Biodiversity of benthic invertebrates and organic matter processing in shallow marine sediments: an experimental study

The main objective of this study was to measure the impact of benthic invertebrate diversity on processes occurring at the water-sediment interface. We analyzed the effects of interactions between three shallow water species (Cerastoderma edule, Corophium volutator, and Nereis diversicolor). The impacts of different species richness treatments were measured on sediment reworking, bacterial characteristics, and biogeochemical processes (bromide fluxes, O₂ uptake, nutrient fluxes, and porewater chemistry) in sediment cores. The results showed that the three species exhibited different bioturbation activities in the experimental system: C. edule acted as a biodiffusor, mixing particles in the top 2 cm of the sediments; C. volutator produced and irrigated U-shaped tubes in the top 2 cm of the sediments; and N. diversicolor produced and irrigated burrow galleries in the whole sediment cores. C. edule had minor effects on biogeochemical processes, whereas the other species, through their irrigation of the burrows, increased the solute exchange between the water column and the sediment two-fold. These impacts on sediment structure and solute transport increased the O₂ consumption and the release of nutrients from sediments. As N. diversicolor burrowed deeper in the sediment than C. volutator, it irrigated a greater volume of sediments, with great impact on the sediment cores.Most treatments with a mixture of species indicated that observed values were often lower than predicted values from the addition of the individual effects of each species, demonstrating a negative interaction among species. This type of negative interaction measured between species on ecosystem processes certainly resulted from an overlap of bioturbation activities among the three species which lived and foraged in the same habitat (water-sediment interface). All treatments with N. diversicolor (in isolation and in mixture) produced similar effect on sediment reworking, water fluxes, nutrient releases, porewater chemistry, and bacterial characteristics. Whichever species associated with N. diversicolor, the bioturbation activities of the worm hid the effect of the other species. The results suggest that, in the presence of several species that use and modify the same sediment space, impact of invertebrates on ecosystem processes was essentially due to the most efficient bioturbator of the community (N. diversicolor). In consequence, the functional traits (mode of bioturbation, depth of burrowing, feeding behaviour) of an individual species in a community could be more important than species richness for some ecosystem processes.     

Macrofauna distribution and bioturbation on tidal confluences of the dutch Wadden Sea

Macrofauna density and bioturbation intensity (measured with X-ray radiography), were studied in the Dutch Wadden Sea near Ameland and in Mok Bay, Texel, in September 1988. The bivalveMacoma balthica and the polychaeteHeteromastus filiformis were widely distributed in the areas studied. The cockleCerastoderma edule and the polychaeteScoloplos armiger did not occur in areas with high clay content, while the molluscsHydrobia ulvae andMya arenaria preferred fine sediments. The polychaeteNereis diversicolor preferred the higher parts of the intertidal. In the Ameland area disturbance of primary sediment structures was highest near the shore and near the middle of the tidal confluence where physical reworking was low. The sheltered Mok Bay sediments were completely reworked by deposit-feeders. Bioturbation intensity and deposit-feeder (bioturbator) density were, however, not highly correlated. A number of stations showed higher bioturbation towards the surface and this may be related to the high abundance of near surface dwelling macrofauna, due to eutrophication and organic enrichment of the sediments in recent years.     

Effects of population density on the sediment mixing induced by the gallery-diffusor Hediste (Nereis) diversicolor O.F. Müller, 1776

The aim of this work was to quantify the intensity of sediment mixing induced by the gallery-diffusor (functional bioturbation group) Hediste diversicolor as a function of density, using particles tracers (luminophores). In order to assess the impact of density on sediment reworking, a 1-D model was used to obtain sediment reworking coefficients such as Db (biodiffusion-like) and r (biotransport). Densities used in this experiment corresponded to population densities observed in the sampling area (Saint-Antoine Canal, Gulf of Fos, France): 144, 288, 577, 1153 indiv/m². At first, results showed that neither luminophore maximum burying depth nor the more marked tracer accumulation areas were influenced by density. Thus density did not seem to have any influence on size of galleries or complexity of structure. Then, density-dependent relations with Db (biodiffusion-like mixing) and r (biotransport) were highlighted with an observed process intensity rate twice as high at highest worm density. On the other hand, Db and r per capita coefficients were negatively influenced by density. Db and r per capita at highest density were equal to ∼ 20% of individual Db and r obtained at the lowest density. Finally, this study showed the importance of density which appears to be a key parameter in the functioning of the sedimentary ecosystem.     

