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Bioturbation in the Deep Ocean Robert A Limno!. Oceanogr., 37(1), 1992, 90-104 © 1992, by the American Society of Limnology and Oceanography, Inc. Experimental tests for particle size-dependent bioturbation in the deep ocean Robert A. Wheatcroft' School of Oceanography, WB- 10, University of Washington, Seattle 98195 Abstract The potential for particle size-dependent bioturbation rates was experimentally tested at 1,240 m in the Santa Catalina Basin (eastern Pacific). Spherical glass bead tracers in five size classes (8- 16, 17-31, 32-62, 63-125, and 126-420 m) were spread over the sediment surface and tube cored 997 d later. Downcore concentrations of glass beads were enumerated in each of the five size categories and Page's L-test was used to test the null hypothesis of equal vertical penetration of all size classes of tracer. In all cores the null hypothesis was rejected; finer tracers penetrated deeper into the sediment. In two of the three cores, vertical biodiffusivities were computed from concen- tration profiles of downcore tracers. These also showed size dependence, with biodiffusivities ranging from 1 cm2 yr' for the 8-1 6-pm fraction to 0.1 cm2 yr' for the 1 25-420-pm size class. These data demonstrate that vertical bioturbation rates are particle size-dependent in Santa Catalina Basin. The likely cause is preferential ingestion and downward transport of fine particles by deposit feeders. Nearly all particles that reach the floor ofplacement of sediment grains also destroys the ocean are displaced several times by an- or blurs sedimentary signals (Wheatcroft imals before they are buried to become part1990). Therefore, a better understanding of of the sedimentary record. This mixing ofbioturbation, especially its rates, styles, and sediment or bioturbation has profound ef-variability would benefit a wide range of fects on a wide range of phenomena. Ratesaquatic disciplines. of organic matter decomposition, for ex- In the past scientists have approached ample, as well as the dissolution of nearlybioturbation from one of two directions. On all sedimentary constituents (e.g. CaCO3 andthe one hand, benthic ecologists (e.g. Rhoads Si02) are markedly influenced by the rate 1967) have typically focused on a single spe- of sediment mixing (Berner 1980). Similar-cies (usually from a shallow-water, temper- ly, the distribution of solid and liquid phaseate site) and observed rates and styles of nutrients is mediated by bioturbation, im-sediment reworking under a range of con- plying that there are likely to be strong feed- ditions (e.g. changing temperature). Follow- backs between the style and rate of mixinging nearly two decades of such autecologic and the distribution of infauna. The dis-studies there is a large body of literature (summarized by Thayer 1983; Wheatcroft et al. 1990) demonstrating that sediment 'Present address: Applied Ocean Physics and En- displacement is tremendously complex. gineering Department, Woods Hole Oceanographic In- stitution, Woods Hole, Massachusetts 02543. Deposit feeding and hence particle displace- ment rates have been shown to be tem- Acknowledgments I thank C. Smith and P. Jumars for aid during the perature- (Rhoads 1967), particle-shape- initiation of this research and dialogue throughout the (Whitlatch 1974), and particle-size- (Powell entire project. This work could not have been accom- 1977) dependent. Moreover, there seem to plished without the aid and expertise of the master and be ontogenetic changes in a species' particle crew of the RV Atlantis II, especially the Alvin group. selectivity, as well as interactions between Help with on-deck processing of cores was provided by D. Penry, R. Pope, L. Self, and a host of others. different environmental cues, that further Previous versions of the manuscript benefited from complicate mixing. comments by J. Bourgeois, C. A. Butman, P. Jumars, The other approach to bioturbation has and A. Nowell. I thank all of these people. been taken primarily by geochemists who This work was supported by Office of Naval Re- search contract N000l4-90-J- 1078 to P. Jumars andrequire a community-wide estimate of the A. Nowell, and is contribution 1889 from the School sediment mixing rate to characterize the role of Oceanography, University of Washington. of bioturbation as a mass transfer mecha- 90 Particle size-dependent bioturbation 91 nism. This goal is achieved by making var- to be associated with finer particles. Coch- ious simplifying models of bioturbation.ran (1985) also found a disagreement be- Currently the most widely used model istween mixing coefficients calculated from based on an analogy with standard Fickianthe profiles of the bomb-produced radioiso- diffusion (Guinasso and Schink 1975;topes, '37Cs and 239'240Pu vs. 210Pb. He sug- Wheatcroft et al. 