Environ. Sci. Technol. 2000, 34, 1594-1602
Terrestrial Meiofauna and Contaminated Land Assessment M A R C U S W . T R E T T , †,‡ BEATRIZ CALVO URBANO,‡ SIMON J. FORSTER,† JUDITH D. HUTCHINSON,† RICHARD L. FEIL,† SIMON P. TRETT,† AND J . G E O R G E B E S T * ,†,§ Physalia Limited, Consultant & Forensic Ecologists, Sedgefen House, 37 Meadow Walk, Harpenden, Hertfordshire AL5 5TF, England, Nebalia S.L., Consultorı´a Medioambiental, Calle del Espejo, 15, 4° izquierda, Madrid 28013, Spain, and Huntsman Tioxide, Tees Road, Hartlepool TS25 2DD, England
Following the development of aquatic pollution monitoring techniques using metazoan meiofauna (microscopic interstitial invertebrates), the value of this group in the assessment of contaminated terrestrial sites has been investigated. Communities present in a former explosives burning ground were sampled at 30 sites using coring techniques and examined in the laboratory. Nematoda were numerically dominant at each of the sites and were the most diverse invertebrate group present (60 species). Structurally modified assemblages of these were identified and correlated significantly with elevated soil heavy metal concentrations. Assemblages associated with elevated metal concentrations exhibited increased dominance and reduced species richness. Using principal components analyses (PCA) and redundancy analyses (RA), the relationships between individual species, the metals, and the structures of the nematode communities were described. Tolerant and resistant species were also identified. It is concluded that the technique is a valuable method of assessing metal contamination status, and it is recommended that further studies be made of sites contaminated with organic materials.
Introduction In Europe, emphasis has been placed on the identification and assessment of actual ecological effects of industrial operations as part of liability management and risk assessment systems. This includes consideration of the historical legacy of contaminated environments. In aquatic habitats, the process of establishing the nature and extent of impacts has been based traditionally on examination of benthic communities. The communities of larger animals (macrofauna) frequently fail to provide suitable and sufficient data to asses effects; the distribution of the organisms can be too patchy (mean-to-variance ratios small), their densities too low, or the animals are simply too mobile to examine and monitor impacts reliably. In response to this, in the past 20 years, many pollution ecologists have turned to the meiofauna (1-11). These microscopic animals live in the spaces between * Corresponding author phone: +44 (0)1642 545 348; fax: +44 (0)1642 546 016; e-mail:
[email protected]. † Physalia Limited, Consultant & Forensic Ecologists. ‡ Nebalia S.L., Consultorı ´a Medioambiental. § Huntsman Tioxide. 1594
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sediment or soil particles and can be defined as those organisms that will pass through a 1 mm mesh sieve but are retained on a 38 µm sieve (see refs 10-13). In aquatic environments, analyses of meiofaunal assemblages have overcome many of the problems associated with macrofaunal studies. The advantages of meiofaunal analyses in the assessment of prevailing environmental conditions have been described by several authors (see, for example, refs 14-16). The principal advantages can be summarized as the following. High Abundance. Four out of five of all metazoan animals on Earth belong to one of the meiofaunal groups (Nematoda (17, 18)), and these can be present at densities of up to 40 million m-2 (19, 20). Owing to their high densities, statistically valid sampling can be achieved more easily than with macrofauna, even with small, easily processed samples (a single core of 10 cm can yield as many as 50 nematode species and 2000 individuals (20)). High Biodiversity. The numbers of meiofaunal species belonging to a single phylum in a given habitat can be an order of magnitude greater than the associated macrofauna. This diversity will encompass an extremely