Invertebrate bioturbation can reduce the clogging of sediment: an experimental study using infiltration sediment columns

1. Invertebrate bioturbation can strongly affect water‐sediment exchanges in aquatic ecosystems. The objective of this study was to quantify the influence of invertebrates on the physical characteristics of an infiltration system clogged with fine sediment. 2. Two taxa (chironomids and tubificids) with different bioturbation activities were studied in experimental slow infiltration columns filled with sand and gravel and clogged with a 2 cm layer of fine sediment at the surface. We measured the effects of each taxon separately and combined on hydraulic head, water mobility and sediment reworking. 3. The results showed that invertebrates could reduce sediment clogging and this effect was linked to the functional mode of bioturbation of each group. Tubificid worms dug networks of galleries in the fine sediment, creating pathways for water flow, which reduced the clogging of sediment. In contrast, the U‐shaped tubes of chironomids were restricted to the superficial layer of fine sediments and did not modify the hydraulic conductivity of experimental columns. The combination of invertebrates did not show any interactive effects between tubificids and chironomids. The occurrence of 80 tubificids in the combination was enough to maintain the same hydraulic conductivity that 160 worms did in monospecific treatment. 4. The invertebrates like tubificid worms can have a great benefit on functioning of clogged interfaces by maintaining high hydraulic conductivity, which contributes to increased water‐sediment exchanges and stimulates biogeochemical and microbial processes occurring in river sediments.     

Ecosystem engineering at the sediment–water interface: bioturbation and consumer-substrate interaction

In soft-bottom sediments, consumers may influence ecosystem function more via engineering that alters abiotic resources than through trophic influences. Understanding the influence of bioturbation on physical, chemical, and biological processes of the water–sediment interface requires investigating top-down (consumer) and bottom-up (resource) forces. The objective of the present study was to determine how consumer bioturbation mode and sediment properties interact to dictate the hydrologic function of experimental filtration systems clogged by the deposition of fine sediments. Three fine-grained sediments characterized by different organic matter (OM) and pollutant content were used to assess the influence of resource type: sediment of urban origin highly loaded with OM and pollutants, river sediments rich in OM, and river sediments poor in OM content. The effects of consumer bioturbation (chironomid larvae vs. tubificid worms) on sediment reworking, changes in hydraulic head and hydraulic conductivity, and water fluxes through the water–sediment interface were measured. Invertebrate influences in reducing the clogging process depended not only on the mode of bioturbation (construction of biogenic structures, burrowing and feeding activities, etc.) but also on the interaction between the bioturbation process and the sediments of the clogging layer. We present a conceptual model that highlights the importance of sediment influences on bioturbation and argues for the integration of bottom-up influence on consumer engineering activities. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Impact of Oil on Bacterial Community Structure in Bioturbated Sediments

Oil spills threaten coastlines where biological processes supply essential ecosystem services. Therefore, it is crucial to understand how oil influences the microbial communities in sediments that play key roles in ecosystem functioning. Ecosystems such as sediments are characterized by intensive bioturbation due to burrowing macrofauna that may modify the microbial metabolisms. It is thus essential to consider the bioturbation when determining the impact of oil on microbial communities. In this study, an experimental laboratory device maintaining pristine collected mudflat sediments in microcosms closer to true environmental conditions – with tidal cycles and natural seawater – was used to simulate an oil spill under bioturbation conditions. Different conditions were applied to the microcosms including an addition of: standardized oil (Blend Arabian Light crude oil, 25.6 mg.g⁻¹ wet sediment), the common burrowing organism Hediste (Nereis) diversicolor and both the oil and H. diversicolor. The addition of H. diversicolor and its associated bioturbation did not affect the removal of petroleum hydrocarbons. After 270 days, 60% of hydrocarbons had been removed in all microcosms irrespective of the H. diversicolor addition. However, 16S-rRNA gene and 16S-cDNA T-RFLP and RT-PCR-amplicon libraries analysis showed an effect of the condition on the bacterial community structure, composition, and dynamics, supported by PerMANOVA analysis. The 16S-cDNA libraries from microcosms where H. diversicolor was added (oiled and un-oiled) showed a marked dominance of sequences related to Gammaproteobacteria. However, in the oiled-library sequences associated to Deltaproteobacteria and Bacteroidetes were also highly represented. The 16S-cDNA libraries from oiled-microcosms (with and without H. diversicolor addition) revealed two distinct microbial communities characterized by different phylotypes associated to known hydrocarbonoclastic bacteria and dominated by Gammaproteobacteria and Deltaproteobacteria. In the oiled-microcosms, the addition of H. diversicolor reduced the phylotype-richness, sequences associated to Actinobacteria, Firmicutes and Plantomycetes were not detected. These observations highlight the influence of the bioturbation on the bacterial community structure without affecting the biodegradation capacities.     

