A Pooled Response Strategy for Combining Multiple Lines of Evidence to Quantitatively Estimate Impact

The impacts of sediment contaminants can be evaluated by different lines of evidence, including toxicity tests and ecological community studies. Responses from 10 different toxicity assays/tests were combined to arrive at a “site score.” We employed a relatively simple summary measure, pooled P-values where we quantify a potential decrement in response in a contaminated site relative to nominally clean reference sites. The response-specific P-values were defined relative to a “null” distribution of responses in reference sites, and were then pooled using standard meta-analytic methods. Ecological community data were also evaluated using an analogous strategy. A distribution of distances of the reference sites from thecentroid of the reference sites was obtained. The distance from each of the test sites from the centroid of the reference sites was then calculated, and the proportion of reference distances that exceed the test site difference was used to define an empirical P-value for that test site. A plot of the toxicity P-value versus the community P-value was used to identify sites based on both alteration in community structure and toxicity, that is, by weight-of-evidence. This approach provides a useful strategy for examining multiple lines of evidence that should be accessible to the broader scientific community. The use of a large collection of reference sites to empirically define P-values is appealing in that parametric distribution assumptions are avoided, although this does come at the cost of assuming the reference sites provide an appropriate comparison group for test sites.     

Development of a sediment bioassay to determine bioavailability of PAHs to fish

Recent work has shown that MFO induction and loss of control of steroid hormone production occurs in fish after exposure to pulp mill effluents, PCBs, PAHs, and some pesticides. We had recently developed laboratory assays to evaluate the effluents on these responses, but were lacking a protocol for a sediment assay. This paper describes the development of a sediment test capable of demonstrating MFO induction in fish. MFO responses were evident in rainbow trout within 4 days of exposure to contaminated sediments. Further testing showed that fish were responding to chemicals from the sediments, but not from bottom water, and a survey of sediment from thirteen contaminated areas showed that MFO induction more closely paralleled PAH levels in the sediments than the observed PCB concentrations. The sites showing MFO induction were also the sites where sediment toxicity was demonstrated with laboratory bioassays using Daphnia magna and Hyalella azteca. The protocol has been further refined to describe the quantity of sediment required and duration of testing. This test will enable us to study the biochemical effects of exposure to contaminated sediments. The protocol could also be used to prioritize areas of contamination and to evaluate dredging impacts and remediation success.     

Contrasting distributions of bacteriophages and eukaryotic viruses from contaminated coastal sediments

Viruses are ubiquitous, abundant and play an important role in all ecosystems. Here, we advance understanding of coastal sediment viruses by exploring links in the composition and abundance of sediment viromes to environmental stressors and sediment bacterial communities. We collected sediment from contaminated and reference sites in Sydney Harbour and used metagenomics to analyse viral community composition. The proportion of phages at contaminated sites was significantly greater than phages at reference sites, whereas eukaryotic viruses were relatively more abundant at reference sites. We observed shifts in viral and bacterial composition between contaminated and reference sites of a similar magnitude. Models based on sediment characteristics revealed that total organic carbon in the sediments explained most of the environmental stress-related variation in the viral dataset. Our results suggest that the presence of anthropogenic contaminants in coastal sediments could be influencing viral community composition with potential consequences for associated hosts and the environment.     

Microbiological examination of Lake Erie and Lake Ontario sediments

Diver collected cores from three sites in Lake Erie, west of Cleveland, Eastern basin and near Buffalo, and one from Lake Ontario near the mouth of the Niagara River, were sectioned to 20 cm and examined for sulfur cycle and nitrogen cycle bacteria, heterotrophic bacteria, insoluble organic and inorganic phosphate solubilizing bacteria and manganese oxidizing bacteria. Eh, pH, nitrogen, organic carbon and percentage moisture determinations were also made. Data from this study support and confirm other microbiological data collected from Lower Great Lakes sediment and also support the hypothesis that microbial flora of similar type sediments are numerically similar irrespective of the sediments source within the Lower Great Lakes.     

