Response of invertebrate assemblages to increased groundwater recharge rates in a phreatic aquifer

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Response of invertebrate assemblages to increased groundwater recharge rates in aphreatic aquiferAuthor(s): Thibault Datry, Florian Malard, Janine GibertSource: Journal of the North American Benthological Society, 24(3):461-477. 2005.Published By: The Society for Freshwater ScienceDOI: http://dx.doi.org/10.1899/04-140.1URL: http://www.bioone.org/doi/full/10.1899/04-140.1

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461

J. N. Am. Benthol. Soc., 2005, 24(3):461–477q 2005 by The North American Benthological Society

Response of invertebrate assemblages to increased groundwaterrecharge rates in a phreatic aquifer

THIBAULT DATRY1, FLORIAN MALARD2, AND JANINE GIBERT3

Unite Mixte de Recherche Centre National de la Recherche Scientifique (UMR CNRS) 5023, Ecologie desHydrosystemes Fluviaux, Universite Claude Bernard Lyon 1, Bat. Forel, 43 Bd 11 Novembre 1918,

F-69622 Villeurbanne Cedex, France

Abstract. Low organic matter supply and reduced spatiotemporal heterogeneity severely constrainbiodiversity in groundwater. At the aquifer scale, spatial variation in the groundwater recharge rateis considered a key factor in generating groundwater patches with distinctly different food suppliesand spatiotemporal heterogeneity. Our study addressed the role of groundwater recharge in sustain-ing biodiversity in a phreatic aquifer by testing differences in the density and species richness ofinvertebrate assemblages among 11 reference sites and 13 sites artificially recharged with storm water.The vertical distribution pattern of invertebrate assemblages was examined using well clusters at oneshallow water-table recharge site and one nearby reference site. Groundwater recharge elevated dis-solved organic C (DOC) concentrations in groundwater and increased spatiotemporal physicochem-ical heterogeneity. The thickness of the vadose zone (VZT) controlled DOC input to groundwater atrecharge sites but did not reduce spatiotemporal heterogeneity. The higher density and richness ofinvertebrate assemblages at the well-cluster recharge site than at the well-cluster reference site alsowas controlled by VZT, suggesting that organic matter supply was a primary factor determiningbiodiversity patterns in groundwater. Invertebrate density increased and species composition shiftedwith increasing depth below the groundwater table at the shallow water-table well-cluster site. Sometaxa, including several epigean species, preferentially occurred near the water table, whereas others,including several hypogean species, colonized deeper groundwater layers.

Key words: stormwater, dissolved organic carbon, dissolved oxygen, environmental heterogeneity,hypogean invertebrates, faunal gradient, hyporheic zone.

Little research has been conducted to identifyfactors controlling biodiversity patterns withingroundwater (Danielopol 1989, Dumas et al.2001, Mauclaire and Gibert 2001) compared tothe amount of research on surface and shallowsubsurface freshwater systems (i.e., hyporheicor parafluvial zones). Groundwater systems dif-fer fundamentally from surface and shallowsubsurface freshwater systems in that they oftenexhibit relatively low organic matter standingstocks and spatiotemporal heterogeneity (Gibertet al. 1994, Baker et al. 2000). Organic C supplyfrom the surface environment can limit the rateof many ecological processes in groundwater,including aerobic respiration (Jones et al. 1995a,Baker et al. 2000), nitrification (Holmes et al.1994, Jones et al. 1995b), denitrification (Starrand Gillham 1993), and methanogenesis (Joneset al. 1995c). Habitat availability is severely re-

1 Present address: CEMAGREF, Groupement deLyon, 3 bis quai Chauveau, CP 220, 69336 Lyon cedex09, France. E-mail: [email protected]

2 E-mail addresses: [email protected] [email protected]

stricted in groundwater and habitat heteroge-neity can be relatively low in space and time,particularly with respect to temperature and O2

(Gibert et al. 1994).Spatial variation in the groundwater recharge

rate is a primary factor generating areas withdistinctly different food supplies and spatiotem-poral heterogeneity at the scale of a regionalaquifer. Groundwater recharge is the process bywhich rain water and surface water seepthrough the soil into an underlying aquifer,thereby providing water supply to groundwater(Lerner et al. 1990, de Vries and Simmers 2002).Increasing use of artificial infiltration basins todispose of storm water and tertiary effluent intothe subsurface may induce locally high ground-water recharge rates in unconsolidated sedi-ments (Pitt et al. 1999, Datry et al. 2004). Labileorganic C generated in surface water and soil istransported downward into the aquifer by infil-trating water and is consumed by microorgan-isms as groundwater moves either vertically orhorizontally away from the recharge zone(Champ et al. 1979, Ronen et al. 1987, Malard

462 [Volume 24T. DATRY ET AL.

and Hervant 1999). Dissolved organic C (DOC)input into groundwater is expected to stimulatethe production of microbial biofilms, which arethen grazed by invertebrates (Cummins andKlug 1979, Barlocher and Murdoch 1989). Theamount of organic matter reaching the ground-water table depends on DOC production in thesurface environment, water infiltration rate, andsignificance of adsorption and biodegradation ofDOC in the vadose zone. DOC concentrations ingroundwater are distinctly higher in areas ofshallow groundwater because the residencetime of water in the vadose zone is insufficientto allow microbial degradation of DOC (Starrand Gillham 1993, Pabich et al. 2001). Areas ofintensive groundwater recharge also shouldshow high spatiotemporal heterogeneity be-cause the infiltration of surface water inducesstrong temporal fluctuations in temperature andwater chemistry that are attenuated with in-creasing depth and distance from the rechargearea (Datry et al. 2004, Malard et al. 2004).

