Seabed Radionuclides, Bioturbation and Benthic Community Structure at the Hawaii Ocean Time-Series Station ALOHA

C. R. Smith, D. J. DeMaster, R. H. Pope, S. P. Garner, D. J. Hoover and S. E. Doan

Abstract

We are characterizing sediment dynamics and benthic community structure at the Hawaii Ocean Time-Series (HOT) Station ALOHA (22 45'N, 158W) to allow seafloor processes at this site to be compared with those at presumably more eutrophic equatorial stations along 140W. Ongoing studies include evaluation of bioturbation, sedimentation and carbon-burial rates using a variety of particle associated radionuclides (e.g., ²³⁴Th, ²¹⁰Pb, ¹⁴C). In addition, megafaunal and macrofaunal abundance and biomass are being evaluated as indices of labile organic-carbon flux to the seafloor. Preliminary results from box-core and multiple-core samples collected at HOT Station ALOHA in early August 1992 are as follows. Seabed inventories of excess ²³⁴Th (half life = 24 d) were substantially higher at HOT than at 9 N along the 140 W JGOFS transect, but somewhat lower than inventories measured at 0, 2, and 5 N. This suggests substantially higher mass fluxes to the seafloor over the previous 100 d at ALOHA than at 9 N. Excess ²³⁴Th, which appeared to be associated with phytodetrital aggregates, penetrated to sediment depths of 4-5 cm at ALOHA indicating bioadvective transport of ²³⁴Th-enriched phytodetritus into the seabed on time scales << 100 d. In addition, we measured high rates of diffusive mixing (up to 15 cm²/yr) in the top centimeter of Station ALOHA sediment. Epibenthic megafaunal densities at ALOHA were substantially lower than at any equatorial stations (0, 2, 5, and 9 N), in accordance with oligotrophic expectations. Preliminary macrofaunal abundance data for ALOHA should be available by symposium time. In summary, Station ALOHA has many of the trappings of an oligotrophic benthic habitat; nonetheless, short-term rates of particle flux and sediment mixing can approach those of more eutrophic abyssal settings.