broad range of physiologies and feeding types and provide a more balanced assessment of effects of prevailing conditions on food webs and community processes than studies of macrofaunal communities alone. Range of Tolerances. With representatives found in all extreme environments examined so far, e.g. hot, volcanic springs, anoxic sediments, sediments of hydrothermal vents, sea ice, and recovered from industrially contaminated sediments, meiofauna include species that are tolerant of a range of different environmental stresses. At the same time, the group includes sensitive, stress-intolerant species. This enables changes throughout a wide spectrum of stress conditions to be assessed. Low Mobility. Living in the pore spaces between the sediment particles, communities of meiofauna are continuously subjected to the constraints of any noxious materials that enter their environment. Therefore the community structure is more directly related to the physicochemistry of the habitat sampled than in the case of the macrofauna. Range of Life-Cycle Times. With life-cycles ranging from as little as 6 days to over 2 years, meiofaunal communities integrate and reflect the effects of short-term as well as longerterm influences more rapidly than macrofauna. Direct Interface with Pore Water Contaminants. Unlike many larger organisms, the majority of meiofauna exploits the interstitial spaces in sediments and is in intimate contact with the pore water. As such, exposure to pore water contaminants is more likely to be a significant factor in determining the survival of meiofaunal species and may be useful to establish quality criteria based on equilibrium models. Faunal surveys of industrially contaminated terrestrial sites have not received the same attention as those of aquatic systems. This reflects problems associated with the communities of the larger terrestrial invertebrates that parallel those seen in the aquatic macrofauna. In this case, the densities and diversity of invertebrates are often naturally low, their mobility can be exceptionally high, particularly among arthropod groups, and their contact with soil-bound contaminants is frequently limited (21, 22). Specific problems are also associated with the use of different sampling techniques (23). Bongers (24) and Kappers and Van Esbroek (25) had already described changes in the structure of the communities 10.1021/es990064d CCC: $19.00
2000 American Chemical Society Published on Web 03/15/2000
hand-corer inserted to a depth of 10 cm. Samples were also collected at each site for the examination of particle size distributions (PSD) and chemical analyses. Laboratory Procedures. PSD determinations were undertaken using a dry sieving technique and yielded percentage compositions for the size fractions > 2000 µm, 500-2000 µm, 250-500 µm, 106-250 µm, 63-106 µm, and < 63 µm. The metals present in the < 63 µm soil fractions for each site were examined using an inductively coupled plasma determination (ICP) technique based on nitric acid digests. Metal concentration data were collected for Al, As, B, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, V, and Zn. For purely comparative purposes, total metal burden indices (TMBIs) for soil samples were calculated as m
TMBIk )
∑ i)1
FIGURE 1. Location map of the contaminated explosives burning ground, Stevenston, Scotland, showing the positions of the sampling stations used in the meiofaunal study. of the most abundant metazoan meiofaunal taxon, the Nematoda, in response to physical and chemical disturbance of soils. The present study arose out of an investigation undertaken for the international chemical company, ICI. This was designed to evaluate a meiofauna technique for inclusion in their terrestrial site assessment procedures. The method described here does not attempt to examine all meiofaunal groups. Instead, the technique provides a practical basis for assaying the contaminant status of soil pore-water using those meiofaunal groups that are most easily extracted and preserved using easily standardizable, nonspecialist procedures.