Our study addressed the role of increasedgroundwater recharge rates in sustaining bio-diversity in a regional phreatic aquifer by ex-amining differences in the density and speciesrichness of invertebrate assemblages among ref-erence sites and sites artificially recharged withstorm water. The vertical distribution of inver-tebrate assemblages was compared between areference site and a shallow water-table re-charge site. We expected that increased DOC in-puts and physicochemical spatiotemporal het-erogeneity at the recharge sites would lead toincreased density and species richness of inver-tebrate assemblages.

Methods

Study sites

Study sites were located in the glaciofluvialaquifer of the city of Lyon, France. The aquifersurface is 450 km2, and the aquifer is drainedby the Rhone River (Fig. 1A). The aquifer con-sists of 3 corridors that are separated by mo-raine hills of low hydraulic conductivity (i.e.,1025–1028 m/s; BURGEAP 2001). Quaternaryglaciofluvial sediments of high hydraulic con-ductivity (i.e., 1023–1022 m/s) are progressivelyreplaced by modern alluvial sediments of simi-lar hydraulic conductivity near the Rhone River.

Monitoring sites. Twenty-four sites were se-

lected. Thirteen recharge sites were in ground-water areas artificially recharged with stormwater, and 11 reference sites were in ground-water areas fed exclusively by direct infiltrationof rain water at the land surface (Fig. 1A, Table1). Sites were selected to obtain 2 categories ofvadose zone thickness (VZT) (,10 m and .10m) for both recharge and reference sites. Eachsite had a monitoring well with a perforatedcasing intersecting the water-table region of theaquifer. At recharge sites, monitoring wells werein the immediate vicinity of stormwater basinsto intersect the groundwater zone artificially re-charged with storm water. The annual infiltra-tion rate was obtained for each basin by divid-ing the annual quantity of rain water falling onthe catchment of the basin by the surface areaof the infiltration basin (Table 1). For compari-son, the annual infiltration rate of water at ref-erence sites was estimated as 420 L m22 y21 (i.e.,700 mm rain/y, 40% of which was lost byevapotranspiration).

Well-cluster sites. One recharge site and onenearby reference site in each VDZ category (n5 4) were implemented with a cluster of short-screen monitoring wells to determine the influ-ence of recharge on the temporal and verticalheterogeneity of groundwater physicochemistry.The shallow water-table recharge site (VZT ,10m; site 1 in Fig. 1A, B, and Table 1) had a VZTof only 3 m, and groundwater in alluvial de-posits was artificially recharged with storm wa-ter from an infiltration basin draining a catch-ment area of 2.5 ha. The deep water-table re-charge site (VZT .10 m; site 11 in Fig. 1A andTable 1) had a VZT of 19 m and groundwaterin glaciofluvial deposits was artificially re-charged with storm water from an infiltrationbasin draining a catchment area of 180 ha. De-tailed information on the hydrological andchemical characteristics of these 2 recharge sitesare reported in Barraud et al. (2002), Datry(2003), Datry et al. (2003, 2004), and Malard etal. (2004). The reference site in each VZT wasclose to the recharge site (site 14 in VZT ,10 mand site 20 in VZT .10 m) and had similar hy-drogeological characteristics to the recharge siteexcept for the absence of an infiltration basin.

Well clusters consisted of 3 to 5 wells sepa-rated from each other by ,2 m. At the shallowwater-table recharge site, 4 wells were installedat depths of 4, 5, 6, and 7 m below the soil sur-face (Fig. 1B). The screened area began 0.5 m

2005] 463GROUNDWATER BIODIVERSITY IN RECHARGE ZONES

FIG. 1. A.—Locations of recharge and reference sites. Arrows indicate direction of groundwater flow. Darkand light gray patterns show the moraine hills and highly urbanized areas, respectively. B.—Cross-sectionalillustration of well clusters (wells are numbered) used for sampling groundwater and invertebrates at theshallow water-table recharge (site 1) and reference (site 14) sites. asl 5 above sea level.


TABLE 1. Characteristics of recharge (below storm-water infiltration basins) and reference sites. Rechargeat reference sites was derived only from the naturalinfilatration of rainwater (420 L m22 y21), so catchmentareas and infiltration rates are not provided. B 5 re-charge site, R 5 reference site, VZT 5 vadose zonethickness.