Macrofaunal community bioturbation along an estuarine gradient

A multidisciplinary study of the impact of bioturbation on sediment dynamics has been underway for some time at the Plymouth Marine Laboratory. The current programme has been founded upon the careful selection of six sites in the River Tamar, representative of important combinations of physical and biological variables. The paper presents preliminary results illustrating the contrasting importance of the speciesNereis diversicolor andNephtys hombergi as agents of bioturbation, given their different distributions across the six sites. Thus bioirrigation byN. diversicolor increases up the estuary and is greatest in the region of high suspended bed-load due to tidal pumping, where the consequences for chemical exchange processes between sediments and the water column may be most important. The sediment mixing effects ofN. hombergi are likely to be greater towards the seaward end of the estuary but ultimately the particular sediment types and the macrofaunal communities they support dictate the level of bioturbation in ways that do not necessarily relate to simple axial gradients along the estuary.     

Relative influence of bioturbation and predation on organic matter processing in river sediments: a microcosm experiment

1. Our objective was to measure the effects of bioturbation and predation on the physical characteristics and biogeochemical processes in river sediments. 2. We investigated the impacts of tubificid worms tested separately and together with an omnivore (Gammarus pulex), which does feed on tubificids, on sediment distribution, water flux, sediment organic carbon, biofilm biomass and microbial activities, and the concentrations of dissolved oxygen, dissolved organic carbon, PO, NO, NO and NH in slow filtration sand–gravel columns. We hypothesised that gammarids, which exploit the top 2–3 cm of the sediment, would modify the impact of worms at the sediment surface. 3. In experiments both with and without gammarids, bioturbation by the tubificids modified both the distribution of surface particles in the sediment column and water flux. In addition, microbial aerobic (oxygen consumption) and anaerobic (denitrification and fermentative decomposition of organic matter) processes in the sediment were stimulated in the presence of tubificid worms. However, G. pulex did not affect either the density or bioturbation activity of the tubificid worms. 4. Bioturbation by the benthos can be a major process in river habitats, contributing to the retention of organic matter in sediment dynamics. The presence of at least one predator had no effect on bioturbation in sediments. In such systems, physical heterogeneity may be sufficient for tubificids to escape from generalist predators, though more specialised ones might have more effect.     

Quantification of sediment reworking by the Asiatic clam Corbicula fluminea Müller, 1774

Active organisms modify the substratum in which they dwell. This process, called “bioturbation”, affects the way that biogeochemical fluxes are mediated at the substratum–water interface. In the frame of this work, the bioturbation potential of the Asiatic clam Corbicula fluminea was characterized and quantified. We measured the displacement of fluorescent particles by C. fluminea burying in a size-based experimental design in order to explore the effects of body-size on sediment reworking. Our results stress that C. fluminea belongs to the functional group of biodiffusors, and that C. fluminea can be considered as an intermediate sediment reworker. We suggest that bioturbation was mainly induced by the pedal-feeding activity of the clams. Results also showed that, though large clams induced displacement of particles deeper into the sediment, small clams showed the highest net sediment reworking activity. This result was in contrast to the initial hypothesis of biovolume as the main driver for particle displacement by bioturbating organisms. Life-history traits and specific features of pedal-feeding could explain the observed pattern.     

The influence of sediment composition and leaf litter on the distribution of tubificid worms (Oligochaeta)

Summary The horizontal distribution of three species of tubificid worm (Tubifex tubifex, Limnodrilus hoffmeisteri and L. udekemianus in the sediment of a small stream was examined. Worm distribution was most strongly correlated with the distribution of leaf litter. This relationship was examined in the laboratory by means of choice chambers. It was concluded that; 1) the addition, to an inorganic substratum, of conditioned leaf material, enhanced its attractiveness to the worms; 2) a combination of silt-clay and leaf material was preferred by the worms to mixtures of leaf material and coarser inorganic substrata 3) some leaf species were more attractive to the worms than others; 4) there were differences between the worm species in their preferences for the various leaf species; 5) none of the three tubificid species was exclusively associated with its preferred leaf species; 6) worms would switch to less attractive leaf species if preferred alternatives were unavailable; 7) leaf material only attracted the worms once it had become conditioned; 8) this appeal was lost when the leaves were autoclaved; and 9) substratum choice was independent of worm size.We believe the worms were attracted to the leaves because the associated microfloras provided a concentration of bacterial food. Differences between the three tubificid species in their preferences for the various leaf species probably reduced trophic competition.     

