Over 50 species from seven phyla of invertebrates have been identified as infauna from within lagoon sediments at Enewetak Atoll. These infauna, as well as representatives from at least 15 families of mobile fishes, disrupt the soft benthos and contribute to the process of sediment
mixing. Callianassid shrimp and hemichordate worms (enteropneusts) appear to have the greatest impact, both visually as well as quantitatively (i.e., in terms of depths and rates of bioturbation). Three different field techniques were used to measure rates of sediment processing by callianassids
and/or enteropneusts at Enewetak and Bikini Atolls. For some individual callianassid mounds, sediment production exceeded 1,300 cc/day; some individual enteropneusts could process over 700 cc/day. Mean estimates per m2 yielded rates as high as 800 cc/m2/day for callianassids
and ca. 600 cc/m2/day for enteropneusts. Although callianassids are known to alter the grain size distribution of sediments, we have observed that enteropneusts do little, if any, sorting and consequently do not appear to alter sediment grain size in their environment. The potential
consequences of massive bioturbation and mixing of lagoon sediments is discussed in light of long-lived transuranic radionuclide elements that are deeply buried in the lagoon. Deep bioturbation provides a mechanism whereby these radionuclides are pumped to the sediment/water interface and
this process could facilitate their re-entry into higher level marine food chains.
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