Epibenthic, agglutinating foraminiferans in the Santa Catalina Basin and their response to disturbance

Abstract

There are five common species of large (0.5–6 cm long) epibenthic, agglutinating foraminiferans in the Santa Catalina Basin (1200–1350 m). This paper describes their basic ecology and response to mound disturbance. Combined, the five species attain mean densities of 200–300 individuals per m 2 and their protoplasm has an average biomass of 199.5 mg m −2. Individual species occur at densities ranging from 7 to 100 m −2, and each species has a different population size structure. Protoplasm comprises <2% of test volumes. Analysis of excess 234Th revealed no indication of particle sequestering within tests, and acridine orange direct counts of bacteria provided no evidence of microbial gardening or enhancement associated with tests. Twenty-five per cent of tests examined had metazoan associates; approximately half of these were polychaetes. Experiments were carried out to investigate the response of the epibenthic foraminiferal assemblage to disturbance from large, biogenic mounds, a common feature on the Santa Catalina Basin floor. Three branched forms, Pelosina cf. arborescens, P. cf. cylindrica and a mud-walled astrorhizinid, were most abundant on background sediments, less common on natural mounds and absent from artificially-created mounds exposed for 10.5 months. Two spherical species, Oryctoderma sp. and a different mud-walled astrorhizinid, were present at similar densities on artificial mounds (9.5–10.5 months old), natural mounds and undisturbed sediments, but Oryctoderma sp. attained largest sizes on mounds. These two species appear to be opportunistic taxa that can colonize and grow rapidly on mound sediments. This study suggests that disturbance, in this case that by sediment mound builders, is an important source of spatial heterogeneity in deep-water foraminiferal communities. Where sediment mounds occur, foraminiferal assemblages will experience disequilibrium dynamics.