Evidence for control of abyssal agglutinated foraminiferal community structure by substrate disturbance: Results from the HEBBLE Area

Abstract

Deep-sea agglutinated foraminifera have been quantitatively examined from 28 coretop samples from seven box cores collected in June, 1983, at the HEBBLE (High Energy Benthic Boundary Layer Experiment) Site (40°27′N, 62°20′W, 4815–4825 m depth) and HEBBLE Shallow Site (40°53′N, 63°44′W, 4185 m depth) on the Nova Scotian continental rise. Samples from the relatively tranquil HEBBLE Shallow Site can be distinguished from the HEBBLE Site on the basis of their microfauna. A Q-mode VARIMAX factor analysis of species-frequency data delineates three major faunal assemblages: 1. (1) A faunal assemblage from the HEBBLE Site dominated by Ammobaculties sp. cf. A. americanus. This assemblage exhibits lowest species diversity (suggesting a disturbed community) and strongly associated with box cores containing fine-grained, laminated sediments. 2. (2) An assemblage associated with box cores from the HEBBLE Meadow dominated by Saccammina tubulata, Psammosphaera sp., cf. P. fusca, a coarsely grained species of Psammosphaera and Reophax sp. 1. 3. (3) A fauna occurring at the HEBBLE Shallow Site, dominated by Hormosinella distans, with Reophax bacillaris and Ammobaculites agglutinans unique to the assemblage. This fauna displays a higher ratio of calcareous to agglutinated species, significantly higher species diversity, lower variability and a greater proportion of hormosinids and species utilizing finer grains than samples from the HEBBLE Site, which is interpreted as reflecting environmental stability. Marked heterogeneity exists among agglutinated foraminiferal assemblages within box cores. At the HEBBLE Meadow, within-box-core faunal variability exceeds between-box-core variability, suggesting that environmental factors influencing the distribution of species are patchy on a scale of approximately 40 cm.