ecological and ecotoxicological surveys of moderately contaminated floodplain ecosystems in the...

Ecological and ecotoxicological surveys of moderatelycontaminated floodplain ecosystems in The Netherlands

J. de Jongea,* , J.M. Brilsb, A.J. Hendriksa, W.C. Mac

aInstitute for Inland Water Management and Waste Water Treatment, PO Box 17, NL-8200 AA Lelystad, The NetherlandsbAquaSense, PO Box 95125, NL-1090 HC Amsterdam, The Netherlands

cInstitute for Forestry and Nature Research, PO Box 23, NL-6700 AA Wageningen, The Netherlands

Abstract

Floodplains of the rivers Rhine and Meuse in the Netherlands are moderately polluted with heavy metals and toxiccompounds. In these floodplain areas a number of nature rehabilitation programmes are being planned and executed. Thequestion arises whether or not the contaminants of concern pose a risk to organisms exposed, and, if so, whether this will hinderthe expected ecosystem recovery. This study focuses on the effects of contaminants on aquatic and terrestrial macro-inverte-brates in the field situation. In three shallow lakes in the floodplains of the river Rhine, showing different degrees of contam-ination, chemical analyses and laboratory bioassays have been carried out on sediment samples together with intensivebiological field surveys (a so called TRIAD-approach). A nearly identical set-up was chosen to assess 18 terrestrial floodplainsites. The concentrations of several contaminants exceed generic critical risk levels. Risk assessment models show that actualeffects on top predators cannot be excluded. Species of lower trophic status might be at risk as well. In the aquatic compartmentschemical, ecotoxicological and ecological results point in the same direction. They can be summarised as moderate biologicaleffects, which can mainly be attributed to the elevated levels of a mixture of priority pollutants (metals and polyaromatichydrocarbons). No large differences were found among the three lakes. At terrestrial sites, however, fewer effects were found infield surveys and bioassays, while the chemical analyses revealed the same levels of pollution as in the lakes study. Thissuggests a lower bioavailability of the contaminants than expected. In conclusion, no extreme effects have been found. Thisconclusion only partly supports the predictions made by present day-risk assessment models. The advantages of naturerehabilitation, that is, improvement of ecological quality through habitat diversity, seem to be larger than the disadvantagesof increasing toxic stress by exposing a larger diversity of species to floodplain contaminants. To confirm this statementrehabilitation programmes should include overall as well as in-depth monitoring studies.q 1999 Elsevier Science Ltd andAEHMS. All rights reserved.

Keywords:Macro-invertebrates; TRIAD-approach; Bioassays

1. Introduction

The Dutch part of the river Rhine is strongly influ-enced by human activities. The river bed is narrowdue to dikes, the main channel is canalised anddeepened and sandbanks have been removed. Nowa-

days floodplains are mostly used for agricultural andfarming purposes. Recently nature developmentprogrammes have been set up to change the flood-plains into more natural, dynamic systems. Secondarychannels, marshes and floodplain forests will berestored (Cals, 1998). Beside this, the level of thefloodplains will be lowered in order to create moredischarge capacity to avoid flood disasters, which

Aquatic Ecosystem Health and Management 2 (1999) 9–18

1463-4988/99/$20.00q 1999 Elsevier Science Ltd and AEHMS. All rights reserved.PII: S1463-4988(99)00017-2

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* Corresponding author.

nearly occurred in 1993 and 1995 (Ministry of Trans-port, Public Works and Water Management, 1996).

However, the floodplains of the river Rhine aremoderate to heavily polluted with heavy metals andtoxic organic compounds. This has been mostly due tohistorical contamination in the sixties and seventieswhen contaminated suspended matter settled in thefloodplains. Although the water quality has improved,there is still a certain level of contamination (Beurs-kens et al., 1993; IRC, 1993).