Site Type VZT (m)

Catchmentarea(ha)

Infiltrationrate

(L m22 y21)

12345

BBBBB

3477.89.2

2.5185285145

33.7

23,333323,750221,667131,818214,455

6789

10

BBBBB

13.513.514.918.118.4

457451

180315

384,14666,994

282,747120,632635,447

1112131415

BBBRR

1919.320.3

55

1805045

120,632233,333828,947

1617181920

RRRRR

557.17.8

17.821222324

RRRR

1919.619.928.2

from the lower end of the well and ended 1 mfrom the lower end of the well (see Fig. 1B).These wells sampled water at depths of 1, 2, 3,and 4 m below the groundwater table, respec-tively (Fig. 1B). At the shallow water-table ref-erence site, 3 wells were installed at depths of7, 8, and 9 m below the soil surface. These wellssampled water ;1.5, 3, and 4 m below thegroundwater table, respectively (Fig. 1B). Thewells at 8 and 9 m were screened as describedabove, but the height of the screen at the shal-lowest well was 2.5 m instead of 0.5 m becauseinformation about the degree of fluctuation ofthe groundwater table was not available whenthe wells were installed. At the deep water-tablerecharge and reference sites, 5 wells were in-stalled at depths of 20, 21, 22, 23, and 24 m.These wells sampled water 1, 2, 3, 4, and 5 m

below the groundwater table, respectively. De-tailed information on the well-cluster designand installation procedure is reported in Datryet al. (2004) and Malard et al. (2004).

Spatiotemporal heterogeneity of groundwaterchemistry

Differences between recharge and referencesites in the spatiotemporal heterogeneity ofgroundwater physicochemistry were assessedby recording specific conductance, temperature,and dissolved O2 (DO) in well clusters at theshallow and deep water-table sites for 10 mousing YSI 600 XLM multiparameter loggers (Yel-low Springs Instrument Co., Yellow Springs,Ohio). Based on manufacturer information, theaccuracy and resolution of probes were 62%and 1 mS/cm for specific conductance, 60.15%and 0.018C for temperature, and 62% and 0.01mg/L for DO, respectively. Physicochemicalvariables were measured at 1-h intervals andloggers were transported to the laboratory every15 d for data transfer, data examination, probemaintenance, and calibration. Rainfall data (1-hintervals) were obtained from the nearest stationof the Meteorological Survey of Lyon. Specificconductance was used as a conservative mea-sure to detect stormwater inputs into ground-water because specific conductance of storm wa-ter (130 mS/cm) was much lower than that ofgroundwater (.540 mS/cm).

DOC, PO432, and DO

Monitoring sites. Groundwater samples werecollected from all 24 sites in October 2001 andApril, June, and October 2002 to test for differ-ences in nutrient (DOC, PO4

32) and DO concen-trations between reference and recharge sites (n5 96). Groundwater was pumped from eachwell from ;0.5 m below the water table with apneumatic piston pump equipped with 2 inflat-able packers (discharge rate ;10 L/min) (UWI-TEC Company, Mondsee, Austria). The first 50L of pumped water were used for sampling in-vertebrates (see below). Groundwater was col-lected in burned glass bottles for determinationof DOC and in polypropylene bottles for anal-ysis of PO4

32. Water samples were stored at 48C,returned within 4 h to the laboratory, and fil-tered through a 0.45-mm membrane filter. DOwas measured in the field with an O2 meter


(WTW OXI 330 meter, Weilheim, Germany),whose readings had been checked previouslywith the Winkler method. The ascorbic acidmethod was used for determination of PO4

32

(APHA 1998). DOC was measured by wet oxi-dation and CO2 detection after removal of in-organic C with orthophosphoric acid (APHA1998).

Well-cluster sites. Groundwater was collectedfrom well clusters at shallow water-table sites on3 dates during dry weather (5 November 2001,8 March and 18 November 2002) and 2 datesduring rainfall (30 November 2001 and 15March 2002) to determine the influence of re-charge on the vertical pattern of nutrient andDO concentrations. A surface suction pump(discharge rate ;10 L/min; Bou and Rouch1967) was used to sample groundwater. Thesampling protocol and physicochemical analy-ses were the same as those described above.

Density and species richness of invertebrateassemblages

Monitoring sites. Invertebrate samples werecollected from all 24 sites in October 2001 andApril, June, and October 2002 to test for differ-ences in density and species richness betweenreference and recharge sites (n 5 96). The first50 L of water extracted with the pneumatic pis-ton pump were filtered through a 100-mm meshnet. Faunal samples were preserved in 4% form-aldehyde. Preliminary tests indicated that inver-tebrate density and the number of additionaltaxa decreased logarithmically with increasingamount of pumped water. Pumping .50 L ofwater did not yield additional taxa.

Well-cluster sites. Invertebrate samples werecollected from well clusters of the shallow wa-ter-table site on 3 dates during dry weather (5November 2001, 8 March and 18 November2002) and 2 dates during rainfall (30 November2001 and 15 March 2002) to determine the influ-ence of recharge on the vertical distribution pat-tern of invertebrates (n 5 35). Invertebrates werecollected with a surface suction pump (dis-charge rate ;10 L/min; Bou and Rouch 1967)following the sampling procedure describedabove.

Faunal samples were processed using a dis-secting microscope. Invertebrates (except Oli-gochaeta) were counted and identified to spe-cies or genus. A distinction was made between

hypogean and epigean invertebrates. Hypogeaninvertebrates are obligate groundwater taxa thatcomplete their entire life cycle exclusively insubsurface water (i.e., stygobites in Gibert et al.1994). Epigean invertebrates are taxa that haveno affinities for groundwater but occur acciden-tally in alluvial sediments (i.e., stygoxenes in Gi-bert et al. 1994) or taxa that exploit groundwaterresources during at least a part of their life cycle(i.e., stygophiles in Gibert et al. 1994).