Materials and Methods Study Site. A plot of land that formed the former burning grounds at ICI Explosives (ICI Nobel Enterprises), at Ardeer in Scotland, was selected for the study (Figure 1). The fencedoff land was located on coastal sand dunes near to the explosives factory. Here, packaging and explosive materials were burnt at high temperatures in the middle of the open ground. Prior to its use, there had been no industrial activity or buildings on the site. Burning operations ceased approximately 8 years before the study commenced, and the central region where burning and deactivation of explosives had taken place could not be distinguished from surrounding vegetated areas. Analyses of the sandy dune soils undertaken by the laboratories at the explosives company had shown that organic compounds (residues of explosives materials) were no longer detectable and did not pose an environmental problem. However, the central region of the site was shown to be contaminated with heavy metals, which had originated either from the explosives themselves or from materials associated with their production. Sampling. A “herringbone” grid of 30 sampling stations was established on the former burning ground using a differential Geographical Position System (GPS) coupled with measurement from fixed points (Figure 1). At each site a single soil core sample was collected using a 65 mm diameter
n
[Vik/(
∑V
ik)]
k)1
where the TMBI for each station (k) is calculated for the ranges k ) 1 to n stations and for metals (i) ranging from 1 to m metals with soil concentration values V (ppm). Specific metal concentration values (ppm) were used in the data analyses (see below). Meiofaunal Analyses. No single separation and fixation technique is suitable for all soil meiofaunal species, particularly groups such as the more delicate and less motile Protozoa. The following general protocol was adopted for the separation and processing of the samples. Individual soil core samples were homogenized in approximately 1 L dechlorinated tap water. Initial separation was carried out using modified Whitehead tray separators for 48 h at 20 °C (26). Separated meiofauna was then elutriated onto 38 µm sieves and fixed in two changes of 7% formaldehyde solution (aqueous) for 24 h (27). Residual materials were examined under a dissecting microscope to confirm complete elution of interstitial fauna. Random samples of residue were reeluted to check that residual meiofauna accounted for < 4% of total extracted. Where appropriate, arthropod species were dissected in glycerol-ethanol solutions for taxonomic study (see ref 28). For microscopy, body parts were mounted in Berlese’s thin formulation medium (29, 30). After clearing (ca. 3 days at 20 °C) specimens were identified and enumerated. Remaining meiofauna, principally Nematoda, Protozoa, Rotifera, and Tardigrada, were processed to glycerol using the Seinhorst method (31) at 40 °C in a vacuum oven and mounted on long coverslip slides. All microscopic examination was carried out using Zeiss Nomarski and Nikon differential interference contrast microscopes (DIC). For Nematoda, the first 100 specimens encountered under highpower were identified and counted. Where necessary, specimens were removed and mounted on Cobb slides (32) for complete identification. The presence of other species observed during subsequent counting (i.e. those species representing < 1% of the nematode community) was also noted. Data Analyses. Data for all groups of organisms were converted to numbers of individuals of each species present at each site L-1 soil. Simpson’s index was used to examine diversity of the nematode community data as this index is less sensitive to the presence and absence of rare taxa in the species-rich meiofaunal assemblages. Dominance-diversity structures of nematodes assemblages were examined and compared using k-dominance analyses (33), and maturity indices were calculated using the methods of Bongers (24, 34, 35) as applied by Yeates (36) and by Wasilewska (37). Analyses of species assemblages was carried out using TWINSPAN (38) and DECORANA (39). Data for the DECORANA studies were log(n+1) transformed. Rare species were not down-weighted, and all axes were rescaled. For TWINVOL. 34, NO. 8, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Ranges and Means of Metal Concentrations (ppm) Recorded in the Burning Ground Sediments Compared with Published Values for “Typical” Soil Concentration Rangesa metal aluminum arsenic boron cadmium chromium copper iron mercury manganese nickel lead vanadium zinc