The question arises whether or not this pollution isa problem for the development of natural areas. Riskassessment based on laboratory experiments andecotoxicological models shows that the present levelsof contamination indeed may pose a risk to aquatic aswell as to terrestrial ecosystems. The toxicants can actdirectly on individuals or via secondary poisoningthrough the foodchain on top predators (Dogger etal., 1992; Hendriks, 1994; Van de Guchte, 1995).To what extent these estimated risks actually resultin site specific effects on ecosystems has been littleinvestigated.

In this article, the results of two studies on thedirect, actual effects of contaminants on macro-invertebrate communities in both aquatic and terres-trial parts of the floodplain will be discussed.Chemical analyses, laboratory bioassays and biolo-gical field surveys were carried out to investigate thecorrelation between pollution and macro-inverte-brates. This combination of techniques, which each

yield additional information, is generally called theTRIAD-approach (Van de Guchte, 1992; Maas et al.,1993).

2. Methods

Aquatic investigations were carried out in threesmall shallow lakes, A, B and C, situated in thefloodplain of the Dutch river Waal (a Rhine branch),in November 1995 (Fig. 1). Lake A (ca. 0.5 km2) isan old cut of a secondary channel; lake B (ca.0.75 km2), also isolated, has been a ‘clay win put’;lake C (ca. 2 km2) has been used for winning sandand is in open connection with the river Waal. Plansare being made to connect lake B and C with eachother and to the river Waal, to make an artificialsecondary channel. In each lake, ca. 5 m from theborder, at a water depth of 1 m, sediment sampleswere taken for testing in bioassays and for chemicaland biological analyses.

For the terrestrial investigations 18 sites wereselected from rough herbage biotopes in the flood-plains of the river Waal. Using historical data of sedi-ment contamination, a range of contamination levelswas obtained among these 18 sites. On each site, soilsamples were taken for bioassays and chemicalanalyses, and macro-invertebrate surveys wereperformed.

J. de Jonge et al. / Aquatic Ecosystem Health and Management 2 (1999) 9–1810

Fig. 1. Sampling sites of the aquatic surveys, near the river Waal (a Rhine branch). Site A: cut of a secondary channel (isloated), site B: ‘claywin put’ (isolated), site C: ‘sand win put’ (connected to the river).^ indicate sampling point.

2.1. Physical and chemical characterisation ofsediment and soil

2.1.1. Aquatic sitesAt each site, a composite sample was taken from

the upper 10 cm of the sediment. Grain size fractions, 2, , 16, , 63, , 210 and . 210mm and thecontent of organic matter were determined. Sedimentcontaining less then 20% organic matter and a,63%mm grain size between 35–55% was charac-terised as sandy silt. When, 63%mm grain size wasbetween 10–35%, the sediment was considered to besilty sand. Sediments were designated as stable whenmoisture content was below 50% and unstable when itwas over 50% (see sediment characterisation, Rein-hold-Dudok van Heel et al., 1999).

Chemical analyses were performed on sedimentsamples for seven heavy metals (Cd, Cr, Cu, Ni, Pb,Zn, Hg) arsenic, polyaromatic hydrocarbons (PAHs)(16 of EPA), 7 polychlorinated biphenyls (PCB),organochlorine pesticides, mineral oil, EOX andnutrients. Concentrations of metals in the sedimentswere normalised to standard sediment (10% organicmatter and 25% silt) by means of equations using,2 mm grain size fraction (silt) and organic mattercontent on a dry weight basis as parameters. Standar-disation of concentrations of organic compounds wasbased on organic matter content (Van der Kooij et al.,1991). After normalisation of contaminant concentra-tions, sediments were classified as class 0 (not orslightly polluted) up to class 4 (heavily polluted)using the Dutch sediment quality criteria (Ministryof Transport, Public Works and Water Management,1989).

2.1.2. Terrestrial sitesOn each site a top layer of 25 cm was sampled for

analyses. The upper layer of litter was removed. Soilgrain size fractions of, 2, , 16 and , 63mm,CaCO3, pH and organic matter were determined.Chemical analyses of soil and litter layer and methodsof normalisation to standard soil were the same as forthe aquatic sediment samples.