Data analysis

Two-way repeated measures analysis of var-iance (ANOVA) and Scheffe post-hoc multiplecomparison tests were used to test for differ-ences in concentrations of DOC, PO4

32, and DO,and richness and density of invertebrate assem-blages between recharge and reference sites.The design was a 2 (recharge or reference sites)3 2 (VZT .10 m or VZT ,10 m) 3 4 (samplingdate as repeated measure) ANOVA. The statis-tical significance of sampling date was testedusing Wilk’s Lambda multivariate test. One-wayrepeated measures ANOVA was used to test forthe effect of recharge on specific conductance,concentrations of DOC, PO4

32, and DO, andrichness and density of invertebrates at the shal-low water-table well-cluster sites. The designwas a 2 (recharge or reference sites) 3 5 (sam-pling date as repeated measure) ANOVA. Soluteconcentrations and invertebrate densities werelog10(x11) transformed prior to statistical anal-ysis to minimize differences among variances.Differences among means were tested usingpost-hoc Scheffe comparison tests. Significancefor all statistical analyses was accepted at a ,0.05. ANOVAs were done with Statistica 6 soft-ware (Statsoft, Tulsa, Oklahoma).

Taxon densities were log(x11) transformedand principal components analysis (PCA) wasused to examine differences in the compositionof invertebrate assemblages between sites (re-charge or reference sites) and depths at the shal-low water-table well-cluster sites. Average factorscores of samples for each depth at recharge andreference sites were plotted on the factor mapand were connected with lines to scores for eachsampling date. PCA and graphical displayswere done using ADE-4 software (Thioulouse etal. 1997).


Results

Spatiotemporal heterogeneity of groundwaterchemistry

Specific conductance, temperature, and DO ingroundwater varied little over time and depthin well clusters at the shallow and deep water-table reference sites (Fig. 2A–C). At both refer-ence sites, the annual amplitudes of tempera-ture and DO 1 m below the water table were,1.58C and 2.5 mg/L O2, respectively (Fig. 2A,B). In contrast, all variables showed marked var-iations over time and depth at the recharge sites(Fig. 2A–C).

Recharge markedly altered the thermal re-gime of groundwater (Fig. 2A). Seasonal pat-terns in groundwater temperature at the re-charge sites were typically more pronouncedand seasonal maxima and minima occurred ear-lier than at reference sites. The annual ampli-tudes of temperature were ;138C and ;8.58C atthe shallow and deep water-table recharge sites,respectively.

The effect of stormwater infiltration on DOpatterns typically was more pronounced at theshallow water-table recharge site than at othersites (Fig. 2B). Cold winter rainfalls elevated DOconcentrations, whereas warm summer rainsdecreased DO. However, DO concentrations ingroundwater never fell below 1 mg/L, except inNovember 2001 at the shallow water-table re-charge site when DO groundwater was tempo-rarily suboxic at a depth of 2 m.

Each rainfall event (Fig. 2D) produced aplume of low-salinity storm water, thereby gen-erating repeated decreases in specific conduc-tance (Fig. 2C). At both recharge sites, temporalvariation in specific conductance was stronglyattenuated with depth, indicating that storm-water flow was restricted to the first 3 m belowthe water table.

DOC, PO432, and DO

Monitoring sites. DOC was significantlyhigher and DO was significantly lower at re-charge sites than at reference sites (Table 2, Fig.3A, B). DOC and DO differed significantly be-tween recharge and reference sites only withVDZ ,10 m (Scheffe test, p , 0.010). PO4

32 didnot differ between recharge and reference sites(ANOVA, p 5 0.055), but PO4

32 was higher at

recharge sites than at reference sites with VZT,10 m (Scheffe test, p 5 0.012, Fig. 3C). Therecharge effect did not interact with time forany response variable except DOC, indicatingthat differences among recharge and referencesites varied little over time (Table 2). DO wassignificantly lower (Scheffe test, p 5 0.002) andDOC and PO4

32 significantly higher (Scheffetest, p , 0.05) at recharge sites with VZT ,10m than at recharge sites with VZT .10 m (Fig.3A–C). DO and DOC did not differ betweenVZT categories at reference sites.

Well-cluster sites. DOC and PO432 concentra-

tions were significantly higher at the shallowwater-table recharge site than at the shallow wa-ter-table reference site (Table 3, Fig. 4A, B). Con-centrations decreased with increasing depth be-low the water table at the recharge site duringrainfall events. Specific conductance was mark-edly lower and DOC concentration markedlyhigher 1 and 2 m below the water table than 3and 4 m below the water table during rainfall(Fig. 4A, C). Few differences were found amongdepths for specific conductance, DOC, andPO4

32 concentrations at the reference and re-charge sites during dry weather (Fig. 4A–C).

Density and richness of invertebrate assemblages

A total of 5672 invertebrates was collectedfrom groundwater. Twenty-eight taxa wereidentified, and 18 were hypogean (Table 4). Theaverage density was 44 invertebrates/50 L ofpumped water (SD 5 117, range 5 0–1015, n 5128). The 3 most abundant taxa were the hy-pogean Pseudocandona zschokkei (Ostracoda) andSalentinella juberthieae (Amphipoda), and the epi-gean Acanthocyclops venustus (Cyclopoida). Mosttaxa had low densities, but only 8 taxa occurredin ,5% of the samples.