Bioturbation experiments in the Venice Lagoon

Short experiments (14–21 days) were carried out during autumn 1998 and spring 1999 at one selected site of the Venice Lagoon to measure bioturbation activities and mixing rates, as well as to obtain quantitative information on benthos functionality. Fluorescent sediment particles (luminophores, 63–350 m) were introduced as pulse inputs at the sediment surface. The concentration–depth profiles of the tracer were simulated with a new advection–diffusion–non local model applied under non-steady state conditions. This allowed the quantification of the mixing parameters associated with different mechanisms: biodiffusion (D _b), bioadvection (W) and non-local mixing (Ke, z₁, z₂). A parameter RS (removed sediment) was also calculated to account for the flux of sediment due to non-local transport. Results show that bioturbation was dominated by biodiffusion in autumn and by bioadvection in spring. Mean mixing parameters Db, W, and RS changed from 3.09 to 0.87 cm² y^–1, from 0.93 to 15.50 y^–1 and from 5.85 to 7.79 g cm^–2 y^–1, respectively.     

A New Approach for the Modelling of Sediment Reworking Induced by a Macrobenthic Community

A new model of bioturbation has been developed to describe short term sediment reworking induced by macrobenthic communities. The design of the model had to consider the mixing processes, firstly, at the organism level, and secondly, at community level. This paper describes the mixing mode of the four types of bioturbators defined by the authors: the biodiffusors, the upward-conveyors, the downward-conveyors and the regenerators. The mathematical formulation of these sub-models consists of ordinary differential equations. They take into account the size of the bioturbated zone, the output fluxes to the water column, tracer decay, physical mixing due to local currents and the type and intensity of the bioturbation processes. These sub-models make it possible to describe correctly the mixing events that have occurred in cores with each type of bioturbator. They also provide the basis for general bioturbation model, that will take into account the respective degrees of involvement of (i) the different bioturbation processes and their characteristics, (ii) the interference between the different processes, and (iii) make possible to predict the particle reworking in order to include it in studies of organic matter in early diagenesis. This revised version was published online in June 2006 with corrections to the Cover Date.     

水丝蚓对太湖沉积物有机磷组成及垂向分布的影响

以太湖常见底栖动物——水丝蚓为研究对象,借助室内流动培养装置和31P核磁共振技术(phosphorus-31 nuclearmagnetic resonance spectroscopy)(31P-NMR),研究生物扰动对太湖沉积物有机磷组成及垂向分布的影响。结果表明:短时间内水丝蚓扰动对沉积物有机磷化合物种类组成影响并不显著,但会引起上层沉积物中稳定性较差的有机磷化合物磷脂和DNA含量出现显著降低,同时沉积物中总磷和有机磷的垂向分布亦发生明显改变。此外,水丝蚓扰动下沉积物含水率、孔隙率和碱性磷酸酶活性显著增加,使活性较高的有机磷化合物分解加速,最终导致表层沉积物中的磷脂与DNA含量降低。     

Particle reworking and solute transport by the sediment-living polychaetes Marenzelleria neglecta and Hediste diversicolor