2.2. Aquatic investigations

2.2.1. BioassaysThe acute toxicity of the sediment samples was

assessed by performing bioassays using the bacteria(Vibrio fischeri), a rotifer (Brachionus calyciflorus)and a crustacean (Thamnocephalus platyurus) as testorganisms.

Chronic toxicity was judged from bioassays usingwaterfleas (Daphnia magna) and midge larvae (Chir-onomus riparius). All bioassays were performedaccording to the guidelines as described in Maas etal. (1993), Van de Guchte et al. (1993), den Besten etal. (1995), Creasel (1990, 1992).

The results were classified as having ‘no effect’( 2 2 ), ‘moderate effect’ (1 2 ) or ‘strong effect’( 1 1 ). For Daphnia magna, Chironomus ripariusand Vibrio fischeri the same criteria were used asdescribed in Reinhold-Dudok van Heel et al. (1999).

The results of Thamnocephalus platyurusandBrachionus calycifloruswere classified as follows:

• no effect (2 2 ): EC50 $ 100 volume% porewater (or %effect in undiluted pore water is, 50)

• moderate effect (1 2 ): EC50 . 50 and,100 volume% pore water (or %effect inundiluted pore water is$ 50 and , 100)

• strong effect (1 1 ): EC50# 50 volume% porewater (or %effect in undiluted pore water is equalto 100).

2.2.2. BioaccumulationTo assess the biological availability of specific

chemical substances for biota, bioaccumulation testswere carried out by exposing oligochaetes (Tubifex) tothe sampled sediments. The tests were carried outaccording to Maas et al. (1993). At the beginning ofthe test (non-exposed animals) and after 28 d of expo-sure the oligochaetes were analysed for the same listof toxicants as given for ‘chemical analyses’. Thebiota-sediment accumulation factor (BSAF) wasdetermined following the method of den Besten(1996). The BSAF of heavy metals is based on contentin dry weight of oligochaetes and sediment; the BSAFof organic pollutants is based on content in fat of theoligochaetes and in organic carbon in the sediment.The predicted BSAF was based on equilibrium parti-tioning equations (den Besten, 1996). The ratiobetween measured and predicted BSAF gives an indi-cation of the bioavailability of toxicants for biota. Theresults were classified as follows:

J. de Jonge et al. / Aquatic Ecosystem Health and Management 2 (1999) 9–18 11

whereM�measured BSAF andP� predicted BSAF.

In this article the results are presented for toxicantswhich showed a substantial increase in internalconcentration (. 30%) at the end of the test (t �28 d), when compared to the beginning of the test(t � 0).

2.2.3. Field surveysAt each site macro-invertebrates were sampled

taking five replicas using an Ekman–Birge corer(0.0225 m2). Each core was flushed over a 500mmsieve and the remaining parts on the sieve wereconserved in 5% formaldehyde. In the laboratory,organisms were separated from organic material byeye and determined where possible to species level.From each site the mean of the five replicas was deter-mined and the results are presented as the number ofspecies within taxonomic groups (diversity) and asabundances (number of animals m22), for chirono-mids, oligochaetes and bivalves. These groups repre-sented 70% of the total number of species and 99% ofthe total number of individuals (Brils et al., 1997). Theresults are classified as not disturbed (2 2 ), moder-ately disturbed (1 2 ) or strongly disturbed (1 1 )communities, according to the criteria based on datafrom polluted and clean reference areas in the Nether-lands (den Besten et al., 1995; Poorter et al., 1996),but modified for the type of sediment (Table 1).

The incidence of mentum deformities ofChiro-nomusspecies was also studied, using the criteria ofWarwick (1988).