Monitoring sites. The total density of inver-tebrates (Fig. 5A) and hypogean taxa was sig-nificantly higher at recharge sites than at refer-ence sites (Table 2), and the interaction betweenrecharge and VZT was significant. Densities dif-fered significantly between recharge and refer-ence sites only when VZT ,10 m (Scheffe test,p , 0.05; Fig. 5A). Mean density was 62 6 100invertebrates/50 L of pumped water at rechargesites (n 5 20) and 9 6 13 invertebrates/50 L atreferences sites (n 5 24) with VZT ,10 m. Rich-ness did not differ significantly between re-charge and reference sites, but the interaction of


FIG. 2. Time series (1-h intervals) for temperature (A), dissolved O2 (DO) concentration (B), specific con-ductance (C), and rainfall (D). Temperature, DO, and specific conductance were measured at depths of 1, 2,and 4 m below the water table in well clusters at shallow water-table (vadose zone thickness [VZT] ,10 m)(left column) and deep water-table (VZT .10 m) (right column) sites. Arrows indicate reference and rechargetime series.


TABLE 2. Results of 2-way repeated measures anal-ysis of variance for testing the effects of recharge andvadose zone thickness (VZT) on dissolved O2 (DO),dissolved organic C (DOC), density of invertebratesand hypogean taxa, and species richness. Time was arepeated factor (4 dates). Only significant effects areshown.

Source df F p

DORechargeVZTTimeRecharge 3 VXT

1131

7.9217.80

3.624.73

0.0110.0010.0180.042

DOCRechargeRecharge 3 time

13

7.9411.99

0.011,0.001

Invertebrate densityRechargeVZTRecharge 3 VZT

111

6.9230.50

8.32

0.016,0.001

0.009

Hypogean densityRechargeVZTRecharge 3 VZT

111

5.9828.30

6.32

0.011,0.001

0.007

Species richnessVZTRecharge 3 VZT

11

15.604.51

0.0010.046

FIG. 3. Mean (61 SD) dissolved organic C (DOC)(A), dissolved O2 (DO) (B), and PO4

32 (C) at recharge(n $ 5) and reference (n $ 5) sites for vadose zonethickness (VZT) ,10 m and .10 m. Sampling dates:1 5 October 2001, 2 5 April 2002, 3 5 June 2002, and4 5 October 2002.

recharge and VZT was significant. Richness wassignificantly higher at recharge sites than at ref-erence sites with VZT ,10 m, (Scheffe test, p 50.026; Fig. 5B). Recharge effect and time did notinteract, indicating that differences in densityand richness between recharge and referencesites varied little over time. Recharge sites withVZT ,10 m had significantly more inverte-brates and species than recharge sites with VZT.10 m (Scheffe test, p , 0.01; Fig. 5A, B), butdensity and richness did not differ betweenVZT categories for reference sites (Scheffe test,p , 0.05; Fig. 5A, B).

Well-cluster sites. Density and richness of in-vertebrate and hypogean assemblages were sig-nificantly greater at the shallow water-table re-charge site than at the shallow water-table ref-erence site (Table 3, Fig. 6A, B). Density andrichness did not vary with time. Mean densityand richness were 168 6 230 invertebrates and7 6 2 species/50 L of pumped water at the re-charge site (n 5 20) vs 18 6 14 invertebrates

and 3 6 2 species/50 L at the reference site (n5 15). The density of invertebrate assemblagesdecreased with increasing depth at shallow wa-ter-table sites, regardless of recharge or weatherconditions (Fig. 6A). Mean density decreasedfrom 411 6 378 invertebrates/50 L of water to33 6 16 invertebrates/50 L at 1 and 4 m, re-spectively, at the recharge site and from 32 6 8


TABLE 3. Results of 1-way repeated measures anal-ysis of variance for testing differences in dissolved C(DOC), PO4

32, and density and richness of invertebrateassemblages between the shallow water-table refer-ence site and recharge site. Time was a repeated factor(4 dates). Only significant effects are shown.

Source df F p

DOCPO4

32

Invertebrate densityHypogean densitySpecies richnessHypogean richness

111111

37.4314.03

8.1913.4635.92

8.35

0.0020.0140.0350.0140.0020.034

FIG. 4. Vertical profiles (mean 61 SD) for dissolved organic C (DOC) (A), PO432 (B), and specific conductance

(C) in groundwater in well clusters at the shallow water-table recharge site and shallow water-table referencesite during dry weather (n 5 3) (upper row) and rainfall (n 5 2) (lower row).

to 5 6 4 invertebrates/50 L at 1 and 4 m, re-spectively, at the reference site. Richness showedno vertical pattern (Fig. 6B).