Rapid flow through the sediments of a headwater stream in the southern Appalachians

SUMMARY. 1. The flow of water through the sediment layer (underflow) of streams can influence nutrient uptake dynamics and the supply of materials to microbes, meiofauna and macroinvertebrates living within stream sediments. We examined the extent of underflow in Hugh White Creek, a headwater stream in the southern Appalachian Mountains and compared underflow at different depths and at different sites within the stream.
2. Initially rhodamine dye was used to trace the flow of water through the sediments; however, the dye was strongly absorbed by the sediments in Hugh White Creek. Thus rhodamine was not suitable as a tracer of water flow. Chloride reacted conservatively in laboratory experiments and was used as a tracer.
3. The tracer infiltrated the sediments within 5 min to depths of 10 cm at all six sites. Chloride infiltration tended to decrease with depth of sediments at all sites although there was no consistent statistical pattern in chloride concentration with depth for the different sites. Equilibrium between the water column and sediments was reached rapidly, within minutes for the sites with coarse sediments and within a few hours for sites with finer sediments. Minimum rates of chloride infiltration into the sediments ranged between 1.0 cm min⁻¹ for the sites with cobble substrate to 0.2 cm min⁻¹ for sites with fine sand sediments associated with debris dams. These data suggest that underflow may be a major component in the functioning of Appalachian mountain streams.     

A Comparison of Three Weight-of-Evidence Approaches for Integrating Sediment Contamination Data within and Across Lines of Evidence

Multiple lines of evidence (LOE) are often considered when examining the potential impact of contaminated sediment. Three strategies are explored for combining information within and/or among different LOE. One technique uses a multivariate strategy for clustering sites into groups of similar impact. A second method employs meta-analysis to pool empirically derived P-values. The third method uses a quantitative estimation of probability derived from odds ratios. These three strategies are compared with respect to a set of data describing reference conditions and a contaminated area in the Great Lakes. Common themes in these three strategies include the critical issue of defining an appropriate set of reference/control conditions, the definition of impact as a significant departure from the normal variation observed in the reference conditions, and the use of distance from the reference distribution to define any of the effect measures. Reasons for differences in results between the three approaches are explored and strategies for improving the approaches are suggested.     

An Integrated Case Study for Evaluating the Impacts of an Oil Refinery Effluent on Aquatic Biota in the Delaware River: Integration and Analysis of Study Components

A series of statistical and graphical techniques incorporating a “weight of evidence” approach were used to interpret results from an integrated Triad case study designed to determine potential environmental impacts to aquatic biota in the Delaware River that may be linked to PAHs found in Motiva's oil refinery effluent. Sediment concentrations of various metals, PCBs and LMW PAHs exceeding both ERL and ERM sediment quality guidelines (SQGs) were reported in the study area. However, most chemical contaminants did not exceed their respective SQGs. Results from a long-term sediment coring study indicated that there was no evidence of significant historical PAH contamination of sediments related to Motiva's exceedences. PAHs comprising the Motiva “fingerprint” were found in the surficial sediments at four near-field sites but non-Motiva PAH concentrations (background) were shown to be significantly higher at other far-field sites (non-Motiva influence). Chronic sediment toxicity appears to have significant relationships to the patterns of most PAH isomers, certain PCB isomers, and certain metals. However, sediment toxicity does not appear to be related to the PAH isomers that are characteristic of Motiva's effluent nor to the near-field sites. Impacted benthic communities were reported in the study area, primarily at one near-field and two far-field sites. However, there were no apparent relationships between benthic community health and sediment contaminants. The status of benthic communities does not appear to be related to PAHs derived from the Motiva effluent. The “weight of evidence” analysis developed from a systematic and comprehensive series of statistical and graphical assessments indicates that, although the study area displayed some degree of sediment contamination, chronic sediment toxicity, and benthic health impacts, these environmental effects generally could not be related to Motiva's exceedences.     

How does the vertical distribution of chlorophyll vary in littoral sediments of small lakes?

1. In this study, sediment chlorophyll profiles at twenty littoral stations in three oligo‐mesotrophic lakes were compared to test whether the vertical distribution of chlorophyll is related to site characteristics (light availability, temperature, physical disturbances) and whether these profiles differ between shallow and deep portions of the littoral zone.
2. The magnitude of chlorophyll peaks at the sediment surface did not vary with light intensity. Chlorophyll peaks in the shallow littoral zone had a weak tendency to decrease with increasing effective fetch. The magnitude of chlorophyll peaks at deeper sites was more closely related to water temperature than to substrate slope.
3. High chlorophyll concentrations were measured down to 1–3 cm in the sediments, both at shallow (< 2.5 m) and deep (4–10 m) stations. The depth to which high chlorophyll was found in sediments did not vary with effective fetch or sediment water content, two indices of wave disturbance in the shallow littoral zone, or with substrate slope, an index of sediment stability in the deep littoral zone. Sediment mixing is apparently not related to common indices of physical disturbances.
4. Between 8 and 100% of sediment surface chlorophyll was 'retained' 4–5 cm into the sediments. The proportion of chlorophyll 'retained' in littoral sediments increased with increasing depth, increasing lake productivity (total phosphorus concentration) and increasing lake pH.
5. Among‐core variability (standard error/mean) in chlorophyll concentration at the sediment surface ranged from less than 1% to 33% at different stations and was highest at shallow, exposed sites. These levels of variability are similar to those found in other periphytic communities.     