2.3. Terrestrial investigations

2.3.1. BioassaysFor bioassays, soil samples were taken from six

sites having different levels of contamination. Beforesampling, the litter layer was separated from theunderlaying soil. Acute toxicity of the soil wasassessed in a bioassay using the springtailFolsomiacandida(24, 48 and 72 h), as described by AquaSense(1996a,b). Chronic toxicity of the soil samples wasjudged using bioassays withF. candida (28 d) andthe earthwormLumbricus rubellus(30 d). The woodlouseTrachelipus rathkei(60 d) was used for testingthe toxicity of the litter layer. The chronicF. candidaassay was performed according to the ISO/TC 190guideline (1992) adapted for testing field samples(AquaSense, 1997). Survival of adults and the numberof juveniles produced were the endpoints measured.The tests usingLumbricus and Trachelipus wereperformed according to methods developed by Maet al. (1998). For earthworms and wood louse thetest parameters consisted of growth, survival andreproduction (number of produced cocoons andnumber of offspring, respectively). No criteria wereavailable to classify the results of the terrestrialbioassays. For each parameter the results fromthe different soil samples were compared to eachother and to a reference control using statistical testmethods.


Criteria Biological availabilityM/P , 1 Not enhanced1 # M/P , 10 EnhancedM/P $ 10 Strongly enhanced

Table 1Standard values for the classification of parameters from macro-invertebrate community surveys. Not disturbed (2 2 ), moderately disturbed( 1 2 ), strongly disturbed (1 1 ) (den Besten et al., 1995)

Stable silty sand, stable sandy silt Unstable silty sand1 1 1 2 2 2 1 1 1 2 2 2

Number of species (N)Chironomids 0–4 5–10 $ 11 0–2 3–5 $ 6Oligochaetes 0–4 5–9 $ 10 0–2 3–5 $ 6Bivalves 0–3 4–8 $ 9 0–2 3–5 $ 6Density (N/m2)Chironomids , 500 500–1500 $ 1500 , 100 100–500 $ 500Oligochaetes , 300 300–1000 $ 1000 , 300 300–1000 $ 1000Bivalves , 200 200–800 $ 800 , 5 5–100 $ 100

2.3.2. BioaccumulationContaminant levels of the heavy metals Cd, Cu, Pb

and Zn, and of PCBs and PAHs were measured inearthworms (Lumbricus rubellus) after 30 d of expo-sure in bioassays and in earthworms sampled at thefield sites (L. rubellus, Aporrectodea caliginosaandAllolobophora chlorotica). Measured concentrationsof heavy metals were compared to theoreticallyexpected levels of internal concentrations as derivedby Ma and Verhallen (1998) by using bioavailabilitymodels.

2.3.3. Field surveysThe terrestrial macro-invertebrates were sampled

using a pyramid trap of 1 m2. Samples were taken inthree periods (July, August and September), eachduring two weeks. Organisms were identified tospecies level were possible. Earthworms weresampled separately from soil surface layers of 50×50 cm with a depth of 25 cm. For terrestrial macro-invertebrates no data from comparable polluted orclean reference areas in the Netherlands wereavailable.

3. Results

3.1. Chemistry of aquatic and terrestrial samplingsites

Table 2 shows, for four heavy metals and threeorganic toxicants, the range of contamination foundin this study. In general, the terrestrial soil was morepolluted than the aquatic sediment, but all were in thecategory of slightly to moderately polluted, accordingto Dutch sediment/soil criteria.

3.2. Aquatic investigations

3.2.1. BioassaysResults of bioassays are presented in Table 3. Expo-

sure of the rotiferB. calyciflorusto pore water resultedin a ‘moderate effect’ at all sites. At site B, the testsusing the crustaceansThamnocephalusandD. magna


Table 2Chemical analyses of aquatic sediment and terrestrial soil from thesampling sites on the floodplains

Aquatic sediment Terrestrial soil

Cadmium (mg kg21) 0.7–1.2 0.2–4.2Copper (mg kg21) 26–44 9–101Lead (mg kg21) 36–60 16–185Zinc (mg kg21) 363–176 90–614S10 PAH (mg kg21) 8.8–12.8 1.3–15.2S6 PCB (mg kg21) 40–124 26–400Organochlorinepesticides (mg kg21)