observed (ppm) range (mean) 2290-34800 4.1-117 0.2-87.5 0.45-14.9 15.2-778 4.0-34500 6460-131000 2000 µm) that distorted the size class distributions. However, statistical analysis of the distributions of each size class within the survey area showed that the data were distributed normally (KolomogorovSmirnov Statistic (58, 59)). This permitted calculation of standard correlation coefficients for the examination of relationships between individual size fractions and species richness and diversity indices (see below). The chemical analyses (see Table 1) identified several sites at which metal concentrations exceeded the “typical” ranges 1596
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FIGURE 2. The distribution of the soil total metal burden indices (TMBI) and the clusters of soil assemblages identified in the multivariate analyses of the nematode communities at the explosives burning ground site. quoted for soils at sites throughout the World by Bowen (41) and Levinson (42). Metals identified with concentrations that exceeded those reported by Bowen and Levinson as typical soil value ranges were As, Cd, Cr, Cu, Hg, Pb, and Zn (see Table 1). Based purely on the total metal burden indices (TMBI values) for each site, soils were categorized as exhibiting “low”, “mildly elevated”, “substantially elevated”, or “high” net metal concentrations (Table 2; see also Figure 2). The sites with elevated net metal concentrations did not correlate with bare ground and sparsely vegetated sampling sites. Meiofaunal Assemblages. Of the 10 major groups of soil invertebrates encountered in the material extracted using the Whitehead tray technique (Table 3), Nematoda were the dominant taxon accounting for over 98% of all the organisms observed during the survey. Nematodes were also the most diverse group that was separated from the samples, and 60 taxa were recorded, representing over 20 families (Table 3). Among the non-nematode meiofaunal groups that were extracted, Rotifera (Bdelloidea) were locally abundant, and densities of one species (?Habrotrocha species) of up to 4500 L-1 soil were recorded. Rotifers were absent from nearly all of the samples from the middle and southern end of the survey area. All remaining meiofaunal groups were present at lower densities than the rotifers (i.e. < 50 L-1 soil). Those observed included representatives of the Protozoa (testate, rhizopod amoebae including Centropyxis and Paracentropyxis species), Acari (< 5 L-1 soil; including a widespread, unidentified prostigmatid species), Tardigrada (Macrobiotus hufelandi-group sensu lato), juvenile and newly emerged
TABLE 3. Summary of the Groups Encountered in the Soil Invertebrate Communities and Standard Community Data for the Nematode Assemblages Faunal Groups Observed Protozoa, Rhizopoda Acarina, Prostigmata Rotifera, Bdelloidea Hexapoda, Collembola Nematoda Hexapoda, Thysanura Annelida, Oligochaeta Hexapoda, Psocoptera Tardigrada, Macrobiotidae Hexapoda, Diptera Nematoda Community Data Summary: 60 Species, Representing 20+ Families mean species richness ) 13.7 ( 4.9 species per station maximum species richness ) 24 species; station B4 minimum species richness ) 6 species; station D3 mean density ) 471.7 ( 476.0 L-1 soil maximum density ) 2,216 L-1; station C3 minimum density ) 25 L-1; station A4 mean codominance ) 52.11% ( 19.60% maximum codominance ) 94.0%; station D3 minimum codominance ) 23.8%; station B4 mean Simpson’s diversity ) 6.00 ( 3.13 maximum diversity ) 12.79; station B1 minimum diversity ) 1.26; station D3
TABLE 4. Nematode Species List and Key to Species Codes Used in the Principal Components Analyses (PCA) and Redundancy Analyses (RA) of the Nematode Assemblages Associated with the Contaminated Land Site, Stevenston, Scotland Plectidae
2 32
Araeolaimida Plectus species 1 Plectus species 2
Dorylaimidae
Trichodoridae
5 14 7 37 35 47 23 53 48
Dorylaimida Dorylaimid species 1 Dorylaimid species 2 Dorylaimus species 1 Dorylaimus species 2 Longidorus species Granulonchus schulzi Mononchus papillatus Prionchulus punctatus Trichodorus similis
Tripylidae
33
Monhysteridae
54 6 25
Longidoridae Mononchidae
Xyalidae