Sediment quality assessment in the delta of rivers Rhine and Meuse based on field observations,bioassays and food chain implications

The quality of sediment was assessed in 46 sites in the delta of the rivers Rhine and Meuse (The Netherlands) by means of physical-chemical analysis, field observations on the macrobenthic community structure, accumulation studies and bioassays using Chironomus riparius, Daphnia magna and Photobacterium phosphoreum. The results of chemical analyses were classified using national criteria for sediment quality. Results of field studies and bioassays were classified using criteria derived from research in reference areas or based on data from literature. Risk assessment was carried out according to the sediment quality triad and by means of multi criteria analysis (MCA). The Triad approach was used to demonstrate causal relations between effects on the macrozoobenthos community structure, effects demonstrated in bioassays and sediment pollution. This was done by making a comparison of sediment contamination levels with toxicity data from literature for the test organisms of the bioassays. Using the MCA method, for each site a numerical value was derived for total environmental risk in the present situation, based on observed effects. In this way, a relative risk ranking of all sites was realized. The MCA values for the present situation were also compared with MCA scores based on estimated risks after remediation in 1995, in order to set priorities for sites where remediation is expected to cause a significant reduction in environmental risk. In most of the 46 sites studied so far, the macrofauna community was poorly developed, judged by a low number of benthic species, low abundances and a high dominance of species regarded as relatively tolerant to chemical pollution. In bioassays high sediment toxicity was demonstrated for 24 sites. Using the sediment quality triad approach, 25 sites were identified as areas where pollution can be held responsible for the effects observed in the field. From a comparison of contaminant concentrations in different types of food with maximum tolerable risk levels, and the application of a bioaccumulation model it was concluded that the sediment pollution also implies high risks for plant-, benthos- or fish-eating birds (secondary poisoning of top predators). In the Nieuwe Merwede highest MCA risk scores were found for shallow parts where highly polluted sediments are found. It is concluded that the sediment quality triad and the MCA provide additional information which can be used to establish priorities for remedial action. Based on an ecotoxicological evaluation of the improved quality of new sediments that will be deposited after removal of the polluted sediments in the Nieuwe Merwede, it is concluded that in this upstream part of the Rhine delta remedial action will be effective.     

Recent histories of six productive lakes in the Irish Ecoregion based on multiproxy palaeolimnological evidence

Palaeolimnological data from six mesotrophic, eutrophic and hypertrophic lakes in the Irish Ecoregion, in the form of microfossil (cladocera, diatoms and pollen) and sediment chemistry data from radiometrically dated sediment cores, were used to reconstruct past variations in lake water quality and catchment conditions. Basal sediments from sediment cores from the six sites ranged in age from the late 18th century to the early 20th century. A weighted averaging partial least squares regression model was developed to reconstruct past epilimnetic total phosphorus concentrations. The results indicate that all but one of the study sites currently are in a far more productive state compared with the beginning of the sediment core record and that those same five lakes have experienced accelerated enrichment post c. 1980. Two of the sites demonstrated long-term enrichment, in one case beginning in the late 19th century, while both eutrophication and oligotrophication have occurred at three sites. The results highlight the difficulties in applying a general temporal end-point for reference conditions and demonstrate that productive lakes in the Irish Ecoregion have complex, locally specific and often long histories of enrichment. These may not be responsive to reduced external loadings of phosphorus and, as a result, restoration could prove particularly challenging. The results also provide evidence of the ways in which palaeolimnological techniques can assist implementation of the EU Water Framework Directive. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Combining limnological and palaeolimnological data to disentangle the effects of nutrient pollution and climate change on lake ecosystems: problems and potential