, 5 6–54

Table 3Results of bioassays with aquatic sediment from shallow poolsA, BandC on the floodplain. No effect (2 2 ), moderate effect (1 2 ),strong effect (1 1 )

Test organism ToxicitySite A Site B Site C

AcuteBacteria (0.5 h, pore water)Vibrio fischeri

2 2 2 2 2 2

Rotifer (1 d, pore water)Brachionus calyciflorus

1 2 1 2 1 2

Crustacean (1 d, porewater)Thamnocephalusplatyurus

2 2 1 2 2 2

ChronicWater flea (15 d, porewater)Daphnia magna

2 2 1 2 1 1

Midge larvae (28 d,sediment/water mixture1:4) Chironomus riparius

2 2 2 2 2 2

Overall toxicity (based onworst effect measured)

1 2 1 2 1 1

Table 4Results of bioaccumulation tests with oligochaetes on aquatic sedi-ment from poolA, B andC

BSAF measured BSAF expected Bioavailability

Site A 0.21 0.04 EnhancedCd 0.15 0.03 EnhancedCrSite BCd 0.45 0.04 Strongly enhancedCr 0.17 0.3 EnhancedHg 0.32 0.9 EnhancedSite CCd 0.18 0.04 EnhancedHg 0.25 0.09 EnhancedPb 0.35 0.03 Strongly enhancedPCB-153 3.18 2.0 EnhancedSDDT 2.75 2.0 Enhanced

also gave a ‘moderate effect’, while a ‘strong effect’was obtained whenD. magnawas exposed to porewater of site C. The judgement of the overall toxicityshows ‘moderate effect’ for sites A and B, and a‘strong effect‘ for site C.

3.2.2. BioaccumulationIn Table 4 the results of the bioaccumulation tests

are presented for those toxicants for which anenhanced bioavailability was found. Compared tothe estimated levels, at sites A and B the availabilityof Cr was enhanced while the availability of Cd wasenhanced at all sites, on site B even strongly (M/Pratio. 10). For site C, Pb was also strongly enhancedand an enhanced bioavailability was found for theorganic pollutants PCB-153 andSDDT.

3.2.3. Field surveysThe number of species and the density of chirono-

mids, oligochaetes, bivalves and gastropods arepresented in Table 5. These are the main groupswhich were found in the samples of the field surveys.Comparing these data to the criteria for the level ofdisturbance (see Table 1) show that especially thebivalves did not match to the expected densities anddiversity (Table 6). On site A the number of species ofoligochaete and on site C the density of chironomidswere strongly disturbed. Most other parameters weremoderately disturbed and only some parameters (chir-onomid density on site A and oligochaete density andnumber of species on sites A and B) were undisturbed.Generally it can be said that the macro-invertebratesurvey showed few species and low densities. At allsites the mentum deformities of the chironomids wereseen, but they did not exceed a 10% incidence (.10%indicates disturbance).

3.3. Terrestrial investigations

3.3.1. BioassaysBioassay results for the reproduction of the earth-

worm, wood louse and spring tail are given in Fig. 2.No clear relationship existed between levels ofcontamination and reproduction. The reference soilfor the wood louse was an artificial substrate, whilefor the springtail and earthworm the reference soiloriginated from a relatively unpolluted (non flood-plain) area. The differences in properties of the refer-ence soil and soil sampled from field sites probablyexplain the differences in reproductivity betweenfloodplain and the reference soils. Contaminationlevels in the litter layer showed a significant negativecorrelation with reproduction parameters for the woodlouse (for cadmiumr �20.743, for zincr �20.714;p , 0.05). For the end points growth and survival, norelation with contaminant levels was found.