oligochaete annelids (Lumbricidae and Enchytraeidae), and single individuals of four different insect groups, including the larvae of cyclorrhaphan Diptera. The low densities of the non-nematode groups extracted using the technique described, combined with their low diversity and/or their patchy distributions, made them unsuitable for use in the multivariate analyses and environmental parameter correlation studies. Nematoda. The nematodes present in the study area included microbivorous species (e.g. Rhabditidae), omnivores (e.g. Dorylaimidae), predators (e.g. Mononchidae and Diplogasteridae), myceliophagous species (e.g. Aphelenchidae), plant-parasitic species (Dorylaimidae/Tylenchida), and saprophytic species/detritivores (e.g. Theristus species and Plectidae (57); see Table 4). The lowest species richness value (number of species per site) was recorded in the sample from site D3 (six species). Comparatively low numbers of species were also observed in the samples from the northern end of the survey area (particularly at sites A4 and A5). The peak nematode density (2216 L-1 soil, equivalent to over 660 000 m-2) was recorded at sampling site C3 where a single species (Tylenchid species 4 (code 38)) accounted for 85% of the individuals recorded. Similar elevated dominance (codominance; combined percent abundance of the two most abundant species) values were noted for several of the other nematode assemblages in this region and correlated with modified communities (Figure 3). The results of k-dominance analyses showed that high dominance corresponded to the communities with reduced diversity (see Figure 4). Maturity index values (MI) for the nematode assemblages calculated using Bongers’ protocol (24) ranged from 1.98 at site A4 to 3.79 at site C5. The lowest values correlated poorly with the communities that were otherwise identified as “modified” or “stressed”. Nonparametric Spearman rank correlation confirmed that there was no correlation between maturity indices and the associated total metal burden indices (p > 0.05). For the present survey area, correlation analyses between individual soil particle size fractions and the species richness and diversity values for the nematode communities failed to demonstrate the existence of statistically significant relationships (threshold value 0.361 at 95% level for 28 d.f.). Based on community structure, the TWINSPAN studies enabled three clusters of nematode assemblages to be defined (clusters I, II, and III; Figure 5). The coherence of these clusters was upheld by the results of the DECORANA studies. Distributions of the three clusters of nematode assemblages
Cephalobidae
Diplogasteridae Rhabditidae
Teratocephalidae
Aphelenchidae Aphelenchoididae Criconematidae
Heteroderidae
Hoplolaimidae Neotylenchidae Tylenchidae
Enoplida Tripyla affinis Monhysterida Monhystera disjuncta Prismatolaimus stenurus Theristus species
Rhabditida Acrobeles ciliatus Acrobeloides buetschlii Cephalobus species (?persegnis) Eucephalobus species Diplogaster species Caenorhabditis species (cf. elegans) 19 Rhabditid species 1 58 Rhabditis species 34 Rhabditonema species (?propinquum)
3 26 8 24 50 9
Teratoceophalida 30 Euteratocephalus palustris 29 Teratocephalus terrestris 11 56 18 15 13 4 40 17 20 10 52 21 49 42 45 60 12 57 16 31 22 36 1 28 37 38 44
Tylenchida Aphelenchus species Aphelenchoidid species Cryptaphelenchus species Criconemoides species Hemicriconemoides species Hemicycliophora species Macroposthonia species (?annulata) Heterodera species 1 Heterodera species 2 Heteroderid species 1 Heteroderid species 2 Heteroderid species 3 (?Meloidodera) Heteroderid species 4 Dolichodorus species Rotylenchus buxophilus Scutellonema bradys Nothotylenchus species Paurodontus gracilis Pseudhalenchus species Sychnotylenchus species Telotylenchus species Tylenchorhynchus species Tylenchid species 1 Tylenchid species 2 Tylenchid species 3 Tylenchid species 4 Tylenchid species 5
Unascribed Taxa 27 NEC.7.95.A 43 NEC.7.95.B 46 NEC.7.95.C 55 NEC.7.95.D VOL. 34, NO. 8, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Surface plot showing the distribution of codominance values (percent abundance) for the nematode assemblages present in the explosives burning ground soils. Note elevated values for the assemblages at sites identified with the highest total metal burden indices. Site viewed from north. tion of the nematode communities. The absence of species such as Acrobeles ciliatus and some members of the family Rhabditidae from cluster III assemblages was noted. Similarly, the omnivorous species, Dorylaimus species 1, was also absent from these assemblages. In contrast, when present, the densities of two other plant-parasitic species, Rotylenchus buxophilus and Tylenchid species 2, were frequently high in the cluster III assemblages.
