234th and 210pb Evidence for Rapid Ingestion of Settling Particles by Mobile Epibenthic Megafauna in the Abyssal Ne Pacific

Alternative method for the calculation of mean time for the assessment of secondary production by true cohort analysis. Abstract-Particle-scavenged 234Th and *'OPb can be used to trace the fate of particulate matter reaching the deep-sea floor. We used this technique to demonstrate rapid ingestion of particles arriving at the sea floor (at a depth of 4,100 m) by mobile epibenthic holothuroids (Abyssocucumis abyssorum and Oneirophuntu mutubilis). Excess 234Th and *'OPb activities of sediment trap material, detrital aggregates from the sea floor, and animal gut contents all were similar while activities of surface sediments (top O-5 mm) were considerably lower. A simple calculation using the excess *'OPb concentration of two potential food sources, sediment trap material and surface sediments, shows that-91% of gut content material of A. ubyssorum must have come from material similar to that found in the sediment trap cups. By setting the sediment trap material as age 0, apparent ages of CO-20 d, >lOO d, and 12-13 d were estimated for the aggregates, surface sediments, and A. ubyssorum guts, respectively. The population of A. ubyssorum at this site could potentially process 0.2-4% of the vertical mass flux (m-* d-l) during the high particle flux period between June and October 1994. The percentage of vertical flux processed could be substantially greater when all of the mobile epibenthic megafauna species are considered. Biological processes have an important role in determining the degree of recycling or burial of particles reaching the deep-sea floor. Pelagic-benthic coupling between settling particles and microbial and infaunal components of the abys-sal benthos has been documented by measurements of sediment community oxygen consumption and growth rates of bacteria and protozoa (Goo-day and Turley 1990). Much less is known about the degree of trophic coupling between settling material and the larger fauna living on the abyssal sediment surface. Only a few bioturbation studies (e.g. Wheatcroft 1991; Hoover and Smith unpubl.), several reports of large epibenthic animals feeding on fresh detrital material (e.g. Billett et al. 1988; Pfannkuche and Lochte 1993), and several studies of mega-fauna1 activity and respiration rates (e.g. LaFonde 1967; Smith 1983; K. L. Smith et al. 1993) address the potential role of mobile epibenthic megafauna in determining the fate of sinking particles. Epibenthic megafauna (animals living on the sediment surface and large enough to be visible in photographs) generally are not as abundant as their smaller infaunal counterparts and often are not considered in studies …