PCA. Axis 1 of the PCA explained 18.4% ofthe variance in the taxonomic data and separat-ed taxa along a density gradient (Fig. 7A). Themost abundant taxa, including S. juberthieae, Ni-phargus renei (Amphipoda), A. venustus, P.zschokkei, and Cryptocandona kieferi (Ostracoda),contributed 61.8% of the factor weights to theaxis and were located on the positive side of theaxis. Axis 1 separated samples on the basis ofrecharge (Fig. 7B). Samples collected at refer-ence sites had negative scores on Axis 1. Re-charge-site samples collected at a depth of 2 mbelow the water table were plotted near refer-ence-site samples because they contained fewerindividuals. Nine taxa, including Oligochaeta,Acanthocyclops robustus (Cyclopoida), Diacyclops

languidoides ssp. (Cyclopoida), N. renei, P. zschok-kei, A. venustus, C. kieferi, S. juberthieae, and Acan-thocyclops rhenanus (Cyclopoida), were signifi-cantly more abundant at the recharge site thanat the reference site (1-way repeated ANOVA, p, 0.05). The cyclopoid Eucyclops serrulatus wasthe only taxon significantly more abundant atthe reference site (p 5 0.015).

Axis 2 of the PCA explained 10.6% of the var-iance in the taxonomic data and separated taxaon the basis of habitat affinity (Fig. 7A). Theepigean taxa E. serrulatus, Paracyclops fimbriatus(Cyclopoida), Macrocyclops albidus (Cyclopoida),and Oligochaeta contributed 42.5% of the factorweights to the axis and were located on the neg-ative side of the axis. The hypogean taxa Salen-tinella lescherae (Amphipoda), Microcharon sp. (Is-opoda), and A. rhenanus contributed 22.6% ofthe factor weights to the axis and were locatedon the positive side of the axis. Axis 2 separatedsamples along a vertical gradient (Fig. 7B). Sam-ples from 1 and 2 m below the water table hadlower scores than samples from 3 and 4 m be-low the water table. Several taxa, including mostepigean invertebrates (e.g., E. serrulatus, P. fim-briatus, M. albidus, A. robustus, and D. languidoi-des ssp.), were more likely to be found in theupper layers of groundwater than in the lowerlayers (Fig. 8). In contrast, other taxa, includingmost hypogean species, occurred at severaldepths (e.g., P. zschokkei, S. juberthiae) or were re-stricted to the lower layers of groundwater (e.g.,Microcharon sp., S. lescherae, and A. rhenanus).


TABLE 4. Mean density and frequency of occurrence of taxa collected in groundwater. Density is expressedas no. individuals/50 L of pumped water. HYP 5 hypogean species, EPI 5 epigean species, * 5 ecologicalcategory not assigned.

TaxonEcologicalcategory

Density (no. individuals/50 L water pumped)

Mean RangeFrequency of

occurrence (%)

AmphipodaNiphargus renei Karaman, 1986N. pachypus Schellenberg, 1933N. kochianus Bate, 1859N. fontanus Bate, 1859Niphargopsis casparyi (Pratz, 1866)

HHHHH

0.60.20.10.20.1

0–200–160–60–90–6

10.93.23.14.65.5

Crangonyx subterraneanus Bate, 1859Salentinella juberthieae Coineau, 1968S. lescherae Coineau, 1968?S. delamarei Coineau, 1962

HHHH

,0.17.4

,0.10.1

0–60–1760–30–5

0.830.54.73.9

SyncaridaBathynellidae sp. H 0.2 0–6 5.5

CyclopoidaMacrocyclops albidus (Jurine, 1820)Eucyclops serrulatus (Fischer, 1851)Paracyclops fimbriatus (Fischer, 1853)Acanthocyclops robustus (Sars, 1863)A. venustus (Norman & Scott, 1906)

EEEEE

0.20.8

,0.12.34.2

0–90–240–60–570–195

6.27.82.9

17.215.6

A. sensitivus (Graeter & Chappuis, 1914)A. rhenanus Kiefer, 1936Diacyclops languidus (Sars, 1863)

HHE

0.10.41.3

0–40–60–75

5.111.78.6

D. languidoides ssp.D. zschokkei (Graeter, 1910)

HarpacticoidaParastenocaris sp.Harpacticoida sp.

*H

H*

0.41.7

0.6,0.1

0–160–75

0–160–1

7.87.8

22.61.4

IsopodaMicrocharon sp. H 0.4 0–12 9.4

AsellidaeProasellus walteri (Chappuis, 1948) H 1.7 0–188 5.1

OstracodaPseudocandona zschokkei (Wolf, 1920)Cryptocandona kieferi (Klie, 1938)Pseudocandona albicans (Brady, 1864)

HHE

11.30.80.5

0–8000–400–40

12.96.26.2

Oligochaeta * 1.7 0–50 35.9

Discussion

Organic matter supply and spatiotemporalheterogeneity

High concentrations of infiltrated DOC at re-charge sites were expected to cause higher DOCconcentrations in groundwater at recharge sitesthan at reference sites. Higher infiltration rates

led to increased DOC and PO432 concentrations

for VZT ,10 m. Furthermore, DO concentra-tions were lower at recharge sites than at refer-ence sites for VZT ,10 m, suggesting that DOCsupply stimulated microbial respiration. DOCconcentrations were only slightly higher at re-charge than at reference sites, but the annualflux of DOC at the water table was substantially


FIG. 5. Mean (61 SD) density (A) and richness (B)of invertebrate assemblages at recharge (n $ 5) andreference (n $ 5) sites for vadose zone thickness (VZT),10 m and VZT .10 m. Sampling dates are as inFig. 3.

higher at recharge sites than at reference sitesas a result of elevated infiltration rates at re-charge sites. Datry (2003) estimated that 68 gDOC m22 y21 were transported by water infil-trating into groundwater at the shallow water-table well-cluster site, whereas only 0.2 g DOCm22 y21 were transported by water infiltratinginto groundwater at the reference site.