FIGURE 4. k-Dominance curves for selected nematode assemblages present in the explosives burning ground survey. Examples shown include curves for the least stressed (least modified) assemblage (site B4; 20% dominance, 24 species) and the most stressed community (site D3; 90% dominance, six species). in the survey area and their association with the soil total metal burden indices are shown in Figure 2. Cluster III assemblages were located in the central section of the study area and were bounded to the north and south by cluster II and cluster I assemblages, respectively. Cluster III comprised the most distinctive and heterogeneous assemblages of nematodes; these were isolated at the first TWINSPAN division level, and the component communities were separated from one another at higher levels than those present in the remaining clusters. With the exception of sites C4 and E3, which were classified as substantially increased soil metal sites (Table 2), all of the remaining cluster III sites were categorized as having high soil metal concentrations. The principal metals exhibiting elevated concentrations at these sites were Cd, Cu, Hg, Pb, and Zn. The results of the TWINSPAN study demonstrated that the presence and relative abundances of bacterial-feeding species were important diagnostic features in the classifica1598
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On the basis of community structure alone, axis 1 of the PCA studies separated the nematode assemblages in order of the contamination status of the soils (Figure 6B). In the bi-plot, communities from the least contaminated sites were located toward the right-hand side and the lower quadrant, while those from the soils with the highest heavy metal burdens were clustered toward the upper left. Axis 1 correlated strongly with soil metal concentrations (note length and orientation of the individual correlation lines for the chemical determinands; Figure 6B). Species correlations from the same PCA studies showed that comparatively few nematode species correlated strongly and positively with the elevated metal concentration sites (Figure 6A; species codes given in Table 4). Those that did included Rotylenchus buxophilus (species 45), two, small unidentified species (species codes 46 and 55), Tylenchid species 2 and 4 (species 28 and 38, respectively), and Cryptaphelenchus species (species 18). In contrast, numerous species, presumed to be sensitive, were found to correlate negatively with the elevated metal concentrations. In addition to Dorylaimus species 1 (species 7) already noted above, other examples included Acrobeles ciliatus (species 3), Plectus species 2 (species 32), Prismatolaimus stenurus (species 6), and Dorylaimid species 2 (species 14). Using iterative DOFS, RA identified four soil metals that mapped with the highest proportion of variance in the data sets such that the inclusion of further metals rendered the results of the analysis nonsignificant (p exceeded 0.05). The metals identified were Cd, Al, V, and Cu. RA emphasized the limited numbers of nematode species that correlated posi-
FIGURE 5. Dendrogram showing the three main clusters of nematode assemblages recognized in the TWINSPAN study. See Figure 2 for distribution of clusters of assemblages in the survey area.
FIGURE 6. Bi-plots from the principal components analyses (PCA) shown on the same community axes. A - correlation lines for individual nematode species in relation to the community analysis axes; B - the relationships between the nematode communities and the correlation lines for the soil metals with respect to the same axes. See Table 4 for key to nematode species codes. tively with assemblages from sites with elevated concentrations of these metals (Figure 7A,B). The analyses also highlighted the presence and relative abundance of R. buxophilus (species 45) in the more contaminated soil communities. The particle size distribution data analyses showed a correlation between the strongly modified nematode communities and the finest fractions (< 63 µm and 63-106 µm) as well as with the coarser fractions (500-2000 µm and > 2000 µm). This result is discussed in more detail below.