3.3.2. BioaccumulationBioaccumulation measurements of toxicants in the

earthworms used in bioassays and those sampled inthe field showed a clear correlation between Cd, Cuand Pb levels in the organisms with the concentrationsin the soil. The strongest accumulation was found forCd. Fig. 3 presents the results for the field sites. For Zn


Table 5Results of aquatic macro-invertebrate surveys with and Eckman–Birge corer in pool A, B and C

Parameters Site A Site B Site C

ChironomidsNumber of species 6 6 3Density (total no./m2) 1929 1111 80OligochaetesNumber of species 3 5 3Density (total no./m2) 1484 3367 978BivalvesNumber of species 0 1 1Density (total no./m2) 0 18 9GastropodsNumber of species 3 2 2Density (total no./m2) 80 80 89

Table 6Assessment of the disturbance of macro-invertebrate communitiesin pool A, B and C. No effect (2 2 ), moderate effect (1 2 ),strong effect (1 1 )

Site A Site B Site C

Chironomid species 1 2 1 2 1 2

Chironomid density 2 2 1 2 1 1

Oligochaete species 1 1 1 2 1 2

Oligochaete density 2 2 2 2 1 2

Bivalve species 1 1 1 1 1 1

Bivalve density 1 1 1 1 1 2

Overall disturbance 1 1 1 1 1 1

(not shown) the correlation was not significant.Compared to bioavailability models for heavymetals, the expected and measured concentrations oftoxicants in the earthworms were within the samerange, while for Cd the measured concentrationswere systematically higher (up to a factor of 3). Acorrelation between contaminant levels in the earth-worm and in soil was also seen for PAHs and PCBs,although less clearly. This shows the availability forthe uptake of PAHs and PCB in the field situation.PAH concentrations in earthworms from field siteswere higher when compared to those organismsexposed in bioassays; this suggests that in the bioas-says no equilibrium was reached.

3.3.3. Field surveysThe existing knowledge about the macro-inverte-

brate populations on the terrestrial floodplains israther poor and hence it was not possible to comparethe data from this study to the ‘expected values’.However, based on expert judgement, the fieldsurveys on the 18 sites sampled show, in general,high diversity and high densities (Fig. 4). No correla-tion with contamination levels was found, with anexception of ground-living species of Sphecidae(Hymenoptera). They show a significant negative

correlation with increasing pollution level (r �20.519,p , 0.05).

4. Discussion

The question of whether the contaminants in thesoil and sediment in the Dutch part of the riverRhine hinders nature development in floodplainareas is an important issue. Risk assessment studiesshow that there are risks for several groups of organ-isms at the present levels of contamination (Dogger etal., 1992; Balk et al., 1994; Van de Guchte, 1995).Field studies in the highly polluted Rhine delta showstrong correlations between poorly developed macro-invertebrate communities and pollution (den Besten etal., 1995) even when confounding factors like sedi-ment composition are eliminated (Reinhold-Dudokvan Heel et al., 1999).

Pollution levels in both the aquatic and terrestrialareas studied can be characterised as low to moderate.At the aquatic sites the field surveys of the inverte-brate communities showed in general low densitiesand a low diversity, having a composition dominatedby chironomid and oligochaete species. Only a fewgastropods, bivalves or other taxonomic groups werefound. A factor which might also be responsible for


Fig. 2. Reproduction success of earthworms (Lumbricus rubellus) (cocoons/individuals), wood louse (Trachilepus rathkei) (juveniles/female),and springtails (Folsomia candida) (juveniles/female) in bioassays with soil from floodplain sites.