This experimental study quantified and compared particle-mixing and solute transport by the polychaetes Marenzelleria neglecta (2 g ww, 3200 ind. m^− 2) and Hediste diversicolor (2 g ww, 800 ind. m^− 2) in Baltic Sea sediments. Particle tracers (luminophores) were added to the sediment surface and their vertical distribution in the sediment was measured after 10 d. The rate of particle mixing was quantified using a gallery-diffusion model calculating the biodiffusion coefficient D_b and the non-local transport parameter r. Bioirrigation was measured by adding an inert solute tracer (bromide) to the overlying water 1, 1.5 and 2 d before the end of the experiment, and quantified by calculating the net bromide flux and fitting the bromide profiles to a 1D diffusion model providing an apparent biodiffusion coefficient D_a. The two polychaete worms displayed similar particle-mixing and solute transport efficiencies (based on total biomass) despite different modes of bioturbation. However, H. diversicolor was a more efficient particle-reworker and M. neglecta a more efficient bioirrigator, on an individual level. H. diversicolor buried a higher percentage (13%) of luminophores below the top 0.5 cm surface layer than M. neglecta (6%). D_b did not differ between the two species (2.4 × 10^− 3 cm² d^− 1) indicating a similar rate of diffusive mixing of the top sediment, however, the non-local transport parameter r was 2.5 y^− 1 for H. diversicolor and zero for M. neglecta, suggesting no significant particle-transport below the biodiffusive layer by M. neglecta. The average individual net bromide fluxes obtained were ca. 0.01 mL min^− 1 for H. diversicolor and 0.003 mL min^− 1 for M. neglecta, corresponding to an area-specific rate of ca. 12 L m^− 2 d^− 1 at the used densities. D_a did not differ between the two polychaetes, suggesting a higher individual solute exchange efficiency of M. neglecta considering the much higher ventilation rates reported for H. diversicolor than for Marenzelleria sp. The ongoing colonization of Baltic Sea sediments by M. neglecta at high densities may thus lead to an enhanced soluble release of both nutrients and contaminants. These results add information to the understanding of the potential effects of the invasion of M. neglecta on sediment biogeochemistry when competing with and/or replacing native species.     

Oil decontamination of bottom sediments using <Emphasis Type="Italic">Limnodrilus hoffmeisteri</Emphasis> (Oligochaeta: Tubificidae)

Using aquatic worms (Oligochaeta: Tubificidae) for oil decontamination of bottom sediments has been first considered. In a set of experiments on Limnodrilus hoffmeisteri Claparede, 1862, worms survived the exposure to oil content as high as 16.72 g/kg; they died of natural causes. Vigorous silt agitation by worms (bioturbation) was observed: the silt layer thickened as compared with that in reference aquaria without worms. Besides, it tended to thicken with increasing oil content of sediments. Within 30 days Limnodrilus hoffmeisteri reduced oil content by 1.20 to 1.72 times, or by 16.67 to 41.90% (a<0.05), compared with that in reference aquaria.     

Effect of bioturbation on denitrification in a marine sediment from the West Mediterranean littoral

Thein vitro effect ofNereis diversicolor on denitrification has been studied in PVC tubes filled with a coastal marine sediment defaunated by sieving. The first aim of the experiment was to determine the effect of sediment defaunation on denitrification (denitrifying population and Denitrifying Enzyme Assays). Sieving induced a loss of 70% of the initial DEA. The number of denitrifying bacteria was 10 times lower than inin situ sediment. In the top two centimetres, the DEA rose by 75% of its initial value, after 82 days. Polychaetes were only added after a return to near pre-disturbance levels to ensure that our data on the effects of their addition would not be disturbed by changes in the sediment. Introduction of Polychaetes increased the denitrifying population and DEA in the first layer (0–2 cm) of the sediment after 15 days. After 45 days, the surface of the polychaete burrows in sediment was 1.3 to 1.5 times higher than after 15 days, resulting in an increase in solute exchange between seawater and the top layer of sediment. An inhibitory effect of oxygen on denitrification was detected in the uppermost layer only.     

Seasonal Vertical Migration and Aestivation of <Emphasis Type="Italic">Rhyacodrilus hiemalis</Emphasis> (Tubificidae,Clitellata) in the Sediment of Lake Biwa,Japan

The vertical distribution of the tubificid worm Rhyacodrilus hiemalis Ohtaka, the numerically dominant species of oligochaete in the littoral of Lake Biwa, was studied with special reference to seasonal vertical migration in the lake sediment. Monthly collections of lake sediment cores were made using PVC tubes. Core sections of sample sediments ranged from 76 to 117 cm. The vertical distribution of the worms showed no diurnal variation; therefore diel vertical migration was not evident. Seasonal downward migration started in April, and upward migration started in October. From December to March, almost all worms remained in the near-surface sediment layer (surface to 30 cm deep), while from July to September almost all worms remained deeper than 30 cm. However, few individuals migrated deeper than 90 cm. No discontinuous layers were found in grain size composition, water content, loss on ignition, particulate carbon, nitrogen or phosphorus. In deep sediment there was no free oxygen, as evidenced by negative ORP values. For 4 months in summer, R. hiemalis aestivated, probably utilizing anaerobic respiration. It appeared that R. hiemalis moved deeper in the sediment in response to sediment temperature, because sediment temperatures in the deep layers seemed to converge at around 20–21 °C in the summer months. The life history traits of seasonal vertical migration and summer aestivation perhaps arose as an adaptation to the climatic conditions accompanying the geographical origin of R. hiemalis, and they also serve to minimize predation risk during summer when most invertebrate predatory fishes are active.     