Discussion This study identified structural variation in microscopic interstitial invertebrate communities (meiofauna) present on a contaminated land site. The distribution of these different communities correlated closely with the observed patterns of heavy metal contamination on the former burning ground. Of the invertebrates recorded using the techniques described, the most informative group proved to be the Nematoda. These were the most diverse (species-rich) and abundant interstitial organisms present in the extracted
samples. However, spatial differences were also seen in the distribution and densities of other groups such as the Rotifera. This group of animals, usually common in moist soils (43), was not observed in the majority of the samples extracted from the central and southern areas of the burning ground that corresponded with the most contaminated soils. In the species-rich nematode assemblages, the differences between the groups of communities took the form of modified community structures. Soils with the highest total metal burden indices supported species-poor complements characterized by the increased dominance of species presumed to be tolerant or resistant (10, 11). These included nonspecialist, generalist feeding types such as the nonselective deposit feeders and detritivores (56, 57). The changes in diversity could have resulted from changes in predation pressures or from altered competition arising from the loss of sensitive, intolerant species such as the bacteriovores (10, 56). In the case of the plant-parasitic species, competition is a less likely explanation, and an additional factor may have been indirect effects mediated through the condition of host plants. However, it should be emphasized that there was no VOL. 34, NO. 8, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Redundancy analyses (RA) showing: A - the variation in the nematode communities associated with the species and B the four most significant soil metals identified by forward selection of environmental variables. See Table 4 for key to the nematode species codes. apparent relationship between sites with visually poorer plant stands, distributed over most of the survey area, and the sites with the higher total metal burden indices. The changes in the dominance-diversity characteristics of the communities were seen clearly in the k-dominance analyses. It is suggested here that these changes may parallel those reported in aquatic pollution studies (see refs 33 and 44). As such, k-dominance analyses may be a valuable means of detecting ecological stress in contaminated land sites. Work originally undertaken in The Netherlands successfully detected modified nematode community structures using an index (the maturity index, MI) that summarizes the relative abundances of “colonisers” and “persisters” (24, 34, 35, 45). In the present study, the maturity index failed to distinguish between the more heavily contaminated site communities and those from the least contaminated sites. Neilson et al. (46) similarly failed to find a relationship between MI and median particle diameter, population densities, and a range of sediment heavy metals (Cd, Cr, Cu, Ni, Pb, and Zn). Particle size is a fundamental determinant of nematode community structure ((47, 48) see below). This fact combined other observations of “poor-fit” made by the present authors in marine and estuarine pollution monitoring studies suggests that caution may be necessary in the application of this index. There was an exceptionally good agreement between the pattern of relationships between the nematode assemblages recognized in the multivariate analyses and the concentrations of soil metals. This was in keeping with the observation that structurally different communities corresponded with different degrees of soil contamination. PCA and RA studies confirmed the existence of correlations between the nematode community structures and metal concentrations. These techniques also identified species that were “tolerant” of elevated soil metal concentrations and distinguished these from the larger numbers of species that were apparently sensitive to this type of stress. The combination of PCA and RA has been used widely by the present authors in commercial aquatic meiofauna studies. In these surveys, heavy metal contamination appears to exert differential effects on the populations of some microbivorous (bacterial-feeding) nematode species. The present study upholds these findings, and bacterial feeders, such as Acrobeles ciliatus and some 1600
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members of the family Rhabditidae, were absent from the more contaminated sites while populations of closely related, predatory, and saprophytic species (detritivores) persisted. This supports the hypothesis that elevated metal concentrations may modify populations of soil microflora. Similar conclusions have been reached by Peterson and his collaborators in aquatic community studies (49). A publication on the terrestrial microbial communities associated with the burning ground meiofauna is currently in preparation. The RA study using forward selection of environmental variables identified soil aluminum as an important correlate with the nematode assemblage structures. This finding is at odds with the published literature and none of the aluminum concentrations recorded was found to exceed the range reported for typical soils (10 000-300 000 ppm (41)). Subsequent discussions with staff at the adjacent explosives factory revealed that aluminum-cased electrochemical devices were routinely desensitized on the burning grounds. It would appear therefore that aluminum was a coincidental correlate. Its distribution would have been spatially identical to that of the more toxic elements, while its comparatively high concentrations would have led to its selection in the RA studies as a strong correlate with the patterns of variation recorded in the nematode community structures. From the correlation analyses, the soil metals could be ranked in order of their ecological importance (described here in terms of the modification of the nematode assemblages). In the sandy soils of the burning ground, vanadium, cadmium, and copper were particularly significant, while iron, mercury, and chromium were least important. This demonstrates an advantage of the present approach over most laboratory-based methods, which would not necessarily have discovered the relative ecological significance of these metals. The nematode communities described in the present study will have taken account the bioavailability of the metals, their oxidation states in the environment, synergistic interactions with other materials, and the integrated effects of these factors on complex foodwebs leading to the highest level consumers in the assemblages. Differential effects of contaminants on naturally variable soil predator-prey systems and associated hostparasite populations cannot be predicted with any certainty from laboratory toxicity testing or screening procedures.