the low diversity and density at site C is the fluctu-ating water level. This sampling site is partly dry atsome periods during the year. The aquatic bioassayswith field sediments show the most pronouncedeffects at site C, but also some minor effects werefound on sites A and B. The enhanced bioavailabilityof toxicants, especially at site C, can explain the nega-tive effects demonstrated in the bioassays and fieldsurveys. These effects were somewhat greater thanwould be expected when the low to moderate pollu-tion levels measured were normalised to standardsediment, using equilibrium partitioning theory. Theavailability of Cd at all sampling sites indicated a riskfor top predators through biomagnification via thefoodchain. In summary, the results of the fieldsurveys, bioassays and the bioavailability of somepollutants indicate that the actual pollution levelmay act as a toxic stress, which hinders the develop-ment of a healthy macro-invertebrate community,especially on site C. This is not in accordance to theconclusion which would be drawn when attention waspaid only to the Dutch (chemical) Sediment Qualitycriteria (Ministry of Transport, Public Works andWater Management, 1989). In that case it wasexpected that site A would hinder nature developmentthe most, because it is the most polluted site.However, the contamination at site A mainly consistsof oil, which might be less biologically available. Atsite C, and to a lesser degree at site B, pollution levelsof heavy metals were higher, we demonstrated thatespecially metal bioavailability was enhanced. Insituations like this, where knowledge about actualrisks on ecosystems is useful in decision makingframeworks, complementary information to chemicalanalyses seems to be indispensable (Hendriks and Vande Guchte, 1997).

At the terrestrial sites, correlations between pollu-tion levels and measured end-points in the field surveysand bioassays were less evident. Negative correlationswith pollution levels were found only for Sphecidae inthe field surveys, and for reproduction of the wood lousein the bioassays. Generally, the terrestrial invertebratecommunities showed high diversity and high densities.However, the interpretation of these data is more diffi-cult when compared to the aquatic investigationsbecause of the lack of comparable data from unpollutedsites. Although there was a pollution gradient in the 18sampled terrestrial sites the deviation was not very large.


Fig. 3. Relation between heavy metal content in soil and earth-worms (Aporrectodea caliginosa) from floodplains sites.

It may be noted that the terrestrial macro-inverte-brates are less endangered by toxicants whencompared to aquatic species because most of thesoil inhabiting species live in the litter layer and/orare very mobile. Therefore, they may not be in inten-sive direct contact with the polluted soil or if so, for ashort time period only. Another reason might be thatthe invertebrate communities in terrestrial floodplainsare adapted to dynamic circumstances, so they may bea stress-adapted group of organisms which are rela-tively insensitive to pollution. The bioavailability ofheavy metals is relatively low in the floodplain areas,where calcium concentrations are relatively high asare pH levels (pH� 8). This may explain the ratherlow toxic stress. However, further studies at unpol-luted and more strongly polluted sites are necessaryto support the findings that the actual pollution level inthe Dutch floodplains does not inhibit terrestrialmacro-invertebrate communities.

In conclusion, at the actual levels of contaminantsin the Dutch river Rhine floodplains, effects on macro-invertebrate communities are likely, but not so strongas to justify a delay of the execution of nature devel-opment projects. However, it is advisable to supportthese projects with monitoring programmes whichshould include overall as well as in-depth studies.Attention should be paid to chronic, sublethal effectsat the community and ecosystem levels. As well,attention should be paid to sensitive indicator specieswhich are expected to return to the floodplains after

the rehabilitation of their habitats. Changing land use(from terrestrial to aquatic and vice versa) during theconstruction of the nature development projects, candrastically change the bioavailability of toxicants.These effects of low and moderately polluted sedi-ments on the density and diversity of macro-inverte-brates may also have indirect effects on speciesinteractions in foodwebs. This aspect was notincluded in this study, nor was the secondarypoisoning effects of the toxicants via biomagnificationin the foodchain. We recommend that to enableadequate interpretations of field surveys, data onpolluted areas and of comparable, non-polluted sitesare very much needed. In the case of floodplains thistype of information can hardly be found in the Nether-lands. Use of ‘reference’ rivers abroad could give usvery useful insights.

Acknowledgements

The authors thank all persons which whereinvolved in the research projects, especially E. Rein-hold, P. den Besten, J. Faber, H. Siepel and A. Klink.C. van de Guchte is greatly thanked for his sugges-tions on the manuscript. The research is carried out byorder of the Directorate East Netherlands of the DutchMinistry of Transport, Public works and WaterManagement.


Fig. 4. Number of species of spiders (Araneida), Hymenoptera, worms (Oligochaeta) and beetles (Coleoptera) in field sites of floodplains withan increasing level of contamination.

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