Influence of the mode of macrofauna-mediated bioturbation on the vertical distribution of living benthic foraminifera: First insight from axial tomodensitometry

We investigated the influence of bioturbation by macrofauna on the vertical distribution of living (stained) benthic foraminifera in marine intertidal sediments. We investigated the links between macrofaunal bioturbation and foraminiferal distribution, by sampling from stations situated on a gradient of perturbation by oyster-farming, which has a major effect on benthic faunal assemblages. Sediment cores were collected on the French Atlantic coast, from three intertidal stations: an oyster farm, an area without oysters but affected by oyster biodeposits, and a control station. Axial tomodensitometry (CT-scan) was used for three-dimensional visualization and two-dimensional analysis of the cores. Biogenic structure volumes were quantified and compared between cores. We collected the macrofauna, living foraminifera, shells and gravel from the cores after scanning, to validate image analysis. We did not investigate differences in the biogenic structure volume between cores. However, biogenic structure volume is not necessarily proportional to the extent of bioturbation in a core, given that many biodiffusive activities cannot be detected on CT-scans. Biodiffusors and larger gallery-diffusors were abundant in macrofaunal assemblage at the control station. By contrast, macrofaunal assemblages consisted principally of downward-conveyors at the two stations affected by oyster farming. At the control station, the vertical distribution of biogenic structures mainly built by the biodiffusor Scorbicularia plana and the large gallery-diffusor Hediste diversicolor was significantly correlated with the vertical profiles of living foraminifera in the sediment, whereas vertical distributions of foraminifera and downward-conveyors were not correlated at the station affected by oyster farming. This relationship was probably responsible for the collection of foraminifera in deep sediment layers (> 6 cm below the sediment surface) at the control station. As previously suggested for other species, oxygen diffusion may occur via the burrows built by S. plana and H. diversicolor, potentially increasing oxygen penetration and providing a favorable microhabitat for foraminifera in terms of oxygen levels. By contrast, the absence of living foraminifera below 6 cm at the stations affected by oyster farming was probably associated with a lack of biodiffusor and large gallery-diffusor bioturbation. Our findings suggest that the effect of macrofaunal bioturbation on the vertical distribution of foraminiferal assemblages in sediments depends on the effects of the macrofauna on bioirrigation and sediment oxidation, as deduced by Eh values, rather than on the biogenic structure volume produced by macrofauna. The loss of bioturbator functional diversity due to oyster farming may thus indirectly affect infaunal communities by suppressing favorable microhabitats produced by bioturbation.     

Stratigraphic and dynamic effects of sediment reworking by Great Lakes zoobenthos

Recent sediments of the North American Great Lakes are inhabited by numerous species of macrobenthos which alter the physical and chemical properties of sediments and modify interface transport characteristics. Distributions of such radionuclides as cesium-137, lead-210, and isotopes of plutonium exhibit a zone of constant activity extending down from the sediment-water interface from 1 to 15 cm. Recent studies have confirmed that radiometrically determined mixed depths are consistent with the vertical distribution of oligochaete worms and the amphipod,Pontoporeia hoyi. Generally, 90% of the benthos are contained within the radiometrically defined mixed zone. Where comparisons are possible, rates of sediment reworking by ‘conveyor belt’ species are comparable to or exceed sedimentation rates. Systematic variations in the mixed depth occur within depositional basins with greatest depths tending to be associated with least consolidated, organically rich materials. A quantitative steady-state mixing model accounts satisfactorily for observed radioactivity and heavy metal profiles. Bioturbation appears to be an important process, limiting the resolution with which historical records of particle-associated contaminants may be reconstructed from sediment cores. As bioturbation serves to maintain contact of contaminated sediments with overlying water, this time may also characterize the long-term lake recovery for contaminants removed by burial. As the time varies with location, a mean for an entire lake is not well known, but is on the order of 20 years for Lake Huron. Contribution No. 300 of the Great Lakes Environmental Research Laboratory, Ann Arbor, Michigan.