1990). Community-widegested differential mixing of different-sized mixing rates are obtained by measuring theparticles, but a!so stated that selective mo- vertical concentration profiles of variousbility of the radionuclides cou!d not be ruled tracers and fitting theoretical profiles to theout. An additional independent line of ev- observed gradients. The best-fit curves con-idence that suggests mixing is size-depen- tain various parameter values, one of whichdent comes from the studies of CaCO3 ages is biodiffusivity (Db). This coefficient rep-in the mixing layer. Berger and Johnson resents all of the myriad sediment-displac-(1978) hypothesized that discrepancies be- ing activities of the benthos at a given site,tween 14C- and t8O-derived dates might re- and thus, much of the complexity observedsult from greater mixing of the bulk car- by benthic ecologists is incorporated intobonate fraction relative to the large Db. An untested and extremely importantforaminiferans from which the oxygen rec- question is whether that complexity willord was derived. manifest itself in the community-wide mea- An explicit test for size dependency in sures of mixing (e.g. biodiffusivities) in adeep-sea mixing was made by Ruddiman et systematic and predictable manner. That is,al. (1980). They measured the vertical dis- is it necessary or even possible to explicitlypersion of different-sized ash particles and, incorporate biology into the biodiffusionon the basis of visual comparisons of the coefficient? concentration profiles, cautiously conclud- This question is addressed by focusing oned that mixing was not size-dependent. Since an issue that has a long history (consideringthen, their data have been statistically rean- the youthfulness of this field) in studies ofalyzed by Wheatcroft and Jumars (1987), bioturbation: the question of particle sizewho found that there was indeed greater dependence in deep-sea mixing rates. Asmixing of the fine fraction. Although the previously mentioned, several autecologicresults of their reanalysis and the inferential laboratory studies have demonstrated pref-evidence discussed above qualitatively sug- erential ingestion of fine particles by depositgest mixing rates are size-dependent in the feeders (see Taghon 1989). In addition, manydeep sea, the magnitude of the relationship head-down deposit feeders preferentiallyis unclear. My primary goal here is to de- ingest fine-grained sediments resulting inscribe results of an a priori test for particle biogenic graded bedding, whereby one pass- size-dependent bioturbation in the deep es from fine to coarse grains moving downocean. A secondary goal is to discuss animal into the sediment. These observations,activities that displace sediment within the however, have been made on a limited sub- context of their potential for particle size- set of animals, mainly temperate, shallow-dependent mixing. water deposit feeders in sandy environ- ments. To what extent these shallow-waterBackground observations apply to deep-sea bioturbation All animal activities that displace sedi- rates is presently unknown. ment have the potential to produce size- There is some inferential evidence thatdependent transport rates. The direction of deep-sea mixing is indeed size-dependent.bias (i.e. preference for fine or coarse par- Severalrecentradioisotopicstudiesticles), however, often differs within and be- (DeMaster and Cochran 1982; Stordal etal. tween activities; thus, there is the possibility 1985) have reported disparities betweenthat the integrated community-wide effect mixing coefficients derived from various ra-could be neutral. That is why tests for par- dionuclides (e.g. 210Pb vs. 325i or 210Pb vs. ticle size-dependent mixing must be made 239'240Pu). In both cases the radionuc!ide that in the field, where the summed activities of yielded the higher mixing rates was inferredall animals in a given community can be 92 Wheatcrofi assessed. Size-dependent mixing rates (par- gravitationally forced vertical migration oc- ticle migration, sensu Williams and Khancur. The first is downward migration of fine 1973) can be produced actively or passively. particles (termed percolation, Williams and Passive particle migration often occurs inKhan 1973), due to the greater tendency of industrial mixing (Williams and Khan 1973;the fines to settle into voids created in the Stephens and Bridgwater 1978), where it ismoving particle matrix. This process in turn typically caused by spatial heterogeneity inresults in a second upward migration of the local strain rate and resultant porositycoarse particles because the weight of coarse field. Some animal activities (e.g. burrow- particles causes an increase in pressure be- ing) may cause passive particle migration.low it that stops the particle from moving Most
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