1. As long‐term observational lake records continue to lengthen, the historical overlap with lake sediment records grows, providing increasing opportunities for placing the contemporary ecological status of lakes in a temporal perspective. 2. Comparisons between long‐term data sets and sediment records, however, require lake sediments to be accurately dated and for sediment accumulation rates to be sufficiently rapid to allow precise matching with observational data. 3. The critical role of the sediment record in this context is its value in tracking the changing impact of human activity on a lake from a pre‐disturbance reference through to the present day. 4. Here, we use data from a range of lakes across Europe presented as case studies in this Special Section. The seven sites considered all possess both long‐term observational records and high‐quality sediment records. Our objective is to assess whether recent climate change is having an impact on their trophic status and in particular whether that impact can be disentangled from the changes associated with nutrient pollution. 5. The palaeo‐data show clear evidence for the beginning of nutrient pollution varying from the mid‐nineteenth century at Loch Leven to the early and middle twentieth century at other sites. The monitoring data show different degrees of recovery when judged against the palaeo‐reference. 6. The reason for limited recovery is attributed to continuing high nutrient concentrations related to an increase in diffuse nutrient loading or to internal P recycling, but there is some evidence that climate change may be playing a role in offsetting recovery at some sites. If this is the case, then lake ecosystems suffering from eutrophication may not necessarily return to their pre‐eutrophication reference status despite the measures that have been taken to reduce external nutrient loading. 7. The extent to which future warming might further limit such recovery can be evaluated only by continued monitoring combined with the use of palaeo‐records that set the pre‐eutrophication reference.     

Bacterial diversity in organically-enriched fish farm sediments

The bacterial diversity and community structure within both organically enriched and adjacent, unimpacted, near-shore marine sediments at two fish farms in southern Tasmania, Australia, was examined using 16S rRNA gene clone library construction and analysis. Sediments at both caged and reference sites at both farms showed a very high level of microbial diversity. Over 900 clones were analysed and grouped into 631 unique phylotypes. Reference sites were dominated by Delta- and Gammaproteobacteria and the Cytophaga-Flavobacteria-Bacteroides group. Cage site sediments were also dominated by these phylotypes, as well as members of the Alpha- and Epsilonproteobacteria. Diversity and coverage indices indicated that the actual diversity of the sediments was much greater than that detected, despite a large sampling effort. All libraries were shown to be statistically different from one another (P < 0.05). Many phylotypes did not group with cultured bacteria, but grouped with other environmental clones from a wide array of marine benthic environments. Diversity and evenness indices suggested that although both parameters changed after farming, diverse communities were present in all sediments. The response of the microbial community to organic load suggested that random, rather than predictable, succession events determine community composition and diversity, and that sediment type may influence bacterial community and sediment response to organic perturbation.     

Ecological Risk Assessment of Sediments in Sydney Harbour,Nova Scotia,Canada

Contamination within sediments of Sydney Harbour (once a major industrial port) were evaluated using a multiple lines-of-evidence (LOE) ecological risk assessment (ERA) approach prior to divestiture of the harbor. The multiple LOE approach included: (1) measurement of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), metals, metalloids, petroleum hydrocarbons(PHCs), and total organic carbon (TOC) concentrations in surface sediments from multiple Sydney Harbour locations; (2) identification and application of sediment quality guidelines (SQGs) from various jurisdictions; (3) comparisons of harbor sediment chemistry against background/reference sediment chemistry; (4) determining number and frequency of exceedances over SQGs; (5) calculating mean probable effect level-quotients (PEL-Qs); (6) PAH forensic source evaluation; (7) review of previous sediment chemistry and biota tissue data; and (8) characterizing benthic habitat at harbor stations. The ERA determined that current sediments exhibited mostly low probability of adverse effects. Furthermore, contaminated sediments exhibiting a high probability of adverse effects were localized to only a few stations within the harbor. Ongoing natural recovery of harbor sediments is likely responsible for attenuating contaminants that historically were higher than those measured in this study and were previously distributed over much wider areas of the harbor. Results suggest that legacy industrial activities and current urban sewage effluents are the major sources of contamination in Sydney Harbour sediments.     

