organic matter richness of single dimensional fractions, the analysis emphasized patterns unequally affected by specific size fractions abundance and organic matter content: deposit feeders demonstrated to be related to the presence in the substrate of intermediate grain size classes necessary for tube-building. Those intermediate fractions proved to be more adequate descriptors than average grain size or other overall sediment parameters per se of relations actually established between feeding types and the complex nature of bottom sediments. Our results emphasize that for Northern Adriatic Sea benthic communities, sediment organic matter content may represent a factor of minor importance in comparison with other substrate attributes, for which it may be necessary a detailed analysis of sediment structure. Thus, we concluded that the complexity of soft-bottom communities may defy any simple paradigm relating macrobenthic patterns to any single sediment bulk attribute, and we propose a shift in focus towards an higher resolution of both functional groups in macrobenthic associations (as already suggested in other investigations) and of substratum structural description