DOC concentrations did not differ betweenrecharge and reference sites for VZT .10 m.Malard et al. (2004) analyzed time series of rain-fall and groundwater level, and calculated atransit time of only 4 h for water in the 19-mvadose zone at the deep water-table well-clusterrecharge site. The vadose zone efficiently re-tained or degraded DOC despite the high ver-tical velocity of infiltrating water linked to high

sediment permeability and hydraulic head instormwater basins. The amount of DOC reach-ing the water table was higher at recharge sitesthan at reference sites because of increasedgroundwater recharge rates at recharge sites.However, the absence of differences in DOCconcentration between recharge and referencesites for VZT .10 m suggested that most DOCentering groundwater was refractory. The in-crease in the flux of refractory DOC had no de-tectable influence on DO concentration becauseit did not stimulate microbial respiration. Pabichet al. (2001) demonstrated that DOC concentra-tion at the water table of the Cape Cod (USA)aquifer decreased exponentially with increasingVZT; complete degradation of surface-derivedDOC in naturally recharged areas was observedwhen VZT was .5 m. In our study, DOC con-centration in groundwater at recharge sites de-creased linearly as VZT increased (DOC 50.983–0.03[VZT], r2 5 0.19, p , 0.010). Back-ground values (i.e., DOC ,0.5 mg/L) were ob-served when VZT was .17 m.

Recharge with storm water considerably in-creased the temporal and vertical heterogeneityof specific conductance, temperature, and DO ingroundwater. An increase in spatiotemporal het-erogeneity was observed for both VZTs, but var-iation was much higher at the shallow water-table site. The thermal regime of groundwater istypically variable in recharge zones because ofthe convective transport of thermal energy byinfiltrating water (Lapham 1989, Silliman andBooth 1993, Constantz and Thomas 1997). Inour study, groundwater recharge increased theseasonal amplitude of temperature and pro-duced thermal variations of short duration (i.e.,a few days) during rainfall events. The physi-cochemical signature of infiltrating water dis-appeared progressively with increasing depthbelow the water table, thereby generating steepvertical gradients in groundwater physicochem-istry.

Density and richness of invertebrate assemblages

Our hypothesis that low organic matter sup-ply and spatiotemporal heterogeneity might beresponsible for reduced biodiversity in ground-water and that recharge zones would harbormore invertebrates and species was supported.Density and richness were significantly higherat recharge than at reference sites with VZT ,10


FIG. 6. Vertical profiles (mean 61 SD) for density (A) and richness (B) of invertebrate assemblages in ground-water in well clusters at the shallow water-table recharge site and shallow water-table reference site during dryweather (n 5 3) (upper row) and rainfall (n 5 2) (lower row).

m. However, density and richness were nothigher at recharge than at reference sites withVZT .10 m, even though spatiotemporal het-erogeneity of groundwater physicochemistrywas considerably higher at recharge than refer-ence sites. This result suggested that organicmatter supply, rather than spatiotemporal het-erogeneity, was a primary factor determiningbiodiversity patterns in groundwater. Indeed,VZT controlled the amount of DOC reachinggroundwater, but it did not markedly attenuatethe effect of stormwater infiltration on ground-water spatiotemporal heterogeneity. Densityand richness of invertebrate assemblages at re-charge sites decreased logarithmically withVZT (density 5 250.5291.5[VZT], r2 5 0.38, p, 0.010; richness 5 12.624.2[VZT], r2 5 0.60, p, 0.010). The 3 recharge sites (sites 1, 2, and 3)that had the highest number of invertebrates(125 6 135 individuals) and species (7 6 3 taxa)were located in shallow water-table areas (VZT,7 m). DOC is not a direct source of food forgroundwater invertebrates, but an increase inDOC supply can stimulate the production ofbiofilm, which is grazed by invertebrates (Hak-enkamp 1991, Mauclaire et al. 2000). Mosslacherand Notenboom (1999) stated in their review of

groundwater biomonitoring studies that an in-crease in microbial production resulting fromorganic matter enrichment of groundwater ledto increased metazoan density as long as DOremained available.

Our finding that hypogean invertebrate as-semblages were more abundant and diverse inrecharge zones fed by infiltration of surface wa-ter contrasts with biodiversity patterns com-monly observed in the hyporheic zone ofstreams. Most studies that have examined theinfluence of groundwater–surface water ex-changes on the distribution of hyporheos re-ported that hypogean invertebrates were lessabundant in downwelling zones than in up-welling zones (Creuze des Chatelliers and Rey-grobellet 1990, Dole-Olivier and Marmonier1992, Boulton 2000, Fowler and Death 2001,Fowler and Scarsbrook 2002). However, inputsof dissolved and particulate organic matter typ-ically are higher and environmental conditionsare more variable in downwelling zones than inupwelling zones (Findlay et al. 1993, Marmonieret al. 1995, Morrice et al. 2000). Most studies onthe distribution patterns of hyporheos in riverfloodplains also showed that hypogean inver-tebrates occurred preferentially in temporally


FIG. 7. Principle component analysis factor scores of species (A) and samples (B) from well clusters at theshallow water-table recharge and shallow water-table reference sites. In A, numbers in parentheses next to axislabels indicate % of the total variance explained by the axis. See Table 4 for full genus and species names. InB, symbols correspond to the center of gravity for samples (n 5 5) collected at each depth at the recharge andreference sites, respectively. Each sample is connected to the center of gravity with a line. Numbers next tosymbols indicate depth (m) from which the samples were collected.