Field Characterization of Potential Reference Sediments in the Gulf of Mexico: Chemical and Biological Quality

Baseline information on the chemical and biological quality of sediments is provided for six coastal locations in the northern Gulf of Mexico, which were considered possible candidates for regional reference areas. Chemical quality, toxicity and benthic community composition were determined for sediments collected three times from each of 12 sites during an approximate one-year period. Potential contaminants in the usually sand-dominated sediments exceeded individual threshold effects level guidelines proposed for Florida coastal areas in approximately 31% of the samples collected from 8 of 12 sites. No probable effects level guidelines were exceeded. Acute toxicity occurred in 16% or less of the sediment samples and no significant chronic toxicity was observed to the infaunal amphipod, Leptocheirus plumulosus. Approximately, 11% and 17% of the sediments were classified as poor or marginal, based on low benthic taxa abundance and diversity index values, respectively. Sediment quality at many sites was less degraded than that for nearby coastal areas receiving point and non-point source contaminants, which suggests their suitability to serve as reference sediments although further confirmation is recommended. In a broader context, the results of this survey reflect the complexity in field verification of reference conditions for near-coastal sediments. This is attributable largely to the natural variability in their physical, biological, and chemical characteristics and to the lack of biocriteria for benthic macro and meiofauna.     

Establishing Causality between Exposure to Metals and Effects on Fish

This study evaluates causal relationships between chronic exposure of fish to metals and effects at different levels of biological organization based on a weight-of-evidence approach. Criteria for evaluation of causality were strength, consistency, and specificity of the association, as well as biological gradient and plausibility. Field sampling was conducted three times between 1998 and 2000, in Furnas Stream, impacted by an abandoned lead mine, and in three other locations, including two reference and one impacted sites. Levels of Pb, Zn, Cd, and Ag in sediments from the Furnas Stream exceeded background levels, and their concentrations were above sediment quality guidelines. Residual levels of metals in fish tissue were high enough to indicate reduced growth, reproduction and/or survival according to toxicological benchmarks. Lead-induced biochemical changes (ALA-D activity depletion) were observed in two species of siluriform catfish. The condition factor of a predatory catfish was reduced, and the percentage of prey generalists was higher in Furnas than at the noncontaminated sites. Reduction in fish community diversity and density was observed. Integration of data provided supporting evidence that observed effects on fish from the Furnas Stream resulted from long-term exposure to metals, however influences from other stressors cannot be ruled out.     

The relationship between invertebrate assemblages and bio-dependant properties of sediment in urbanized temperate mangrove forests

Relationships between ecological structure, functions and properties of sediments in intertidal mangrove forests and mudflats are poorly understood. Studies have not yielded consistent results, due to complex interactions between biotic and abiotic components. The benthic macrofauna in mangrove forests are important contributors to ecological functions, mediating various sedimentary properties. They vary in abundance and diversity at scales from centimeters to hundreds of metres in what appears to be similar “habitat”, but properties of sediments are often not measured at such small scales, or small-scale variation is ignored as random noise. Yet, there should be relationships between scales of variation in macrofauna and bio-dependant properties of sediments.This paper describes variation in assemblages of benthic invertebrates and bio-dependant properties of sediments at a hierarchical range of spatial scales, within and among different habitats in an urbanized mangrove forest and the associated intertidal mudflats in Sydney Harbour, Australia. Measurements were made on two occasions, in replicate sites in each habitat. These were used to test hypotheses about relationships of scales of variation in the benthos and sediments. The benthos showed relatively little difference in diversity and abundances among habitats compared to variation within and between sites in each habitat. The bio-dependant properties of the sediment showed less variation at small scales and larger among-habitat variation than did the benthos. Variation in the benthos did not clearly correlate with variation in the selected bio-dependant properties of the sediments at any scales. Neither did the properties of the sediment show consistent correlations in any habitat.These data indicate that the properties and processes driving the benthos are not simply related to the properties of the sediment we measured and that there was very large variation in both benthos and bio-dependant properties within small sites. This highlights the necessity of using experimental designs that sample at a hierarchy of scales for benthic assemblages and bio-dependant properties of sediment.     