The role of soil particle size fractions in determining the structure of the nematode communities needs to be interpreted carefully. Motility of nematodes in soils, for example, is a function of interstitial pore size and soil moisture/ retention (53, 54, 55, 60). This, in turn, is a function of the matrix granulometry. Accordingly, soils with higher proportions of fines may select for assemblages comprising smaller nematode species (see ref 61). Similarly, in aquatic systems, diversity in sediments with higher proportions of fine particle fractions (e.g. muds) is lower for nematode and other meiofaunal species than in coarser-grained sediments (e.g. well-sorted sands (15, 50-52)). In terrestrial systems, published data on species richness and diversity in relation to particle size distributions are less clear, possibly reflecting variations in sampling techniques and different extraction efficiencies for different soil types (D. J. Hooper, personal communication). In the present study, standard correlation analyses for the sandy soils of the explosives burning ground showed that none of the particle size fractions was correlated with species richness or diversity. The correlation coefficient for the < 63 µm soil fraction and nematode species richness was the nearest value to the critical threshold (correlation value -0.3500; critical threshold value 0.361 at p ) 0.05 and 28 d.f. (59)). However, the more highly contaminated soils corresponded to sites with some of the higher proportions of fine particle size fractions. This could have reflected adsorption of metals onto charged clay particles or simply related to in situ accumulation of fine materials from the substances that were burned. As the analytical protocol used for the soil metal analyses was based on the < 63 µm particle size fractions, this aspect of the study requires further investigation. Despite this, the existence of differing degrees of correlation between individual metals and the more modified soil nematode communities and the presence of diverse communities at peripheral sites that comprised high proportions of fine fractions (e.g. sites A4 and F1) provide a strong case for a metal contaminant driven selection system. In comparison to the chemical analytical costs, the use of meiofaunal analyses proved to be a cost-effective technique for the identification of sites of actual ecological effects on the former explosives burning ground. The area identified as supporting modified communities was smaller than might otherwise have been predicted given the history of the site and its usage. The mapping of the area enabled remediation measures to be focused and permitted a large reservoir of marginal vegetation around the area to be conserved for recolonization and stabilization of the newly remediated site. Further studies are needed on the successional changes in meiofaunal communities that occur as contaminated terrestrial sites recover either naturally (self-cleaning) or postremediation. The effectiveness of the cleanup measures adopted could also be monitored using the same method. Similarly, investigations might usefully be undertaken on the effects associated with different types of terrestrial contaminants (e.g. organic compounds or materials that alter the physicochemical nature of soils). As in the case of inorganic contamination, experience in aquatic habitats indicates that these should be equally rewarding.
Acknowledgments We would like to thank ICI for support in the form of a Strategic Research Fund Grant (No. 277). We are also grateful to staff at ICI Nobel Enterprises, Ardeer, for access to the burning ground and for undertaking the soil physicochemical analyses. In this connection, particular thanks are due to Peter Cartwright, Robert Crawford, and Janet Harris. Finally, thanks are due to the anonymous ES&T referees for their constructive suggestions and comments.
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Received for review January 20, 1999. Revised manuscript received January 11, 2000. Accepted January 18, 2000. ES990064D