FIG. 8. Vertical distribution of selected species at shallow water-table well-cluster sites. Species were orderedon the basis of their factor scores along axis 2 of the principal component analysis (see Fig. 7A). Asterisksindicate epigean species. Diameters of circles are proportional to the mean no. individuals/50 L of pumpedwater (n 5 5 dates). Crosses indicate sites where no individuals were collected. See Table 4 for full genus andspecies names.

stable hyporheic habitats receiving low organicmatter inputs from surface water (i.e., deep sed-iment layers and sediments of groundwater-fedchannels; Marmonier et al. 1992, Dole-Olivier etal. 1993, Pospisil 1999, Malard et al. 2003). Therole of organic matter supply and spatiotem-poral heterogeneity in determining distributionpatterns of true groundwater taxa in hyporheichabitats may be blurred by competition pro-cesses among taxa. High availability of organicmatter may be attractive for hypogean inverte-brates, but access to food resources is con-strained by biotic interactions with epigean in-vertebrates that densely colonize organic-mat-ter-rich habitats (Brunke and Gonser 1999). Onthe other hand, hypogean taxa may be moresensitive to physical disturbances during floods(bed movement) than epigean invertebrates.

Vertical patterns in invertebrate assemblages

The density of invertebrate assemblages de-creased with increasing depth below the watertable at both reference and recharge sites. Thedensity gradient was not markedly steeper atthe recharge site, despite greater DOC input andgreater temporal variability in the upper layersthan in the lower layers of groundwater. Mau-claire and Gibert (2001) reported that the den-sity of invertebrates in the alluvial aquifer of theRhone River (France) was greater at 1 and 2 mbelow the water table than at 4 and 7 m belowthe water table. Pospisil et al. (1994) showed thatmost cyclopoids occurred immediately belowthe water table in suboxic groundwater (DO,0.3 mg/L) of the Lobau wetland (Austria). Cy-clopoids migrated into the deeper layers of


groundwater only when the aquifer was replen-ished with oxic surface water. In our study, DOwas probably not a limiting factor becausegroundwater remained oxygenated throughoutthe year. We suggest that higher invertebratedensities in the upper sediment layers of oxicgroundwater reflect the tendency of ground-water taxa to develop abundant populations inresource-rich patches located near sites of sur-face production of organic matter.

The composition of groundwater invertebrateassemblages also changed with increasingdepth below the water table. Some taxa, includ-ing most epigean species, preferentially oc-curred in the vicinity of the water table, whereasothers, mainly hypogean species, colonizeddeeper layers of groundwater. This shift in spe-cies partly explained why species richness didnot increase with increasing depth below thewater table. The vertical shift in species com-position was observed at both sites despitemarked differences in DOC input and spatio-temporal heterogeneity between sites. The com-positional shift in invertebrate assemblages withincreasing depth from the water table could re-flect the outcome of biotic interactions amongtaxa, but this hypothesis has yet to be tested.Brunke and Gonser (1999) suggested that thedepth distribution of epigean and hypogeantaxa within the hyporheic zone of the Toss Riv-er, Switzerland, was governed by trade-offs. Ac-cording to Brunke and Gonser (1999), depthpenetration within the sediment by epigean taxawas limited by food supply, whereas coloniza-tion of the upper nutrient-rich layers by hypo-gean taxa was restricted by interference com-petition. This model of community organizationwithin the hyporheic zone of streams also mightprovide a valid explanation of the distributionpattern of invertebrate assemblages at the water-table interface of phreatic aquifers. Our studysuggests that the availability of organic matter,which is considered the main selection pressureleading to adaptations among groundwater or-ganisms (Huppop 2000), is probably a key fac-tor driving biodiversity patterns in groundwa-ter.

Acknowledgements

This work was part of the Experimental Ob-servatory for Urban Hydrology (OTHU) projectfunded by the urban community of Lyon

(COURLY) and the Rhone–Alpes Region. Weare indebted to G. Bouger and L. Vitry for ded-icated assistance with field and laboratory work.We thank the following specialists for their valu-able help in the identification of taxa: M. J. Dole-Olivier (Salentinella), R. Ginet (Niphargus), F.Lescher-Moutoue and J. Mathieu (Cyclopoida),and P. Marmonier (Ostracoda). We thank C.Dahm, M. J. Dole-Olivier, F. Mermillod-Blondin,M. Scarsbrook, and 2 anonymous referees forcomments that improved earlier versions of thismanuscript.

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Received: 16 November 2004Accepted: 21 April 2005

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Response of invertebrate assemblages to increased groundwater recharge rates in a phreatic aquifer

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