Cichlidogyrus sclerosus (Monogenea: Ancyrocephalinae) and its host, the Nile tilapia (Oreochromis niloticus), as bioindicators of chemical pollution

Experimental results showed that the gill monogenean Cichlidogyrus sclerosus and its host, the Nile tilapia Oreochromis niloticus, exhibited significant numerical and physiological responses after exposure to sediments polluted with polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls, and heavy metals in comparison with control fishes. After 15 days of exposure, C. sclerosus abundance significantly increased in treatments with low to fairly high sediment pollutant concentrations, but declined at high sediment pollutant concentrations. Hypertrophy and hyperplasia in secondary gill lamellae and the spleen melanomacrophage centers were significantly higher at extremely high sediment pollutant concentrations compared with the controls. Spleen lymphocyte and monocyte counts were significantly lower at extremely high sediment pollutant concentrations and were significantly correlated with high fluorescent aromatic compound concentrations measured as PAH exposure indicators. A multivariate redundancy analysis showed significant statistical association between sediment pollutant concentration, C. sclerosus abundance, and tilapia physiological variables. The polluted sediments negatively affected monogenean abundance and induced immunosuppression in hosts, consequently increasing histological damage in hosts and allowing persistent C. sclerosus infection. This study documents evidence suggesting that C. sclerosus and its host are indeed excellent models to test environmental quality in tropical freshwater ecosystems.     

Release rates and potential fates of nitrogen and phosphorus from sediments in a eutrophic reservoir

1. Nutrients released from lake sediments can influence water column nutrient concentrations and planktonic productivity. We examined sediment nutrient release [soluble reactive phosphorus (SRP) and ammonia (NH)] at two sites in a eutrophic reservoir (Acton Lake, OH, U.S.A.) that differed in physical mixing conditions (a thermally stratified and an unstratified site). 2. Sediment nutrient release rates were estimated with three methods: sediment core incubations, seasonal in situ hypolimnetic accumulation and a published regression model that predicted sediment phosphorous (P) release rate from sediment P concentration. All three methods were applied to the deeper stratified site in the reservoir; however, we used only sediment core incubations to estimate SRP and NH release rates at the shallow unstratified site because of the lack of thermal stratification. We also compared the total P concentration (TP_S) of sediments and the concentration of P in various sediment fractions at both sites. 3. Anoxic sediments at the stratified site released SRP at rates more than an order of magnitude greater than oxic sediments at the shallow unstratified site. However, P accumulated in the hypolimnion at much lower rates than predicted by sediment core incubations. In contrast, NH was released at similar rates at both sites and accumulated in the hypolimnion at close to the expected rate, indicating that P was ‘lost’ from the hypolimnion through biogeochemical pathways for P, such as precipitation with inorganic material or biological uptake and sedimentation. 4. TP_S was significantly greater at the deeper stratified site and organically bound P accounted for >50% of TP_S at both sites. 5. We examined the magnitude of SRP fluxes into the study reservoir in 1996 by comparing the mean summer daily SRP fluxes from anaerobic sediments, aerobic sediments, stream inflows and gizzard shad excretion. While the SRP release from anaerobic sediments was high, we hypothesise that little of this SRP gained access to the epilimnion in mid‐summer. SRP flux to the reservoir from aerobic sediments was less than from gizzard shad excretion and streams. Large interannual variability in thermocline stability, gizzard shad biomass and stream discharge volumes, will affect SRP loading rates from different sources in different years. Therefore, construction of P budgets for different years should account for interannual variation in these parameters.     

Fungi and Macroaggregation in Deep-Sea Sediments

Whereas fungi in terrestrial soils have been well studied, little is known of them in deep-sea sediments. Recent studies have demonstrated the presence of fungal hyphae in such sediments but in low abundance. We present evidence in this study that one of the apparent reasons for the poor detection of fungi in deep-sea sediments is their cryptic presence in macroaggregates. Fungal biomass carbon from different core sections of deep-sea sediments from approximately 5000 m depth in the Central Indian Ocean was estimated based on direct microscopic detection of fungal mycelia. Treatment of sediment samples with ethylenediamine tetra-acetic acid (EDTA) enabled more frequent detection and significantly higher biomass than in samples without such treatment. Treatment with EDTA resulted in detecting various stages of breakdown of aggregates in the sediments, gradually revealing the presence of fungal hyphae within them. Experimental studies of a deep-sea, as well as three terrestrial isolates of fungi, showed that all could grow at 200 bar and 5 degrees C in a nutrient medium and in deep-sea sediment extract. Hyphae of fungi grown in sediment extract under the above conditions showed various stages of accretion of particles around them, leading to the formation of aggregates. Such aggregates showed the presence of humic material, carbohydrate, and proteins. We suggest that fungi in deep-sea sediments may be involved in humic aggregate formation by processes very similar to those in terrestrial sediments. The importance of such a process in carbon sequestration and food web in the deep sea needs to be examined.